Total synthesis of avermectin B_1a revisited

Abstract

Avermectins were isolated as compounds possessing anthelmintic activity from the culture broth of Streptomycesavermitilis by Ōmura and co-workers. Owing to their potent anthelmintic and insecticidal activities, as well as their unique pentacyclic architecture, the avermectin family attracted keen interest from synthetic organic chemists. We have recently completed a more efficient and straightforward total synthesis of avermectin B_1a, as compared with previous syntheses.

Introduction

In the 1970s, Ōmura’s group at the Kitasato Institute and researchers of Merck Sharp and Dohme Research Laboratories discovered potent antiparasitic agents, the avermectins, from the culture broth of Streptomyces avermitilis (S. avermectinius).^{1, 2} Among this family, avermectin B_1a (1, Scheme 1) is the most potent anthelmintic congener. Ivermectin, a mixture of 22,23-dihydro-avermectins B_1a and B_1b, has been used as an important anthelmintic in veterinary fields and for the control of onchocerciasis and lymphatic filariasis in more than 200 million people worldwide.^{3, 4, 5}

Avermectins are 16-membered macrolactones that consist of a 6,6-spiroacetal north segment attached to the disaccharide oleandrosyl-oleandrosyl, and a unique, highly sensitive hexahydrobenzofuran south segment. Avermectins and structurally related milbemycins⁶ attracted keen interest from synthetic organic chemists. After some difficulties, total syntheses for avermectin B_1a (1),^{7, 8, 9} A_1a,¹⁰ ivermectin (aglycon)¹¹ and milbemycin D¹² and G¹³ were achieved. These successful syntheses used some indirect strategies to control the position of the C3–C4 double bond and C2 stereochemistry.^{7, 8, 9, 10, 12, 13, 14} Construction of the hexahydrobenzofuran and the E,E-diene, however, was less than satisfactory in terms of control of stereo- and regiochemistries. Previously, we developed a straightforward route to the hexahydrobenzofuran segment,¹⁵ which allowed us to complete a total synthesis of milbemycin α₁.¹⁶ We describe herein an improved and efficient approach to the north^{17, 18} and south segments, as well as a stereocontrolled total synthesis of avermectin B_1a (1).

Results and discussion

Synthesis plan

Our renewed plan for the synthesis of avermectin B_1a (1) is shown in Scheme 1. The challenges of the present synthesis are: (1) stereoselective construction of the hexahydrobenzofuran and spiroacetal segments, as well as the C8–C11 E,E-diene; and (2) macrolactone cyclization without disturbing the readily epimerizable C2 stereogenic center and the C3–C4 double bond, which is prone to migrate to the C2 position. Straightforward retrosynthesis of 1 is summarized in Scheme 1. Based on our successful synthesis of milbemycin α₁,¹⁶ seco acid 2 should be generated from oxetane acetal 4, which could be assembled through Julia or Wittig coupling of the north segment 5 and the south segment 6. Oxetane acetal 6 would be prepared from bis-carbonyl compound 7 via an intramolecular aldol reaction. It is noteworthy that the oxetane acetal group is a masked form of the labile β-hydroxy aldehyde, which can maintain the C2 stereochemistry and C3–C4 double bond during construction of 4 and is readily convertible to carboxylic acid 2.

Synthesis of the south segment began with readily available d-sorbitol 8 (Scheme 2). Acid-promoted cyclization,¹⁹ followed by formation of acetonide, afforded tetrahydrofuran 9. Sequential protection of the remaining diol of 9 with t-butyldiphenylchlorosilane (TBDPSCl) and 2-naphtylmethyl (NAP) bromide gave fully protected 10. Selective hydrolysis and oxidative cleavage of the resultant diol furnished aldehyde 11, which was directly used in the next vinylogous Mukaiyama aldol reaction without purification. Treatment of 11 with 2-(trimethylsiloxy)furan 12 in the presence of BF₃·OEt₂ in CH₂Cl₂ at −78 °C afforded the desired adduct 13 as a 5:1 C4-diastereomeric mixture.²⁰ The stereochemical outcome was assumed by the non-chelate Felkin–Anh model.²¹ Hydrogenation of the double bond of 13 by NiCl₂ and NaBH₄, followed by t-butyldimethylsilyl (TBS) protection of the hydroxyl group gave lactone 14 in 90% overall yield. After diisobutylaluminum hydride (DIBAL) reduction of 14, the resultant diol was protected as a bis-triethylsilyl (TES) ether, and then the NAP group was removed by treatment with DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone) to give 15.²² Oxidation of 15 by Dess–Martin periodinane (DMP)²³ in the presence of pyridine afforded bis-carbonyl compound 7 directly as a major product together with ketone 16, which was converted to 7 through conventional hydrolysis and oxidation.

With the requisite 7 in hand, we examined the intramolecular aldol reaction. Upon treatment of 7 with Et₃N in 1,2-dichloroethane, no reaction occurred, even at high temperatures (entry 1; Scheme 2). Treatment with either lithium diisopropylamide or piperidine gave unfruitful results (entries 2 and 3). However, when 7 was reacted with 1,8-diazabicyclo[5.4.0]undec-7-ene in CH₂Cl₂ at 0 °C, cyclized product 17 was formed as a 3.9:1 mixture of C2-diastereomers (entry 4). Owing to the instability of 17 on silica gel, the crude product was used without purification in the next reaction.

Based on our previous results,¹⁶ β-hydroxy aldehyde 17 was applied to an oxetane acetal formation (Scheme 2). Crude 17 was treated at 50 °C with trityl trifluoromethanesulfonate, prepared in situ from trityl chloride and silver trifluoromethanesulfonate (AgOTf),^{16, 24} to successfully produce trityl oxetane acetal 18 as a single isomer in 75% overall yield (2 steps from 7). Selective cleavage of the C4 TES ether of 18 by the action of AcOH-buffered TBAF (tetrabutylammonium fluoride), followed by TPAP (tetrapropylammonium perruthenate oxidation) afforded ketone 19 in 71% yield. The C4a methyl group and the C3–C4 double bond were then installed by the following three steps: (1) hydrazone formation; (2) iodination with iodine in the presence of Et₃N;²⁵ (3) Suzuki–Miyaura coupling with methylboronic acid,²⁶ which led to 20 in 94% yield. The structure of 20 was unambiguously confirmed by X-ray crystallography of the corresponding bis-TBS ether, as shown in Figure 1. Selective removal of the TBDPS group by NaOH²⁷ in the presence of the TBS group gave rise to secondary alcohol 21. Dess–Martin oxidation of 21 afforded the south segment (6). Two carbon elongated south segment 23 was also synthesized from 6 (77% overall yield) in three steps: Horner–Wadsworth–Emmons olefination of 6 with 22, DIBAL reduction, and subsequent Dess–Martin oxidation.

Figure 1

ORTEP drawing of bis-TBS ether of 20.

Next, we synthesized the C22–C28 unit 29 of the north segment (Scheme 3). Enantioselective reduction of ynone 24 was achieved by an asymmetric hydrogen transfer using Noyori catalyst 25 (ref. 28) to give a chiral secondary alcohol (98% ee), which was converted to the corresponding mesylate 26 in 84% overall yield. Diastereoselective coupling between 26 and aldehyde 27 was realized using Et₂Zn in the presence of a catalytic amount of Pd(OAc)₂ and PPh₃ according to the Marshall method.²⁹ Homopropargylic alcohol 28 was obtained in good diastereoselectivity (dr=14:1). Removal of the trimethylsilyl (TMS) group of terminal alkyne 28 with K₂CO₃ in MeOH, followed by TMS protection of the secondary alcohol afforded the C22–C28 unit 29 in 87% yield.

The C15–C21 unit 36 was synthesized from propanediol mono-benzyl ether 30 (ref. 30) (Scheme 3). TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) oxidation of 30 in the presence of PhI(OAc)₂ gave aldehyde 31, which was used directly in the next aldol reaction developed by Oguni et al.³¹ Treatment of 31 with diketene 32 and Ti(Oi-Pr)₄ in the presence of a catalytic amount of chiral Schiff base 33 afforded δ-hydroxy-β-ketoester 34 in 76% yield. Antiselective Evans reduction³² of ketone 34 and acid catalyzed lactonization, followed by TBS protection of alcohol 35, gave the C15–C21 unit 36 in 51% overall yield.

Carefully lithiated terminal alkyne 29 with n—BuLi was coupled with lactone 36 to give ynone in 64% yield (Scheme 3). Treatment of the ynone with citric acid in methanol caused selective cleavage of TMS ether and formation of methyl acetal 37. Acetal 37 was converted to spiroacetal 38 via Lindler reduction,³³ acid treatment and Birch reduction in 59% overall yield. At this stage, the undesired minor diastereomer, which was derived from the antipode of 36, was separable by silica gel column chromatography. The side chain of 38 was elongated via five steps following Danishefsky’s procedure to give the north segment 4.¹⁰

Having both segments, we in turn attempted Julia coupling between C10 and C11 (Scheme 3). The requisite sulfones 43a and 43b were prepared from 4. Selective dihydroxylation of the terminal olefin, oxidative cleavage of the resultant diol, NaBH₄ reduction, Mitsunobu reaction with 1-phenyl-1H-tetrazole-5-thiol or benzothiazole-2-thiol, followed by Mo-catalyzed oxidation of sulfide³⁴ afforded sulfones 43a and 43b. Treatment of the PT sulfone 43a³⁵ with either potassium hexamethyldisilazide or n-butyllithium in THF, followed by the addition of aldehyde 23 caused no reaction (entries 1 and 2; Scheme 3). When a smaller BT sulfone 43b (ref. 36) was used, aldehyde 23 was consumed, but only a complex mixture was formed (entry 3). Thus, it is conceivable that the coupling reaction was prevented by deprotonation of the acidic C8a hydrogen of 23 to form the corresponding enolate. These results led us to abandon the Julia coupling route.

An alternative Wittig route through C8–C9 bond formation was thus examined, as depicted in Scheme 4. The phosphonate 46a, triphenylphosphonium salt 46b and trimethyl phosphonium salt 46c were prepared from aldehyde 42 by conventional means. As the Horner–Wadsworth–Emmons reaction of ketone 6 with phosphonate 22 was successful, as shown in Scheme 2, phosphonate 46a was first reacted with ketone 6 (entries 1 and 2, Scheme 4). Neither of these conditions, however, gave the desired product. When triphenylphosphonium salt 46b was used, as in our previous study of the synthesis of milbemycin,¹⁶ the desired 8,9-E-adduct 44 was only formed at a low yield and 40% of 6 was recovered (entry 3). As steric repulsion between 46b and 6 was likely to hinder the reaction, the smaller trimethylphosphonium salt 46c was used.³⁷ As expected, E-adduct 44 was obtained in 45% yield. Furthermore, we found that addition of hexamethylphosphoric triamide improved the yield up to 80% (entry 5). Under these optimal conditions, 1.7 eq of 46c was sufficient to complete the reaction. As 44 was unstable on silica gel, the reaction mixture was purified after removal of TBS groups by TBAF, giving rise to triol 47 in 56% overall yield.

Before further transformation of 47, we examined the macrolactonization of seco acid 2, which was degraded from natural avermectin B_1a.^{38, 39} Under Yonemitsu–Yamaguchi conditions,⁴⁰ the desired aglycon 49 was obtained in 36% yield. Furthermore, we found that Shiina macrolactonization of 2 using 2-methyl-6-nitrobenzoic anhydride⁴¹ afforded 49 in 67% yield without serious side reactions.

Encouraged by our preliminary experiments, we then attempted a de novo synthesis of aglycon 49 from 47 (Scheme 4). As experienced in the synthesis of milbemycin,¹⁶ removal of trityl group of 47 by formic acid⁴² and subsequent Krause–Pinnick oxidation^{43, 44} must be carried out under carefully degassed conditions using an argon atmosphere, as well as degassed reagents and solvents. Otherwise, we ended up with a complex mixture. Thus, the β,γ-unsaturated aldehyde 48 was very likely to be oxidized quickly by molecular oxygen and decomposed.¹⁶ In practice, after 47 was treated with formic acid in degassed t-BuOH and 2-methyl-2-butene, the reaction mixture was directly added to a degassed aqueous solution of NaClO₂ and NaH₂PO₄ to give the seco acid 2. Subsequent Shiinamacrolactonization of 2 produced the avermectin B_1a aglycon 49 in 54% overall yield.

As shown in Scheme 5, disaccharide 3 was prepared according to Nicolaou’s procedure³⁹ with some modifications. l-rhamnal 52 was synthesized from l-rhamnose 50 by the modified Koreeda method.^{45, 46} Protection of diol 52 with benzoyl and TBS groups, respectively, followed by methyl ether formation gave 53,⁴⁷ which was treated with NIS and then reduced to afford oleandrose 54. Treatment of 54 with thiophenol in the presence of BF₃·OEt₂, followed by TBAF deprotection, afforded a 1.2:1 anomeric mixture of thioglycosides (55α and 55β). Fluoride 56 was also prepared by DAST. Treatment of 55α and 56 under modified Mukaiyama conditions^{39, 48} (SnCl₂, AgOTf and 2,6-di-t-Bu-4-Me-pyridine (DTBMP) in Et₂O) gave C1″α-disaccharide 3 in 46% yield together with C1″ β-isomer (9%).

Glycosylation was tested using aglycon 58 derived from natural avermectin B_1a.³⁹ Aglycon 58 and fluoride 57, prepared from thioglycoside 3 by DAST and N-bromosuccinimide (NBS), were treated with SnCl₂ and AgClO₄ in Et₂O (entry 1; Scheme 5). The desired disaccharide 59, however, was obtained only as a minor product (~10%) along with hydrolyzed monosaccharide 60. The addition of DTBMP as an acid scavenger improved the yield of 59α to 22% in addition to C1′ β-isomer (10%), whereas a significant amount of glycal 61 was isolated. Thus, we assumed that generation of 61 is attributable to the short lifetime of the oxonium cation intermediate formed from 57, and expected that MeCN as a solvent instead of ether might more effectively stabilize the intermediate. We found that MeCN gave 59α in higher yield (45%; its β-isomer: 20%, entry 3).

Finally, the total synthesis of avermectin B_1a (1) was achieved as described in Scheme 5. The C5-hydroxy group of synthetic 49 was selectively protected, and the resultant 58 was coupled with glycosyl fluoride 57 by the action of SnCl₂ and AgOTf in the presence of DTBMP in MeCN to give the desired α-glycoside 59α in 45% yield. Removal of TBS groups by buffered HF·pyridine in MeCN⁹ gave avermectin B_1a (1) in 88% yield. Spectroscopic data of synthetic samples were consistent with those of the natural product.

In conclusion, we achieved an efficient and straightforward total synthesis of avermectin B_1a (1). The highlights of total synthesis include: (i) a vinylogous Mukaiyama aldol reaction to install the C1–C4 carbons (11+12→13); (ii) an intramolecular aldol reaction to form the octahydrobenzofuran (7→17) (iii) a formation of trityl oxetane acetal to protect a labile β-hydroxy aldehyde (17→18); (iv) a regioselective installation of C3–C4 double bond (19→20); (v) a highly diastereoselective construction of the north segment via coupling of the C22–C28 alkyne and the C15–C21 lactone (29+36→4); (vi) an E-selective Wittig reaction using trimethylphosphonium salt to connect the polyfunctionalized north and south segments (6+46c→44); (vii) an efficient Shiina macrolactonization using a genuine seco acid (2→49); (viii) a modified synthesis of the disaccharide and improved glycosylation to afford avermectin B_1a (57+58→1). In particular, we should emphasize that a unique and powerful ‘trityl oxetane acetal’ protecting group strategy worked well to prevent serious epimerization and double bond migration of the sensitive β-hydroxy carbonyl tetrahydrobenzofuran moiety of 1.

Experimental Procedure

All reactions sensitive to air or moisture were carried out under argon or nitrogen atmosphere in dry, freshly distilled solvents under anhydrous conditions, unless otherwise noted.

Analytical TLC was performed using E Merck Silica gel 60 F254 precoated plates (Merck, Frankfurt, Germany). Column chromatography was performed using 100–210 μm Silica Gel 60N (Kanto Chemical Co., Tokyo, Japan), and for flash column chromatography, 40–50 μm Silica Gel 60N (Kanto Chemical Co.) was used. ¹H- and ¹³C NMR spectra were recorded on Agilent 400MR (400 and 100 MHz, respectively; Agilent, Santa Clara, CA, USA) or Bruker AVANCE III 700 (700 and 175 MHz, respectively; Bruker, Billerica, MA, USA) spectrometer. Chemical shifts are reported in terms of chemical shift relative to solvent signals (¹H NMR: CHCl₃ (7.26), C₆D₅H (7.16); ¹³C NMR: CDCl₃ (77.16), C₆D₆ (128.06)). Signal patterns are indicated as: s=singlet; d=doublet; t=triplet; q=quartet; quint.=quintet; m=multiplet; br=broad peak. IR spectra were recorded on a Perkin-Elmer Spectrum BX FT-IR spectrometer (Perkin-Elmer, Waltham, MA, USA). High-resolution ESI-FT mass spectra were measured on Thermo Orbi-trap instrument (Thermo Scientific, Waltham, MA, USA). Melting points were measured on Yanaco MP-S3 micromelting point apparatus. The carbon numbering of compounds is corresponding with avermectin B_1a.

Structure and synthesis plan for avermectin B_1a (1).

Synthesis of south segment. Reagents and conditions: (a) HCl·pyridine, 140 °C; (b) TsOH·H₂O, acetone, room temperature (rt), 53% (2 steps); (c) t-butyldiphenylchlorosilane (TBDPSCl), imidazole, dimethylformamide (DMF), 0 °C, 71%; (d) NAPBr, NaH, THF, rt; (e) TsOH·H₂O, THF, H₂O, reflux, 85% (2 steps); (f) NaIO₄, MeCN, H₂O, 0 °C; (g) 12 (1.3 eq), BF₃·OEt₂, CH₂Cl₂, −78 °C; citric acid, MeOH, rt, 65% (2 steps), C4-epimer: 13%; (h) NiCl₂·6H₂O, NaBH₄, MeOH, 0 °C, 93%; (i) tert-butyldimethylsilyl triflate (TBSOTf), 2,6-lutidine, CH₂Cl₂, rt, 97%; (j) diisobutylaluminum hydride (DIBAL), CH₂Cl₂, 0 °C, 96%; (k) TESOTf, 2,6-lutidine, CH₂Cl₂, rt, 97%; (l) DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone), CH₂Cl₂, pH 7, phosphate buffer, 0 °C, 90%; (m) Dess–Martin periodinane (DMP), pyridine, CH₂Cl₂, 0 °C to rt, 16: 24%, 7: 54%; (n) pyridinium p-toluenesulfonate (PPTS), MeOH, CH₂Cl₂, −50 to −20 °C, 91%; (o) DMP, NaHCO₃, CH₂Cl₂, 0 °C to rt, 84%; (p) trityl chloride (TrCl), silver trifluoromethanesulfonate (AgOTf), 2,6-lutidine, (CH₂Cl)₂, 50 °C, 75% (2 steps); (q) tetrabutylammonium fluoride (TBAF), AcOH, THF, 0 °C to rt; (r) tetrapropylammonium perruthenate (TPAP), NMO, 4 ÅMS, (CH₂Cl)₂, rt, 71% (2 steps); (s) H₂NNH₂·H₂O, Et₃N, EtOH, 60 °C; (t) I₂, Et₃N, THF, 0 °C, 55% (2 steps); (u) Pd(dppf)Cl₂, MeB(OH)₂, CsCO₃, DMF, toluene, H₂O, 60 °C, 94%; (v) NaOH, MeOH, 65 °C, 53%; (w) DMP, pyridine, CH₂Cl₂, 0 °C to rt, quant.; (x) 22, NaH, THF, −90 °C to rt; (y) DIBAL, Et₂O, −78 °C, 81% (2 steps); (z) DMP, pyridine, CH₂Cl₂, 0 °C to rt, 95%.

Synthesis of north segment and attempted Julia coupling. Reagents and conditions: (a) 25, i-PrOH, 40 °C, 88%; (b) MsCl, Et₃N, CH₂Cl₂, −85 °C, 96%; (c) 27, Pd(OAc)₂, PPh₃, Et₂Zn, THF, −90 to 0 °C, 78% (dr=14:1); (d) K₂CO₃, MeOH, rt; (e) trimethylsilyl (TMS)-imidazole, CH₂Cl₂, 0 °C to room temperature (rt), 87% (2 steps); (f) TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl), PhI(OAc)₂, CH₂Cl₂, rt; (g) diketene 32, 33, Ti(Oi-Pr)₄, CH₂Cl₂, −40 °C, 54% (2 steps, 76% ee); (h) NaBH(OAc)₃, acetic acid (AcOH), MeCN, −60 to 0 °C, 83%; (i) pyridinium p-toluenesulfonate (PPTS), CH₂Cl₂, reflux, 75%; (j) TBSCl, imidazole, DMF, 40 °C, 82%; (k) 29 (1.2 eq), n-BuLi, THF, −78 °C; 36, 74% from 36 (dr=7:1); (l) citric acid, MeOH, rt, 83% (dr=7:1); (m) H₂, Lindler cat. MeOH, rt, 92% (dr=7:1); (n) PPTS, Et₂O, rt, 86% (dr=5:1); (o) Na, NH3, THF, −78 °C, 75%; (p) Dess–Martin periodinane (DMP), NaHCO₃, CH₂Cl₂, rt; (q) Ph₃P=C(Me)CO₂Et, THF, reflux, 87% (2 steps); (r) diisobutylaluminum hydride (DIBAL), CH₂Cl₂, −60 °C, 98%; (s) DMP, NaHCO₃, CH₂Cl₂, rt; (t) 40, toluene, 4ÅMS, −80 to −30 °C, 86% (2 steps); (u) TBSCl, imidazole, DMF, rt, quant.; (v) OsO₄, NMO, t-BuOH, THF, H₂O, rt; (w) NaIO₄, MeOH, H₂O, rt, 78% (2 steps); (x) NaBH₄, MeOH, 0 °C, 95%; (y) DIAD, PPh₃, 1-phenyl-1H-tetrazole-5-thiol (PTSH) for 43a, benzothiazole-2-thiol (BTSH) for 43b, DMF, rt; (z) Mo₇O₂₄(NH₄)₆·4H₂O, 30% aq. hydrogen peroxide (H₂O₂), EtOH, 43a: 33% (2 steps), 43b: 67% (2 steps).

Synthesis of avermectin aglycon 49. Reagents and conditions: (a) Ph₃P=CHCO₂Et, toluene, room temperature (rt); (b) diisobutylaluminum hydride (DIBAL), Et₂O, −78 °C, 75% (2 steps); (c) MsCl, LiBr, Et₃N, THF, 0 °C to rt, 62%; (d) P(OEt)₃, KI, acetone, MeCN, 60 °C, quant.; (e) PPh₃, MeCN, 70 °C, quant.; (f) PMe₃, MeCN, rt, quant.; (g) TBAF, THF, 45 °C, 56% (2 steps); (h) HCO₂H, 2-Me-2-butene, t-BuOH; (i) NaClO₂, NaHPO₄, H₂O, rt; (j) 2-methyl-6-nitrobenzoic anhydride (MNBA), DMAP, CH₂Cl₂, rt, 54% (3 steps).

Total synthesis of avermectin B_1a (1). Reagents and conditions: (a) Ac₂O, HBr, AcOH, 0 °C to room temperature (rt); Zn, CuSO₄, NaOAc, H₂O, 77%; (b) K₂CO₃, MeOH, rt, 87%; (c) BzCl, pyridine, CH₂Cl₂, 0 °C; (d) TBSCl, imidazole, DMF, 40 °C, 73% (2 steps); (e) MeLi, THF, −80 °C; MeOTf, −90 to −55 °C, 85%; (f) NIS, MeCN, H₂O, 0 °C to rt; Na₂S₂O₄, NaHCO₃, DMF, H₂O, rt, 92% (α:β=7:3); (g) PhSH, BF₃·OEt₂, CH₂Cl₂, 0 °C; (h) TBAF, THF, 40 °C, 55α: 31% (2 steps), 55β: 25% (2 steps); (i) DAST, THF, −40 °C, quant.; (j) SnCl₂, AgOTf, 2,6-di-t-Bu-4-Me-pyridine (DTBMP), 4ÅMS, Et₂O, −30 °C to rt, 3: 46%, C1'' β-isomer: 9%; (k) DAST, NBS, CH₂Cl₂, −30 °C, ~85%; (l) TBSCl, imidazole, DMF, rt, 78%; (m) SnCl₂, AgOTf, DTBMP, 4ÅMS, MeCN, −40 °C to rt, 59α: 45%, 59β: 20%; (n) HF·pyridine, pyridine, MeCN, rt, 88%.

Diol 9

A mixture of d-sorbitol 8 (10.4 g, 57 mmol) and HCl·pyridine (10.4 g, 91 mmol) was heated to 140 °C. The reaction mixture was stirred at this temperature overnight, and then passed through a short pad of silica eluting with EtOAc/MeOH=5/1. The solution was concentrated to provide tetraol S1 (12.6 g, impure), which was used in the next reaction.

To a solution of tetraol S1 (6.9 g, ca. 31 mmol) in acetone (62 ml) was added TsOH·H₂O (2.9 g, 15 mmol) at room temperature. The reaction mixture was stirred overnight, then quenched with EtOAc and saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (EtOAc) to yield diol 9 (3.34 g, 53%, 2 steps) as a white solid. 9: m.p. 72 °C; [α]_D²⁸=−11.1 (c 0.41, CHCl₃); IR (KBr) ν: 3424, 2933, 1454, 1370, 1257 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 4.31 (1H, ddd, J=6.3, 5.5, 5.5 Hz, H5), 4.30 (1H, dd, J=7.8, 5.5 Hz, H6), 4.27 (1H, m, H8), 4.17 (1H, dd, J=9.8, 3.9 Hz, H8a), 4.17 (1H, dd, J=8.6, 6.3 Hz, H4), 3.99 (1H, dd, J=7.8, 3.9 Hz, H7), 3.97 (1H, dd, J=8.6, 5.5 Hz, H4), 3.75 (1H, dd, J=9.8, 1.2 Hz, H8a), 2.54 (1H, d, J=2.9 Hz, OH), 1.80 (1H, d, J=3.9 Hz, OH), 1.44 (3H, s, acetonide), 1.36 (3H, s, acetonide); ¹³C NMR (100 MHz, CDCl₃) δ: 109.52 (C, acetonide), 81.35 (CH, C7), 77.77 (CH, C8), 77.58 (CH, C6), 73.87 (CH₂, C8a), 73.76 (CH, C5), 67.84 (CH₂, C4), 26.78 (CH₃, acetonide), 25.22 (CH₂, acetonide); HRMS (ESI) m/z calcd for C₉H₁₆O₅Na [(M+Na)]⁺ 227.0890, found 227.0890.

TBDPS ether S2

To a solution of diol 9 (8.3 g, 41 mmol) in DMF (40 ml) was added imidazole (6.9 g, 0.10 mol), followed by TBDPSCl (10.5 ml, 41 mmol) at 0 °C. The reaction mixture was stirred at this temperature for 5 h, and then added additional TBDPSCl (2.5 ml, 9.6 mmol). The reaction mixture was stirred at this temperature for 2 h, then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=1/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=5/1–4/1) to yield TBDPS ether S2 (12.69 g, 71%) as a white solid. S2: m.p. 83 °C; [α]_D²³=−12.5 (c 0.96, CHCl₃); IR (film) ν: 3447, 2932, 2858, 1589, 1472, 1427, 1219 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.65 (2H, m, TBDPS), 7.62 (2H, m, TBDPS), 7.47–7.36 (6H, m, TBDPS), 4.29 (1H, ddd, J=7.6, 6.3, 5.5 Hz, H5), 4.22 (1H, ddd, J=3.9, 1.4, 1.4 Hz, H8), 4.18 (1H, m, H7), 4.14 (1H, dd, J=8.4, 6.3 Hz, H4), 4.07 (1H, dd, J=7.6, 3.3 Hz, H6), 3.99 (1H, dd, J=8.4, 5.5 Hz, H4), 3.96 (1H, dd, J=9.4, 3.9 Hz, H8a), 3.71 (1H, dd, J=9.4, 1.4 Hz, H8a), 2.19 (1H, d, J=3.1 Hz, OH), 1.45 (3H, s, acetonide), 1.35 (3H, s, acetonide), 1.07 (9H, s, TBDPS); ¹³C NMR (100 MHz, CDCl₃) δ: 135.64 (CH, TBDPS), 129.94 (CH, TBDPS), 127.82 (CH, TBDPS), 127.79 (C, TBDPS), 109.33 (C, acetonide), 81.26 (CH, C6), 78.97 (CH, C8), 77.75 (CH, C7), 74.33 (CH₂, C8a), 73.86 (CH, C5), 67.65 (CH₂, C4), 26.84 (CH₃, TBDPS), 26.80 CH₃, acetonide), 25.21 (CH₃, acetonide), 19.08 (C, TBDPS); HRMS (ESI) m/z calcd for C₂₅H₃₄O₅SiNa [(M+Na)]⁺ 465.2068, found 465.2068.

Diol 10

To a solution of S2 (4.0 g, 9.1 mmol) in THF (30 ml) was added NaH (0.73 g, 60% in oil, 18 mmol) at room temperature. The reaction mixture was stirred for 15 min and then added NAPBr (2.2 g, 10 mmol). The reaction mixture was stirred overnight and quenched with saturated aqueous NH₄Cl. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated to provide NAP ether S3, which was used in the next reaction.

To a solution of NAP ether S3 in THF-H₂O (4:1, 30 ml) was added TsOH·H₂O (0.53 g, 2.8 mmol). The reaction mixture was stirred for 3 h under reflux conditions and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=1/1) to yield diol 10 (4.22 g, 85%, 2 steps) as a white amorphous. 10: [α]_D²¹=−52.0 (c 0.98, CHCl₃); IR (film) ν: 3421, 2930, 2857, 1588, 1508, 1458, 1426 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.82–7.73 (3H, m, NAP), 7.71 (2H, m, TBDPS), 7.64 (2H, m, TBDPS), 7.51–7.38 (9H, m, TBDPS, NAP), 7.20 (1H, dd, J=8.4, 1.6 Hz, NAP), 4.38 (1H, d, J=3.7 Hz, H8), 4.25 (1H, d, J=11.9 Hz, NAP), 4.12 (1H, dd, J=8.0, 3.9 Hz, H6), 4.09 (1H, d, J=11.9 Hz, NAP), 4.00 (1H, dd, J=9.4, 3.7 Hz, H8a), 3.99 (1H, m, H5), 3.96 (1H, d, J=3.9 Hz, H7), 3.83 (1H, m, H4), 3.82 (1H, d, J=9.4 Hz, H8a), 3.71 (1H, ddd, J=11.4, 5.7, 5.7 Hz, H4), 2.41 (1H, d, J=5.7 Hz, OH), 2.07 (1H, m, OH), 1.07 (9H, s, TBDPS); ¹³C NMR (100 MHz, CDCl₃) δ: 135.89 (CH, TBDPS), 135.76 (CH, TBDPS), 134.85 (C), 133.27 (C), 133.08 (C), 132.93 (C), 130.17 (CH), 130.06 (CH), 128.44 (CH), 127.97 (CH, TBDPS), 127.91 (CH, TBDPS), 127.84 (CH), 127.70 (CH), 126.31 (CH), 126.27 (CH), 126.11 (CH), 125.19 (CH), 84.92 (CH, C7), 80.47 (CH, C6), 75.71 (CH, C8), 74.39 (CH₂, C8a), 71.69 (CH₂, NAP), 69.82 (CH, C5), 64.88 (CH₂, C4), 26.87 (CH₃, TBDPS), 19.05 (C, TBDPS); HRMS (ESI) m/z calcd for C₃₃H₃₈O₅SiNa [(M+Na)]⁺ 565.2381, found 565.2387.

2(5H)-furanone S4 (ref. 49)

To a solution of furfural (41 ml, 0.50 mol) in CH₂Cl₂ (200 ml) were added Na₂SO₄ (20 g) and N,N-dimethylethanolamine (17 ml, 0.17 mol), followed by formic acid (38 ml, 1.0 mol) at room temperature. Then, 35% hydrogen peroxide (78 ml, 0.8 mol) was added dropwise. The reaction mixture was stirred overnight. The phases were separated and the aqueous phase was extracted with CH₂Cl₂. The combined organic layer was washed with saturated aqueous Na₂S₂O₃, dried over MgSO₄ and concentrated. The residue was distilled under reduced pressure at 75–95 °C to yield 2(5H)-furanone S4 (18.6 g, 44%) as pail yellow oil. S4: IR (film) ν: 3567, 3100, 1943, 1777, 1742, 1599, 1447 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.57 (1H, dt, J=5.6, 1.6 Hz, H3), 6.17 (1H, dt, J=5.6, 2.4 Hz, H2), 4.91 (2H, dd, J=2.4, 1.6 Hz, H4); ¹³C NMR (100 MHz, CDCl₃) δ: 173.65 (C, C1), 152.70 (CH, C3), 121.61 (CH, C2), 72.08 (CH₂, C4).

Furan 12

To a solution of 2(5H)-furanone S4 (5.8 g, 69 mmol) in CH₂Cl₂ (50 ml) was added Et₃N (12 ml, 83 mmol) at 0 °C. Then, TMSOTf (13 ml, 72 mmol) was added dropwise. The reaction mixture was gradually warmed to room temperature, stirred overnight and then diluted with pentane (100 ml). The phases were separated and the organic layer was washed with pH 7.0 phosphate buffer, 0.5 m CuSO₄ aq. and brine. The layer was dried over MgSO₄ and concentrated. The residue was distilled under reduced pressure at 50–60 °C to yield furan 12 (7.10 g, 65%) as a colorless oil. 12: IR (film) ν: 2963, 1618, 1523, 1382, 1256 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 6.82 (1H, dd, J=2.4, 1.2 Hz, H4), 6.21 (1H, dd, J=3.2, 2.4 Hz, H3), 5.10 (1H, dd, J=3.2, 1.2 Hz, H2), 0.30 (9H, s, TMS); ¹³C NMR (100 MHz, CDCl₃) δ: 156.66 (C, C1), 132.37 (CH, C4), 110.97 (CH, C3), 83.21 (CH, C2), −0.27 (CH₃, TMS).

Butenolide 13

To a solution of diol 10 (22.4 g, 41 mmol) in MeCN-H₂O (4:1, 206 ml) was added NaIO₄ (13.3 g, 62 mmol) at 0 °C. The reaction mixture was stirred at this temperature for 2.5 h, and quenched with H₂O. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated to provide aldehyde 11, which was used in the next reaction.

To a solution of aldehyde 11 in CH₂Cl₂ (370 ml) was added BF₃·OEt₂ (11 ml, 62 mmol) at −78 °C. The reaction mixture was stirred at this temperature for 15 min, and then added furan 12 (8.4 g, 54 mmol) in CH₂Cl₂ (40 ml). The reaction mixture was stirred at this temperature for 3 h, then quenched with EtOAc and saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with CH₂Cl₂. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. To this residue in MeOH (200 ml) was added citric acid (10 g) at room temperature. The reaction mixture was stirred for 2 h, then quenched with EtOAc and saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=3/1) to yield butenolide 13 (16.08 g, 65%, 2 steps) and its diastereomer (3.2 g, 13%) as a white amorphous. 13: [α]_D²³=−85.9 (c 1.01, CHCl₃); IR (film) ν: 3443, 3052, 2931, 2250, 1746, 1602, 1509, 1470, 1427 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.89 (1H, m, NAP), 7.75–7.71 (2H, m, NAP), 7.68 (2H, m, TBDPS), 7.62 (2H, m, TBDPS), 7.53 (1H, dd, J=5.9, 1.6 Hz, H3), 7.50–7.38 (9H, m, TBDPS, NAP), 7.18 (1H, dd, J =8.4, 1.8 Hz, NAP), 6.14 (1H, dd, J=5.9, 2.0 Hz, H2), 5.27 (1H, ddd, J=3.7, 2.0, 1.6 Hz, H4), 4.36 (1H, d, J=3.7 Hz, H8), 4.28 (1H, dd, J=8.0, 3.5 Hz, H6), 4.23 (1H, d, J=11.6 Hz, NAP), 4.16 (1H, d, J=11.6 Hz, NAP), 4.09 (1H, m, H5), 4.02 (1H, d, J=3.5 Hz, H7), 4.01 (1H, dd, J=9.8, 3.7 Hz, H8a), 3.84 (1H, d, J=9.8 Hz, H8a), 2.50 (1H, d, J=6.9 Hz, OH), 1.08 (9H, s, TBDPS); ¹³C NMR (100 MHz, CDCl₃) δ: 173.07 (C, C1), 154.52 (CH, C3), 135.84 (CH, TBDPS), 135.72 (CH, TBDPS), 134.72 (C), 133.09 (C), 133.03 (C), 132.95 (C), 130.18 (CH, NAP), 130.05 (C, NAP), 128.31 (CH), 127.98 (CH, TBDPS), 127.90 (CH, TBDPS), 127.85 (CH), 127.65 (CH), 126.50 (CH), 126.20 (CH), 126.07 (CH), 125.45 (CH, NAP), 122.09 (CH, C2), 84.92 (CH, C7), 84.42 (CH, C4), 79.93 (CH, C6), 75.79 (CH, C8), 74.64 (CH₂, C8a), 72.14 (CH₂, NAP), 69.61 (CH, C5), 26.86 (CH₃, TBDPS), 19.03 (C, TBDPS); HRMS (ESI) m/z calcd for C₃₆H₃₈O₆SiNa [(M+Na)]⁺ 617.2330, found 617.2331.

Lactone S5

To a solution of butenolide 13 (1.1 g, 1.9 mmol) in MeOH (19 ml) was added NiCl₂·6H₂O (0.26 g, 0.95 mmol), followed by NaBH₄ (0.14 g, 3.8 mmol) at 0 °C. The reaction mixture was stirred at this temperature for 55 min, then quenched with EtOAc and saturated aqueous NH₄Cl. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=2/1) to yield lactone S5 (1.06 g, 93%) as a white amorphous. S5: [α]_D²²=−31.3 (c 0.90, CHCl₃); IR (film) ν: 3437, 3052, 2931, 2249, 1769, 1589, 1509, 1471 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.79 (1H, m, NAP), 7.76–7.72 (2H, m, NAP), 7.69 (2H, dt, J=6.5, 1.6 Hz, TBDPS), 7.62 (2H, dt, J=6.5, 1.6 Hz, TBDPS), 7.52–7.37 (9H, m, TBDPS, NAP), 7.18 (1H, dd, J =8.4, 1.6 Hz, NAP), 4.76 (1H, ddd, J=8.0, 6.3, 1.8 Hz, H4), 4.36 (1H, d, J=3.9 Hz, H8), 4.24 (1H, dd, J=7.2, 3.5 Hz, H6), 4.24 (1H, d, J=11.7 Hz, NAP), 4.14 (1H, d, J=11.7 Hz, NAP), 4.05 (1H, d, J=3.5 Hz, H7), 4.01 (1H, dd, J=9.6, 3.9 Hz, H8a), 3.91 (1H, ddd, J=7.2, 7.2, 1.8 Hz, H5), 3.82 (1H, d, J=9.6 Hz, H8a), 2.72 (1H, d, J=7.2 Hz, OH), 2.64 (1H, ddd, J=17.6, 9.6, 6.5 Hz, H2), 2.48 (1H, ddd, J=17.6, 10.2, 7.4 Hz, H2), 2.37–2.23 (2H, m, H3x2), 1.08 (9H, s, TBDPS); ¹³C NMR (100 MHz, CDCl₃) δ: 177.66 (C, C1), 135.86 (CH, TBDPS), 135.72 (CH, TBDPS), 134.73 (C), 133.12 (C), 133.10 (C), 133.05 (C), 130.17 (CH), 130.03 (CH), 128.31 (CH), 127.97 (CH, TBDPS), 127.89 (CH, TBDPS), 127.65 (CH), 126.54 (CH), 126.20 (CH), 126.08 (CH), 125.44 (CH, NAP), 85.52 (CH, C7), 80.11 (CH, C4), 79.62 (CH, C6), 75.87 (CH, C8), 74.38 (CH₂, C8a), 71.96 (CH₂, NAP), 71.58 (CH, C5), 28.53 (CH₂, C2), 26.86 (CH₃, TBDPS), 23.98 (CH₂, C3), 19.03 (C, TBDPS); HRMS (ESI) m/z calcd for C₃₆H₄₀O₆SiNa [(M+Na)]⁺ 619.2486, found 619.2486.

TBS ether 14

To a solution of lactone S5 (2.7 g, 4.5 mmol) in CH₂Cl₂ (23 ml) was added 2,6-lutidine (1.6 ml, 14 mmol), followed by tert-butyldimethylsilyltriflate (TBSOTf; 1.6 ml, 6.8 mmol) at room temperature. The reaction mixture was stirred for 1.5 h and then quenched with saturated aqueous NH₄Cl. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=5/1) to yield TBS ether 14 (3.12 g, 97%) as a white amorphous. 14: [α]_D²²=−16.8 (c 1.07, CHCl₃); IR (film) ν: 3052, 2929, 2252, 1777, 1589, 1471 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.78 (1H, m, NAP), 7.73 (2H, dt, J=6.5, 1.6 Hz, TBDPS), 7.72–7.69 (2H, m, NAP), 7.60 (2H, dt, J=6.5, 1.6 Hz, TBDPS), 7.53–7.36 (9H, m, TBDPS, NAP), 7.11 (1H, dd, J =8.4, 1.6 Hz, NAP), 4.80 (1H, ddd, J=7.4, 7.4, 3.5 Hz, H4), 4.38 (1H, d, J=4.1 Hz, H8), 4.18 (1H, dd, J=8.4, 2.5 Hz, H6), 4.12 (1H, dd, J=8.4, 3.5 Hz, H5), 4.03 (1H, dd, J=9.6, 4.1 Hz, H8a), 3.98–3.90 (2H, m, NAP), 3.84 (1H, d, J=9.6 Hz, H8a), 3.79 (1H, d, J=2.5 Hz, H7), 2.58–2.44 (2H, m, H2x2), 2.30–2.14 (2H, m, H3x2), 1.10 (9H, s, TBDPS), 0.88 (9H, s, TBS), 0.05 (3H, s, TBS), −0.04 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 177.38 (C, C1), 136.03 (CH, TBDPS), 135.69 (CH, TBDPS), 135.47 (C), 133.41 (C), 133.07 (C), 133.01 (C), 132.72 (C), 130.17 (CH), 129.99 (CH), 128.00 (CH, TBDPS), 127.88 (CH, TBDPS), 127.72 (CH), 127.62 (CH), 126.08 (CH), 125.79 (CH), 125.30 (CH), 124.89 (CH, NAP), 83.93 (CH, C7), 81.55 (CH, C6), 81.45 (CH, C4), 74.32 (CH₂, C8a), 74.09 (CH, C8), 70.05 (CH₂, NAP), 69.86 (CH, C5), 28.68 (CH₂, C2), 26.87 (CH₃, TBDPS), 25.95 (CH₃, TBS), 23.42 (CH₂, C3), 19.00 (C, TBDPS), 18.27 (C, TBS), −4.14 (CH₃, TBS), −4.49 (CH₃, TBS); HRMS (ESI) m/z calcd for C₄₂H₅₄O₆Si₂Na [(M+Na)]⁺ 733.3351, found 733.3345.

Diol S6

To a solution of 14 (0.59 g, 0.80 mmol) in CH₂Cl₂ (23 ml) was added DIBAL (4.7 ml, 1.02 m in toluene, 4.8 mmol) at 0 °C. The reaction mixture was stirred at this temperature for 30 min and then quenched with saturated aqueous Rochelle salt. After being stirred for 6 h, the phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=5/1–2/1) to yield diol S6 (548 mg, 96%) as a white amorphous. S6: [α]_D²³=−31.2 (c 0.99, CHCl₃); IR (film) ν: 3478, 3053, 2928, 2246, 1589, 1510, 1470 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.78 (1H, m, NAP), 7.75–7.69 (4H, m, TBDPS, NAP), 7.60 (2H, m, TBDPS), 7.53–7.36 (9H, m, TBDPS, NAP), 7.12 (1H, dd, J=8.4, 1.6 Hz, NAP), 4.32 (1H, d, J=3.9 Hz, H8), 4.24–4.16 (2H, m, H5, H6), 4.04 (1H, dd, J=9.4, 3.9 Hz, H8a), 4.00 (1H, d, J=12.1 Hz, NAP), 3.93 (1H, d, J=12.1 Hz, NAP), 3.86 (1H, d, J=9.4 Hz, H8a), 3.79–3.64 (4H, m, H7, H1x2, H4), 3.48 (1H, d, J=10.0 Hz, OH), 2.94 (1H, brs, OH), 1.96 (1H, m, H2), 1.86–1.75 (2H, m, H3x2), 1.55 (1H, ddt, J=14.1, 10.6, 7.0 Hz, H2), 1.10 (9H, s, TBDPS), 0.87 (9H, s, TBS), 0.01 (3H, s, TBS), −0.08 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 135.99 (CH, TBDPS), 135.66 (CH, TBDPS), 135.47 (C), 133.35 (C), 133.07 (C), 132.97 (C), 132.70 (C), 130.19 (CH), 130.02 (CH), 127.99 (CH, TBDPS), 127.89 (CH, TBDPS), 127.86 (CH), 127.75 (CH), 127.61 (CH), 126.06 (CH), 125.75 (CH), 125.11 (CH), 124.76 (CH, NAP), 84.26 (CH, C7), 81.94 (CH, C6), 74.82 (CH, C4), 74.60 (CH₂, C8a), 73.94 (CH, C8), 70.47 (CH₂, NAP), 68.96 (CH, C5), 63.17 (CH₂, C1), 30.78 (CH₂, C3), 29.30 (CH₂, C2), 26.88 (CH₃, TBDPS), 25.82 (CH₃, TBS), 19.00 (C, TBDPS), 17.98 (C, TBS), −4.29 (CH₃, TBS), −4.97 (CH₂, TBS); HRMS (ESI) m/z calcd for C₄₂H₅₈O₆Si₂Na [(M+Na)]⁺ 737.3664, found 737.3663.

Bis-TES ether S7

To a solution of diol S6 (5.6 g, 7.8 mmol) in CH₂Cl₂ (39 ml) was added 2,6-lutidine (4.5 ml, 39 mmol), followed by TESOTf (5.3 ml, 23 mmol) at room temperature. The reaction mixture was stirred for 1 h and then quenched with saturated aqueous NH₄Cl. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=2/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=100/1) to yield S7 (7.15 g, 97%) as a colorless oil. S7: [α]_D²²=−23.6 (c 1.09, CHCl₃); IR (film) ν: 3052, 2930, 1589, 1509, 1462, 1311 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.76 (1H, m, NAP), 7.73 (2H, m, TBDPS), 7.71–7.66 (2H, m, NAP), 7.60 (2H, m, TBDPS), 7.51–7.35 (9H, m, TBDPS, NAP), 7.14 (1H, dd, J=8.6, 1.8 Hz, NAP), 4.32 (1H, d, J=4.1 Hz, H8), 4.17 (1H, dd, J=9.4, 2.6 Hz, H6), 4.07 (1H, dd, J=9.4, 1.8 Hz, H5), 4.03 (1H, dd, J=9.6, 4.1 Hz, H8a), 4.03 (1H, d, J=12.3 Hz, NAP), 3.96 (1H, d, J=12.3 Hz, NAP), 3.86 (1H, m, H4), 3.82 (1H, d, J=9.6 Hz, H8a), 3.78 (1H, d, J=2.6 Hz, H7), 3.68–3.57 (2H, m, H1x2), 1.76 (1H, m, H3), 1.67 (1H, m, H2), 1.61–1.46 (2H, m, H2, H3), 1.11 (9H, s, TBDPS), 1.00 (9H, t, J=7.8 Hz, TES), 0.94 (9H, t, J=7.8 Hz, TES), 0.89 (9H, s, TBS), 0.67 (6H, q, J=7.8 Hz, TES), 0.59 (6H, q, J=7.8 Hz, TES), 0.08 (3H, s, TBS), 0.00 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 136.11 (C), 136.04 (CH, TBDPS), 135.73 (CH, TBDPS), 133.57 (C), 133.24 (C), 133.14 (C), 132.66 (C), 130.05 (CH), 129.86 (CH), 127.91 (CH, TBDPS), 127.80 (CH, TBDPS), 127.73 (CH), 127.66 (CH), 127.58 (CH), 125.89 (CH), 125.54 (CH), 125.00 (CH), 124.85 (CH, NAP), 84.57 (CH, C7), 80.00 (CH, C6), 74.42 (CH, C8), 74.32 (CH₂, C8a), 73.60 (CH, C4), 71.14 (CH, C5), 70.24 (CH₂, NAP), 63.13 (CH₂, C1), 30.10 (CH₂, C2), 29.56 (CH₂, C3), 26.81 (CH₃, TBDPS), 26.07 (CH₃, TBS), 18.97 (C, TBDPS), 18.49 (C, TBS), 7.02 (CH₃, TES), 6.80 (CH₃, TES), 5.19 (CH₂, TES), 4.43 (CH₂, TES), −3.22 (CH₃, TBS), −3.65 (CH₃, TBS); HRMS (ESI) m/z calcd for C₅₄H₈₆O₆Si₄Na [(M+Na)]⁺ 965.5394, found 965.5390.

Alcohol 15

To a solution of S7 (730 mg, 0.77 mmol) in CH₂Cl₂-pH 7 phosphate buffer (10:1, 7.7 ml) was added DDQ (209 mg, 0.92 mmol) at 0 °C. The reaction mixture was stirred at this temperature for 1.5 h, then quenched with saturated aqueous NaHCO₃ and saturated aqueous Na₂S₂O₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. To this residue in MeOH (8 ml) was added NaBH₄ (58 mg, 1.5 mmol) at room temperature. The reaction mixture was stirred for 10 min and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=2/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=100/1–50/1) to yield alcohol 15 (553 mg, 90%) as a colorless oil. 15: [α]_D²²=−20.0 (c 0.98, CHCl₃); IR (film) ν: 3469, 3071, 2929, 1589, 1461, 1427 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.64 (4H, m, TBDPS), 7.44–7.33 (6H, m, TBDPS), 4.73 (1H, d, J=1.2 Hz, OH), 4.31 (1H, dd, J=4.7, 2.0 Hz, H5), 4.26 (1H, dd, J=2.0, 2.0 Hz, H6), 4.22 (1H, d, J=3.5 Hz, H8), 4.21 (1H, m, H7), 3.94 (1H, dd, J=9.4, 3.5 Hz, H8a), 3.74 (1H, m, H4), 3.68–3.57 (3H, m, H8a, H1x2), 1.89 (1H, m, H3), 1.74 (1H, m, H2), 1.49 (1H, m, H2), 1.46 (1H, m, H3), 1.06 (9H, s, TBDPS), 0.99 (9H, t, J=7.8 Hz, TES), 0.96 (9H, t, J=7.8 Hz, TES), 0.85 (9H, s, TBS), 0.64 (6H, q, J=7.8 Hz, TES), 0.60 (6H, q, J=7.8 Hz, TES), 0.11 (3H, s, TBS), 0.10 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 135.67 (CH, TBDPS), 133.62 (C, TBDPS), 129.72 (CH, TBDPS), 127.69 (CH, TBDPS), 79.71 (CH, C7), 79.06 (CH, C8), 77.08 (CH, C6), 76.94 (CH, C5), 74.15 (CH, C4), 72.94 (CH₂, C8a) 62.90 (CH₂, C1), 30.52 (CH₂, C2), 28.03 (CH₂, C3), 26.81 (CH₃, TBDPS), 25.63 (CH₃, TBS), 19.07 (C, TBDPS), 17.90 (C, TBS), 6.92 (CH₃, TES), 6.79 (CH₃, TES), 5.18 (CH₂, TES), 4.43 (CH₂, TES), −4.90 (CH₃, TBS), −5.17 (CH₃, TBS); HRMS (ESI) m/z calcd for C₄₃H₇₈O₆Si₄Na [(M+Na)]⁺ 825.4768, found 825.4755.

Ketone 16 and bis-carbonyl compound 7

To a solution of alcohol 15 (920 mg, 1.1 mmol) in CH₂Cl₂ (11 ml) was added pyridine (1.7 ml, 21 mmol), followed by DMP (2.9 g, 6.9 mmol) at 0 °C. The reaction mixture was allowed to warm to room temperature, stirred for 3.5 h, then quenched with saturated aqueous NaHCO₃ and saturated aqueous Na₂S₂O₃. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=5/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=16: 150/1, 7: 5/1) to yield ketone 16 (220 mg, 24%) as a colorless oil and bis-carbonyl compound 7 (424 mg, 54%) as a colorless oil. 16: [α]_D²¹=−12.4 (c 1.32, CHCl₃); IR (film) ν: 3072, 2930, 1778, 1589, 1461, 1427, 1389 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.75 (2H, dt, J=6.5, 1.4 Hz, TBDPS), 7.62 (2H, dt, J=6.5, 1.4 Hz, TBDPS), 7.46–7.34 (6H, m, TBDPS), 4.22 (1H, dd, J=10.0, 8.6 Hz, H8), 4.14 (1H, d, J=1.0 Hz, H6), 3.90 (1H, dd, J=9.0, 8.6 Hz, H8a), 3.79 (1H, dd, J=5.7, 1.0 Hz, H5), 3.64–3.56 (3H, m, H1x2, H4), 3.55 (1H, dd, J=10.0, 9.0 Hz, H8a), 1.78 (1H, m, H3), 1.68 (1H, m, H2), 1.54–1.37 (2H, m, H3, H2), 1.08 (9H, s, TBDPS), 0.97 (9H, t, J=7.8 Hz, TES), 0.91 (9H, t, J=7.8 Hz, TES), 0.68 (9H, s, TBS), 0.60 (6H, q, J=7.8 Hz, TES), 0.54 (6H, q, J=7.8 Hz, TES), −0.05 (3H, s, TBS), −0.19 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 210.51 (C, C7), 135.80 (CH, TBDPS), 135.57 (CH, TBDPS), 133.49 (C, TBDPS), 132.31 (C, TBDPS), 130.00 (CH, TBDPS), 129.88 (CH, TBDPS), 127.78 (CH, TBDPS), 79.94 (CH, C6), 76.79 (CH, C5), 74.24 (CH, C4), 73.66 (CH, C8), 68.97 (CH₂, C8a) 63.28 (CH₂, C1), 30.53 (CH₂, C₂), 28.48 (CH₂, C3), 26.70 (CH₃, TBDPS), 25.64 (CH₃, TBS), 19.19 (C, TBDPS), 17.77 (C, TBS), 6.84 (CH₃, TES), 6.80 (CH₃, TES), 4.96 (CH₂, TES), 4.43 (CH₂, TES), −4.80 (CH₃, TBS), −4.94 (CH₃, TBS); HRMS (ESI) m/z calcd for C₄₃H₇₆O₆Si₄Na [(M+Na)]⁺ 823.4611, found 823.4611. 7: [α]_D²²=−18.3 (c 1.26, CHCl₃); IR (film) ν: 2930, 1777, 1712, 1589, 1471, 1428 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 9.74 (1H, dd, J=2.0, 2.0 Hz, H1), 7.75 (2H, dt, J=6.5, 1.6 Hz, TBDPS), 7.63 (2H, dt, J=6.5, 1.6 Hz, TBDPS), 7.46–7.34 (6H, m, TBDPS), 4.22 (1H, dd, J=10.0, 8.6 Hz, H8), 4.17 (1H, d, J=1.0 Hz, H6), 3.93 (1H, dd, J=8.8, 8.6 Hz, H8a), 3.84 (1H, dd, J=5.5, 1.0 Hz, H5), 3.60 (1H, m, H4), 3.58 (1H, dd, J=10.0, 8.8 Hz, H8a), 2.48 (1H, m, H2), 2.42 (1H, m, H2), 2.13 (1H, m, H3), 1.93 (1H, m, H3), 1.08 (9H, s, TBDPS), 0.91 (9H, t, J=7.8 Hz, TES), 0.69 (9H, s, TBS), 0.54 (6H, q, J=7.8 Hz, TES), −0.04 (3H, s, TBS), −0.18 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 210.75 (C, C7), 203.06 (C, C1), 135.78 (CH, TBDPS), 135.56 (CH, TBDPS), 133.38 (C, TBDPS), 132.20 (C, TBDPS), 130.05 (CH, TBDPS), 129.94 (CH, TBDPS), 127.80 (CH, TBDPS), 79.80 (CH, C6), 77.00 (CH, C5), 73.71 (CH, C8), 73.11 (CH, C4), 68.94 (CH₂, C8a), 40.89 (CH₂, C2), 26.69 (CH₃, TBDPS), 25.61 (CH₃, TBS), 25.27 (CH₂, C3), 19.18 (C, TBDPS), 17.74 (C, TBS), 6.78 (CH₃, TES), 4.85 (CH₂, TES), −4.88 (CH₃, TBS), −4.97 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₇H₆₀O₆Si₃Na [(M+Na)]⁺ 707.3590, found 707.3590.

Alcohol S8

To a solution of ketone 16 (63 mg, 78 μmol) in MeOH-CH₂Cl₂ (2:1, 1.5 ml) was added pyridinium p-toluenesulfonate(PPTS; 20 mg, 78 μmol) at −50 °C. The reaction mixture was gradually warmed to −20 °C over 1.5 h and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=20/1–10/1) to yield alcohol S8 (48.8 mg, 91%) as a colorless oil. S8: [α]_D²⁰=−16.4 (c 0.92, CHCl₃); IR (film) ν: 3541, 3072, 2929, 1777, 1590, 1471 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.75 (2H, m, TBDPS), 7.62 (2H, m, TBDPS), 7.46–7.34 (6H, m, TBDPS), 4.22 (1H, dd, J=10.0, 8.8 Hz, H8), 4.14 (1H, d, J=1.0 Hz, H6), 3.91 (1H, dd, J=9.0, 8.8 Hz, H8a), 3.82 (1H, dd, J=5.5, 1.0 Hz, H5), 3.68–3.63 (2H, m, H1x2), 3.60 (1H, m, H4), 3.56 (1H, dd, J=10.0, 9.0 Hz, H8a), 1.85 (1H, m, H3), 1.70 (1H, m, H2), 1.58–1.45 (2H, m, H3, H2), 1.08 (9H, s, TBDPS), 0.92 (9H, t, J=7.8 Hz, TES), 0.69 (9H, s, TBS), 0.54 (6H, q, J=7.8 Hz, TES), −0.04 (3H, s, TBS), −0.18 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 210.77 (C, C7), 135.78 (CH, TBDPS), 135.56 (CH, TBDPS), 133.40 (C, TBDPS), 132.25 (C, TBDPS), 130.03 (CH, TBDPS), 129.91 (CH, TBDPS), 127.80 (CH, TBDPS), 79.89 (CH, C6), 76.78 (CH, C5), 73.95 (CH, C4), 73.72 (CH, C8), 68.92 (CH₂, C8a) 63.21 (CH₂, C1), 30.03 (CH₂, C2), 28.66 (CH₂, C3), 26.70 (CH₃, TBDPS), 25.63 (CH₃, TBS), 19.18 (C, TBDPS), 17.76 (C, TBS), 6.83 (CH₃, TES), 4.95 (CH₂, TES), −4.81 (CH₃, TBS), −4.90 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₇H₆₂O₆Si₃Na [(M+Na)]⁺ 709.3746, found 709.3751.

Bis-carbonyl compound 7

To a solution of alcohol S8 (49 mg, 71 μmol) in CH₂Cl₂ (1.5 ml) was added NaHCO₃ (60 mg, 0.71 mmol), followed by DMP (90 mg, 0.21 mmol) at 0 °C. The reaction mixture was allowed to warm to room temperature, stirred for 1.5 h, then quenched with saturated aqueous NaHCO₃ and saturated aqueous Na₂S₂O₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=10/1) to yield dicarbonyl compound 7 (40.7 mg, 84%) as a colorless oil.

Oxetane acetal 18

To a solution of bis-carbonyl compound 7 (2.2 g, 3.2 mmol) in CH₂Cl₂ (64 ml) was added 1,8-diazabicyclo[5.4.0]undec-7-ene (0.96 ml, 6.4 mmol) at 0 °C. The reaction mixture was stirred at this temperature for 3 h and then quenched with saturated aqueous NH₄Cl. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated to provide aldehyde 17 (dr=3.9:1), which was used in the next reaction.

To a solution of aldehyde 17 in (CH₂Cl)₂ (32 ml) was added 2,6-lutidine (2.2 ml, 19 mmol), followed by AgOTf (2.5 g, 9.6 mmol) and trityl chloride (2.7 mg, 9.6 mmol). After being stirred for 3.5 h at 50 °C, the mixture was filtered through filter paper and quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=100/1, 1% Et₃N) to yield oxetane acetal 18 (2.23 g, 75%, 2 steps) as a white amorphous. 18: [α]_D²²=−27.5 (c 0.90, CHCl₃); IR (film) ν: 3057, 2930, 1732, 1597, 1492, 1470 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.84–7.79 (2H, m, TBDPS), 7.76–7.72 (2H, m, TBDPS), 7.46–7.16 (21H, m, TBDPS, Tr), 5.55 (1H, d, J=4.9 Hz, H1), 4.26 (1H, d, J=3.3 Hz, H6), 3.89 (1H, dd, J=9.8, 7.4 Hz, H8), 3.67–3.60 (2H, m, H5, H4), 3.50–3.40 (2H, m, H8ax2), 2.76 (1H, dd, J=8.4, 4.9 Hz, H2), 1.22 (1H, m, H3), 1.17 (9H, s, TBDPS), 0.74 (9H, t, J=8.0 Hz, TES), 0.68 (9H, s, TBS), 0.34 (6H, m, TES), 0.29 (1H, m, H3), −0.13 (3H, s, TBS), −0.28 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 144.93 (C, Tr), 136.10 (CH, TBDPS), 136.02 (CH, TBDPS), 134.11 (C, TBDPS), 133.98 (C, TBDPS), 129.62 (CH, TBDPS), 129.59 (CH, TBDPS), 128.70 (CH, Tr), 127.72 (CH, Tr), 127.63 (CH), 127.54 (CH), 126.78 (CH), 101.19 (CH, C1), 87.32 (C, Tr), 82.08 (CH, C6), 80.39 (C, C7), 75.26 (CH, C8), 70.60 (CH, C4), 69.79 (CH, C5), 68.62 (CH₂, C8a), 42.72 (CH, C2), 27.13 (CH₃, TBDPS), 25.64 (CH₃, TBS), 23.35 (CH₂, C3), 19.61 (C, TBDPS), 17.86 (C, TBS), 6.73 (CH₃, TES), 4.61 (CH₂, TES), −4.93 (CH₃, TBS), −5.56 (CH₃, TBS); HRMS (ESI) m/z calcd for C₅₆H₇₄O₆Si₃Na [(M+Na)]⁺ 949.4685, found 949.4667.

Ketone 19

To a solution of oxetane acetal 18 (20.3 mg, 0.022 mmol) in THF (0.44 ml) were added TBAF (0.33 ml, 1.0 min THF, 0.33 mmol) and AcOH (0.16 ml, 1.0 m in THF, 0.16 mmol) at 0 °C. The reaction mixture was allowed to warm to room temperature, stirred for 1 h and then passed through a short pad of silica, eluting with CH₂Cl₂. The solution was concentrated to provide a mixture of alcohol S9 and oxetane acetal 18, which was used in the next reaction.

To a solution of alcohol S9 and oxetane acetal 18 mixture in (CH₂Cl)₂ (1.1 ml) was added MS4A, followed by NMO (7.7 mg, 0.066 mmol) and TPAP (1.5 mg, 4.4 μmol) at room temperature. The reaction mixture was stirred for 1 h and then filtered through a short pad of silica eluting with CH₂Cl₂ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=18: 100/1, 19: 50/1–10/1, 1% Et₃N) to yield ketone 19 (12.7 mg, 71%, 2 steps) as a white amorphous along with oxetane acetal 18 (~3 mg). 19: [α]_D²⁰=−13.9 (c 1.23, CHCl₃); IR (film) ν: 3068, 2929, 1731, 1589, 1471, 1427 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.89–7.86 (2H, m, TBDPS), 7.76–7.71 (2H, m, TBDPS), 7.46–7.22 (21H, m, TBDPS, Tr), 5.19 (1H, d, J=5.1 Hz, H1), 4.33 (1H, d, J=2.5 Hz, H6), 4.24 (1H, t, J=4.7 Hz, H8), 4.06 (1H, d, J=2.5 Hz, H5), 3.63 (2H, m, H8ax2), 2.72 (1H, dd, J=9.2, 5.1 Hz, H2), 2.04 (1H, dd, J=18.4, 9.2 Hz, H3), 1.17 (9H, s, TBDPS), 0.98 (1H, d, J=18.4 Hz, H3), 0.81 (9H, s, TBS), 0.01 (3H, s, TBS), −0.09 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 207.09 (C, C4), 144.10 (C, Tr), 136.17 (CH, TBDPS), 135.86 (CH, TBDPS), 133.84 (C, TBDPS), 133.27 (C, TBDPS), 129.82 (CH, TBDPS), 129.76 (CH, TBDPS), 128.45 (CH, Tr), 128.07 (CH, Tr), 127.74 (CH), 127.72 (CH), 127.44 (CH), 100.37 (CH, C1), 88.78 (C, Tr), 82.81 (CH, C6), 81.79 (C, C7), 75.99 (CH, C8), 75.71 (CH, C5), 72.36 (CH₂, C8a), 44.56 (CH, C2), 33.75 (CH₂, C3), 26.96 (CH₃, TBDPS), 25.76 (CH₃, TBS), 19.49 (C, TBDPS), 18.34 (C, TBS), −4.86 (CH₃, TBS), −5.35 (CH₃, TBS); HRMS (ESI) m/z calcd for C₅₀H₅₈O₆Si₂Na [(M+Na)]⁺ 833.3664, found 833.3678.

Vinyl iodide S11

To a solution of ketone 19 (73 mg, 89 μmol) in EtOH (1.8 ml) was added Et₃N (62 μl, 0.45 mmol), followed by H₂NNH₂·H₂O (43 μl, 0.89 mmol) at 60 °C. The reaction mixture was stirred at this temperature for 3 h and then quenched with H₂O. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated to hydrazone S10, which was used in the next reaction.

To a solution of hydrazone S10 in THF (1.8 ml) was added Et₃N (124 μl, 0.89 mmol), followed by I₂ (357 μl, 1.0 m in THF, 0.36 mmol) at 0 °C. The reaction mixture was stirred at this temperature for 1 h and then quenched with H₂O. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=100/1, 1% Et₃N) to yield vinyl iodide S11 (45.6 mg, 55%, 2 steps) as a white amorphous. S11: [α]_D¹⁹=−21.9 (c 1.27, CHCl₃); IR (film) ν: 3069, 2929, 2857, 1724, 1647, 1597, 1471 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.86–7.82 (2H, m, TBDPS), 7.79–7.75 (2H, m, TBDPS), 7.48–7.23 (21H, m, TBDPS, Tr), 5.10 (1H, d, J=4.1 Hz, H1), 4.64 (1H, d, J=4.1 Hz, H3), 4.29 (1H, dd, J=6.4, 6.4 Hz, H8), 4.18 (1H, d, J=4.3 Hz, H6), 4.15 (1H, d, J=4.3 Hz, H5), 3.60 (1H, dd, J=8.4, 6.4 Hz, H8a), 3.58 (1H, dd, J=8.4, 6.4 Hz, H8a), 2.69 (1H, dd, J=4.1, 4.1 Hz, H2), 1.19 (9H, s, TBDPS), 0.78 (9H, s, TBS), 0.10 (3H, s, TBS), −0.09 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 144.09 (C, Tr), 136.07 (CH, TBDPS), 135.97 (CH, TBDPS), 135.47 (CH, C3), 133.92 (C, TBDPS), 133.71 (C, TBDPS), 129.75 (CH), 128,46 (CH, Tr), 128.05 (CH, Tr), 127.71 (CH), 127.63 (CH), 127.22 (CH), 100.92 (CH, C1), 99.05 (C, C4), 87.92 (C, Tr), 81.58 (CH, C6), 80.97 (C, C7), 74.89 (CH, C8), 74.62 (CH, C5), 70.23 (CH₂, C8a), 49.68 (CH, C2), 27.11 (CH₃, TBDPS), 25.94 (CH₃, TBS), 19.59 (C, TBDPS), 18.04 (C, TBS), −4.46 (CH₃, TBS), −4.72 (CH₃, TBS); HRMS (ESI) m/z calcd for C₅₀H₅₇IO₅Si₂Na [(M+Na)]⁺ 943.2681, found 943.2681.

Hexahydrobenzofuran 20

To a solution of vinyl iodide S11 (87 mg, 95 μmol) in DMF-toluene-H₂O (2:1:1, 4.8 ml) were added Cs₂CO₃ (618 mg, 1.9 mmol) and MeB(OH)₂ (57 mg, 0.95 mmol), followed by Pd(dppf)Cl₂ (16 mg, 19 μmol) at 60 °C. The reaction mixture was stirred at this temperature overnight and then quenched with H₂O. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=5/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=100/1, 1% Et₃N) to yield hexahydrobenzofuran 20 (72.1 mg, 94%) as a white amorphous. 20: [α]_D¹⁸=−40.3 (c 0.98, CHCl₃); IR (film) ν: 3057, 2929, 2856, 1726, 1597, 1471, 1427 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.87–7.83 (2H, m, TBDPS), 7.79–7.76 (2H, m, TBDPS), 7.46–7.20 (21H, m, TBDPS, Tr), 5.00 (1H, d, J=3.5 Hz, H1), 4.29 (1H, dd, J=5.5, 5.5 Hz, H8), 4.13 (1H, d, J=4.1 Hz, H6), 4.07 (1H, d, J=4.5 Hz, H3), 3.98 (1H, d, J=4.1 Hz, H5), 3.55 (1H, dd, J=9.0, 5.5 Hz, H8a), 3.49 (1H, dd, J=9.0, 5.5 Hz, H8a), 2.70 (1H, m, H2), 1.54 (3H, s, Me4a), 1.17 (9H, s, TBDPS), 0.79 (9H, s, TBS), 0.00 (3H, s, TBS), −0.06 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 144.39 (C, Tr), 136.99 (C, C4), 136.12 (CH, TBDPS), 135.96 (CH, TBDPS), 134.26 (C, TBDPS), 133.80 (C, TBDPS), 129.62 (CH), 128.59 (CH, Tr), 127.92 (CH, Tr), 127.65 (CH), 127.59 (CH), 127.08 (CH), 118.79 (CH, C3), 102.72 (CH, C1), 87.96 (C, Tr), 82.43 (C, C7), 80.03 (CH, C6), 75.93 (CH, C8), 70.88 (CH₂, C8a), 69.81 (CH, C5), 46.56 (CH, C2), 27.10 (CH₃, TBDPS), 25.83 (CH₃, TBS), 20.89 (CH₃, C4a), 19.61 (C, TBDPS), 18.12 (C, TBS), −4.43 (CH₃, TBS), −5.11 (CH₃, TBS); HRMS (ESI) m/z calcd for C₅₁H₆₀O₅Si₂Na [(M+Na)]⁺ 831.3871, found 831.3871.

Diol S12

To a solution of vinyl hexahydrobenzofuran 20 (49 mg, 61 μmol) in THF (1.2 ml) was added TBAF (1.2 ml, 1.0 m in THF, 1.2 mmol) at 40 °C. The reaction mixture was stirred at this temperature for 6 h and then quenched with H₂O. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=3/1–1/1, 1% Et₃N) to yield diol S12 (18.7 mg, 67%) as a white amorphous. S12: [α]_D¹⁹=−82.4 (c 0.94, CHCl₃); IR (film) ν: 3435, 3059, 2929, 1718, 1597, 1491, 1448 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.45–7.41 (6H, m, Tr), 7.35–7.24 (9H, m, Tr), 5.16 (1H d, J=3.5 Hz,, H1), 4.68 (1H, dd, J=3.1, 1.8 Hz, H3), 4.11 (1H, dd, J=8.6, 8.6 Hz, H8a), 4.07 (1H, ddd, J=8.8, 8.6, 8.6 Hz, H8), 4.02 (1H, d, J=4.5 Hz, H6), 3.98 (1H, dd, J=4.5, 2.2 Hz, H5), 3.49 (1H, dd, J=8.6, 8.6 Hz, H8a), 3.26 (1H, ddd, J=3.5, 3.1, 1.8 Hz, H2), 3.22 (1H, d, J=8.8 Hz, OH), 2.31 (1H, d, J=2.2 Hz, OH), 1.76 (3H, dd, J=1.8, 1.8 Hz, Me4a); ¹³C NMR (100 MHz, CDCl₃) δ: 143.88 (C, Tr), 135.62 (C, C4), 128.39 (CH, Tr), 128.11 (CH, Tr), 127.43 (CH, Tr), 121.46 (CH, C3), 101.95 (CH, C1), 88.30 (C, Tr), 82.55 (C, C7), 82.40 (CH, C6), 74.03 (CH, C8), 70.71 (CH₂, C8a), 67.84 (CH, C5), 46.75 (CH, C2), 21.92 (CH₃, C4a); HRMS (ESI) m/z calcd for C₂₉H₂₈O₅ [(M+Na)]⁺ 479.1829, found 479.1829.

Bis-TBS ether S13

To a solution of diol S12 (15.4 mg, 34 μmol) in DMF (0.84 ml) were added imidazole (46 mg, 0.67 mmol) followed by TBSCl (51 mg, 0.34 mmol) at 40 °C. The reaction mixture was stirred at this temperature for 3.5 h and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=5/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=100/1, 1% Et₃N) to yield bis-TBS ether S13 (17.2 mg, 75%) as a white solid. A single crystal for X-ray analysis was prepared by recrystallization from hexane and EtOAc. S13: m.p.=154 °C; [α]_D¹⁹=−61.5 (c 0.90, CHCl₃); IR (film) ν: 2928, 2856, 1731, 1621, 1471, 1448 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ:7.50–7.44 (6H, m, Tr), 7.34–7.21 (9H, m, Tr), 5.00 (1H, d, J=3.5 Hz, H1), 4.29 (1H, dd, J=5.5, 5.5 Hz, H8), 4.28 (1H, d, J=5.5 Hz, H3), 4.05 (1H, d, J=4.1 Hz, H6), 4.03 (1H, d, J=4.1 Hz, H5), 3.85 (1H, dd, J=8.8, 5.5 Hz, H8a), 3.65 (1H, dd, J=8.8, 5.5 Hz, H8a), 2.79 (1H, m, H2), 1.58 (3H, s, Me4a), 0.98 (9H, s, TBS), 0.86 (9H, s, TBS), 0.22 (3H, s, TBS), 0.18 (3H, s, TBS), 0.07 (3H, s, TBS), 0.05 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 144.35 (C, Tr), 137.17 (C, C4), 128.57 (CH, Tr), 127.92 (CH, Tr), 127.12 (CH, Tr), 118.65 (CH, C3), 102.65 (CH, C1), 87.88 (C, Tr), 82.30 (C, C7), 79.54 (CH, C6), 75.66 (CH, C8), 71.33 (CH₂, C8a), 69.75 (CH, C5), 46.66 (CH, C2), 26.02 (CH₃, TBS), 25.86 (CH₃, TBS), 20.88 (CH₃, C4a), 18.45 (C, TBS), 18.18 (C, TBS), −4.23 (CH₃, TBS), −4.28 (CH₃, TBS), −4.60 (CH₃, TBS), −5.08 (CH₃, TBS); HRMS (ESI) m/z calcd for C₄₁H₅₆O₅Si₂Na [(M+Na)]⁺ 707.3558, found 707.3558.

Alcohol 21

To a solution of TBDPS ether 20 (92.5 mg, 0.114 mmol) in THF (1.5 ml) was added 10% solution of NaOH in MeOH (11.4 ml) at room temperature. The reaction mixture was heated to 65 °C, stirred for 3 h at this temperature and then EtOAc and H₂O were added. The phases were separated and aqueous phase was extracted with EtOAc. The combined organic layer was washed with H2O, brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=200/1–100/1) to yield alcohol 21 (34.6 mg, 53%) as a white amorphous. 21: [α]_D²⁰−82.4 (c 0.73, CHCl₃); IR (film) ν: 3528, 2928, 1734, 1560, 1449, 1388 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.46–7.40 (m, 6H, Tr), 7.36–7.23 (m, 9H, Tr), 5.16 (d, J=3.3 Hz, 1H, H1), 4.64 (dd, J=2.7, 1.8 Hz, 1H, H3), 4.16 (ddd, J=9.4, 8.4, 8.4 Hz, 1H, H8), 4.07 (dd, J=8.4, 8.4 Hz, 1H, H8a), 3.93 (d, J=3.7 Hz, 1H, H6), 3.89 (d, J=3.7 Hz, 1H, H5), 3.43 (dd, J=8.4, 8.4 Hz, 1H, H8a), 3.31 (ddd, J=3.3, 2.7, 1.8 Hz, 1H, H2), 3.20 (d, J=9.4 Hz, 1H, OH), 1.68 (dd, J=1.8, 1.8 Hz, 3H, H4a), 0.82 (s, 9H, TBS), 0.04 (s, 3H, TBS), 0.02 (s, 3H, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 144.04 (C, Tr), 137.27 (C, C4), 128.43 (CH, Tr), 128.07 (CH, Tr), 127.39 (CH, Tr), 120.70 (CH, C3), 101.91 (CH, C1), 88.40 (C, Tr), 83.64 (C, C7), 82.29 (CH, C6), 73.71 (CH, C8), 71.02 (CH₂, C8a), 69.85 (CH, C5), 46.88 (CH, C2), 25.76 (CH₃, TBS), 21.70 (CH₃, C4a), 18.04 (C, TBS), −4.44 (CH₃, TBS), −5.11 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₅H₄₂O₅SiNa [(M+Na)]⁺ 593.2694, found 593.2691.

The south segment ketone 6

To a solution of alcohol 21 (7.8 mg, 14 μmol) in CH₂Cl₂-pyridine (1:1, 1.4 ml) was added DMP (17 mg, 41 mmol) at 0 °C. The reaction mixture was allowed to warm to room temperature, stirred for 3 h, then quenched with saturated aqueous NaHCO₃ and saturated aqueous Na₂S₂O₃. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=5/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated in vacuo to give the crude products. The residue was filtered through a florisil short column to give ketone 6, which was used in next reaction without further purification. 6: ¹H NMR (400 MHz, CDCl₃) δ: 7.49–7.45 (m, 6H, Tr), 7.35–7.24 (m, 9H, Tr), 5.08 (d, J=3.7 Hz, 1H, H1), 4.61 (dd, J=3.7, 1.6 Hz, 1H, H3), 4.20 (d, J=3.5 Hz, 1H, H6), 4.19 (d, J=17.6 Hz, 1H, H8a), 4.18 (d, J=3.5 Hz, 1H, H5), 4.12 (dd, J=17.6 Hz, 1H, H8a), 3.34 (ddd, J=3.7, 2.7, 2.0 Hz, 1H, H2), 1.68 (dd, J=2.0, 1.6 Hz, 3H, H4a), 0.81 (s, 9H, TBS), 0.06 (s, 3H, TBS), 0.02 (s, 3H, TBS); HRMS (ESI) m/z calcd for C₃₅H₄₀O₅SiNa [(M+Na)]⁺ 591.2537, found 591.2537.

Alcohol S14

To a solution of 4-trimethylsilyl-3-butyn-2-one 24 (6.60 ml, 40 mmol) in degassed 2-propanol (400 ml) at 35 °C was added Ru[(1 R,2 R)-p-TsNCH(C₆H₅)CH(C₆H₅)NH](η⁶-p-cymene) 25 (240 mg, 0.40 mmol). The reaction mixture was stirred overnight and then the solvent was removed in vacuo. The residue was distilled under reduce pressure at 80–90 °C to yield alcohol S14 (5.01 g, 88%) as a colorless oil. S14: [α]_D²⁴=28.6 (c 2.96, CHCl₃) (ent -S14: lit. [α]_D²³=−27.6 (c 2.97, CHCl₃), 98% ee (S)²⁸); IR (film) ν: 3338, 2961, 2899, 2175, 1406, 1327, 1250, 1119, 1046 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 4.51 (1H, qd, J=6.7, 5.2 Hz, H24), 1.80 (1H, d, J=5.2 Hz, OH), 1.45 (3H, d, J=6.7 Hz, Me24a), 0.17 (9H, s, TMS); ¹³C NMR (100 MHz, CDCl₃) δ: 107.61 (C, C23), 88.41 (C, C22), 58.76 (CH, C24), 24.23 (CH₃, C24a), −0.17 (CH₃, TMS); HRMS (ESI) m/z calcd for C₇H₁₄OSiNa [M+Na]⁺ 165.0706, found 165.0703.

Mesylate 26

To a solution of alcohol S14 (5.01 g, 35 mmol) in CH₂Cl₂ (70 ml) at −90 °C was added Et₃N (9.8 ml, 70 mmol), followed by MsCl (4.1 ml, 53 mmol). After being stirred for 3 h at the same temperature, the reaction mixture was gradually warmed to −20 °C and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with Et₂O. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=15/1) to yield mesylate 26 (7.43 g, 96%) as yellowish oil. 26: [α]_D²⁵=116.1 (c 2.32, CHCl₃); IR (film) ν: 2961, 2177, 1363, 1252, 1718, 1126, 1091, 1024 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 5.26 (1H, q, J=6.8 Hz, H24), 3.12 (3H, s, Ms), 1.63 (3H, d, J=6.8 Hz, Me24a), 0.19 (9H, s, TMS); ¹³C NMR (100 MHz, CDCl₃) δ:101.21 (C, C23), 93.66 (C, C22), 68.58 (CH, C24), 39.10 (CH₃, Ms), 22.46 (CH₃, C24a), −0.49 (CH₃, TMS); HRMS (ESI) m/z calcd for C₈H₁₆O₃SSiNa [M+Na]⁺ 243.0482, found 243.0484.

Aldehyde 27

To a solution of (S)-(-)-2-methyl-1-butanol (7.6 ml, 70 mmol) in CH₂Cl₂ (70 ml) at 0 °C was added TEMPO (1.1 g, 7.0 mmol), followed by PhI(OAc)₂ (24.8 g, 77 mmol). After being stirred for 3 h at room temperature, the reaction mixture was quenched with saturated aqueous Na₂S₂O₃. The phases were separated and the aqueous phase was extracted with CH₂Cl₂. The combined organic layer was washed with saturated aqueous NaHCO₃ and brine, dried over MgSO₄ and concentrated to provide aldehyde 27, which was used in the next reaction without further purification.

Homopropargyl alcohol 28

To a solution of Pd(OAc)₂ (337 mg, 1.5 mmol) in THF (300 ml) was added PPh₃ (393 mg, 1.5 mmol) at −90 °C. Then, aldehyde 27 (~70 mmol), mesylate 26 (6.6 g, 30 mmol) and Et₂Zn (90 ml, 1 m in hexane, 90 mmol) were added sequentially. The reaction mixture was allowed to warm to −20 °C and stirred overnight. After being stirred for 4 h at 0 °C, the reaction mixture was quenched with saturated aqueous NH₄Cl. The phases were separated and the aqueous phase was extracted with Et₂O. The combined organic layer was washed with brine and H2O, dried over MgSO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=100/1–50/1) to yield alcohol 28 (5.00 g, 78%, dr =14:1) as a yellow oil. 28: [α]_D²⁵−1.8 (c 1.02, CHCl₃); IR (film) ν: 3568, 2961, 2935, 2876, 2164, 1458, 1376, 1249 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 3.22 (1H, ddd, J=6.1, 6.1, 4.7 Hz, H25), 2.68 (1H, qd, J=6.9, 6.1 Hz, H24), 1.86 (1H, d, J=6.1 Hz, OH), 1.55–1.41 (2H, m, H26, H27), 1.27 (1H, dqd, J=14.9, 7.3, 1.8 Hz, H27), 1.17 (3H, d, J=6.9 Hz, Me24a), 0.90 (3H, d, J=6.9 Hz, Me26a), 0.90 (3H, t, J=7.3 Hz, Me28), 0.15 (9H, s, TMS); ¹³C NMR (100 MHz, CDCl₃) δ: 108.19 (C, C22), 87.46 (C, C23), 77.11 (CH, C25), 37.55 (CH, C26), 31.88 (CH, C24), 26.58 (CH₂, C27), 17.69 (CH₃, C24a), 13.12 (CH₃, C26a), 11.62 (CH₃, C28), 0.12 (CH₃, TMS); HRMS (ESI) m/z calcd for C₁₂H₂₄OSiNa [M+Na]⁺ 235.1489, found 235.1489.

TMS ether 29

To a solution of alcohol 28 (5.00 g, 24 mmol) in MeOH (17 ml) was added K₂CO₃ (4.90 g, 35 mmol) at room temperature. After being stirred for 1.5 h, the reaction mixture was quenched with Et₂O and H₂O. The phases were separated and the aqueous phase was extracted with Et₂O. The combined organic layer was dried over MgSO₄ and concentrated. The residue was filtered through a short pad of silica gel (hexane/Et₂O=1/1) to yield terminal alkyne S15, which was used in the next reaction.

To a solution of S15 (<24 mmol) in CH₂Cl₂ (235 ml) was added TMS-imidazole (5.2 ml, 35 mmol) at 0 °C. After being stirred for 5 h at room temperature, the reaction mixture was quenched with Et₂O and H₂O. The phases were separated and the aqueous phase was extracted with Et₂O. The combined organic layer was dried over MgSO₄ and concentrated. The residue was purified by open column chromatography (hexane) to yield TMS ether 29 (4.35 g, 87%, 2 steps) as a colorless oil. 29: [α]_D²⁶4.9 (c 1.03, CHCl₃); IR (film) ν: 3306, 3018, 2964, 2399, 1250, 1215, 1055 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 3.43 (1H, dd, J=6.3, 4.7 Hz, H25), 2.62 (1H, qdd, J=7.0, 6.3, 2.5 Hz, H24), 2.04 (1H, d, J=2.5 Hz, H22), 1.55 (1H, m, H26), 1.41 (1H, dqd, J=12.9, 7.4, 2.3 Hz, H27), 1.19 (1H, dq, J=12.9, 7.4 Hz, H27), 1.15 (3H, d, J=7.0 Hz, Me24a), 0.90 (3H, t, J=7.4 Hz, Me28), 0.85 (3H, d, J=6.7 Hz, Me26a), 0.15 (9H, s, TMS); ¹³C NMR (100 MHz, CDCl₃) δ: 87.77 (C, C23), 78.96 (CH, C25), 69.39 (CH, C22), 37.81 (CH, C26), 30.96 (CH, C24), 26.84 (CH₂, C27), 17.95 (CH₃, C24a), 13.60 (CH₃, C26a), 11.75 (CH₃, C28), 0.89 (CH₃, TMS); HRMS (ESI) m/z calcd for C₁₂H₂₄OSiNa [M+Na]⁺ 235.1489, found 235.1492.

Benzyl ether 30

To a solution of 1,3-propanediol (21.0 ml, 0.30 mol) in THF (100 ml) was added NaH (6.00 g, 60% in oil, 0.15 mol) at 0 °C. The reaction mixture was stirred at this temperature for 1 h and then BnBr (12 ml, 0.10 mol) was added. The reaction mixture was gradually warmed to room temperature, stirred overnight, and then quenched with saturated aqueous NH₄Cl. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=4/1–2/1) to yield mono-benzyl ether 30 (17.0 g, 100%) as a colorless oil. 30: IR (film) ν: 3391, 3030, 2944, 2866, 1718, 1453, 1364, 1275, 1074 cm⁻¹; ¹H NMR (500 MHz, CDCl₃) δ: 7.37–7.27 (5H, m, Bn), 4.53 (2H, s, Bn), 3.79 (2H, td, J=5.6, 5.6 Hz, H17), 3.67 (2H, t, J=5.6 Hz, H15), 2.26 (1H, m, OH), 1.87 (2H, tt, J=5.6, 5.6 Hz, H16); ¹³C NMR (100 MHz, CDCl₃) δ: 138.02 (C, Bn), 128.44 (CH, Bn), 127.71 (CH, Bn), 127.64 (CH, Bn), 73,27 (CH₂, Bn), 69.42 (CH₂, C15), 61.95 (CH₂, C17), 32.05 (CH₂, C16); HRMS (ESI) m/z calcd for C₁₀H₁₄O₂Na [M+Na]⁺ 189.0886, found 189.0886.

β-Ketoester 34

To a solution of 30 (3.00 g, 18 mmol) in CH₂Cl₂ (18 ml) was added TEMPO (0.28 g, 1.8 mmol), followed by PhI(OAc)₂ (6.4 g, 20 mmol) at 0 °C. The reaction mixture was allowed to warm to room temperature, stirred for 1.5 h and then quenched with saturated aqueous Na₂S₂O₃. The phases were separated and the aqueous phase was extracted with CH₂Cl₂. The combined organic layer was washed with saturated aqueous NaHCO₃ and brine, dried over MgSO₄ and concentrated to provide aldehyde 31, which was used directly.

To a solution of Schiff base 33 (1.4 g, 3.6 mmol) in CH₂Cl₂ (18 ml) was added Ti(Oi-Pr)₄ (5.3 mmol, 18 mmol) at room temperature. The reaction mixture was stirred for 1 h and then cooled to −40 °C. To the reaction mixture were added aldehyde 31 (<18 mmol) and diketene 32 (2.8 ml, 36 mmol), stirred at this temperature for 48 h and then quenched with EtOAc and 1 n HCl. After being stirred for 2 h at room temperature, the phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=5/1–4/1) to yield β-ketoester 34 (2.98 g, 54%, 2 steps) as a yellow oil. 34: [α]_D³⁰=−13.4 (c 1.00, CHCl₃). The ee was determined to be 76% by HPLC analysis (Chiralpak AD, 5% ethanol and 0.01% trifluoroacetic acid in hexane, 1.0 ml min⁻¹, λ=254 nm, S isomer 15.3 min, R isomer 19.2 min); IR (film) ν: 3503, 2981, 2935, 2868, 1735, 1717, 1313, 1105 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.37–7.27 (5H, m, Bn), 5.05 (1H, sept, J=6.2 Hz, iPr), 4.51 (2H, s, Bn), 4.30 (1H, m, H17), 3.69–3.58 (2H, m, H15x2), 3.44 (2H, s, H20), 3.29 (1H, brs, OH), 2.75–2.65 (2H, m, H18x2), 1.83–1.69 (2H, m, H16x2), 1.25 (6H, d, J=6.2 Hz, iPr); ¹³C NMR (100 MHz, CDCl₃) δ: 202.98 (C, C19), 166.47 (C, C21), 137.89 (C, Bn), 128.36 (CH, Bn), 127.65 (CH, Bn), 127.60 (CH, Bn), 73,19 (CH₂, Bn), 68.98 (CH, iPr), 67.93 (CH₂, C15), 66.57 (CH, C17), 50.16 (CH₂, C20), 49.69 (CH₂, C18), 35.94 (CH₂, C16), 21.60 (CH₃, iPr); HRMS (ESI) m/z calcd for C₁₇H₂₄O₅Na [M+Na]⁺ 331.1516, found 331.1521.

Diol S16

To a solution of NaBH(OAc)₃ (4.10 g, 19 mmol) in MeCN (96 ml) were added AcOH (3.30 ml, 58 mmol) and β-ketoester 34 (2.98 g, 9.6 mmol) at −60 °C. The reaction mixture was gradually warmed to 0 °C, stirred overnight and then quenched with K₂CO₃ and H₂O. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with saturated aqueous NaHCO₃ and brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=2/1–1/1) to yield diol S16 (2.48 g, 83%) as a yellowish oil. S16: [α]_D²⁷=−0.97 (c 1.07, CHCl₃, 76% ee); IR (film) ν: 3447, 2979, 2937, 2866, 1734, 1718, 1707, 1375, 1281, 1107 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.37–7.27 (5H, m, Bn), 5.04 (1H, sept, J=6.3 Hz, iPr), 4.52 (2H, s, Bn), 4.32 (1H, m, H19), 4.15 (1H, m, H17), 3.76–3.61 (2H, m, H15x2), 3.57 (1H, d, J=3.9 Hz, C19-OH), 3.45 (1H, d, J=2.9 Hz, C17-OH), 2.52–2.38 (2H, m, H20), 1.87 (1H, m, H16), 1.73 (1H, m, H16), 1.66–1.57 (2H, m, H18), 1.24 (6H, d, J=6.3 Hz, iPr); ¹³C NMR (100 MHz, CDCl₃) δ: 172.24 (C, C21), 137.82 (C, Bn), 128.46 (CH, Bn), 127.77 (CH, Bn), 127.68 (CH, Bn), 73.37 (CH₂, Bn), 69.16 (CH₂, C15), 68.68 (CH, C17), 68.11 (CH, iPr), 65.61 (CH, C19), 42.39 (CH₂, C18), 41.79 (CH₂, C20), 36.49 (CH₂, C16), 21.79 (CH₃, iPr); HRMS (ESI) m/z calcd for C₁₇H₂₆O₅Na [M+Na]⁺ 333.1672, found 333.1676.

Lactone 35

To a solution of diol S16 (2.48 g, 8.0 mmol) in (CH₂Cl)₂ (40 ml) was added PPTS (100 mg, 0.40 mmol). The reaction mixture was stirred for 5 h under reflux conditions and then introduced additional PPTS (100 mg, 0.40 mmol). The reaction mixture was stirred at this temperature for 1.5 h and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine and dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=1/2–0/1) to yield lactone 35 (1.65 g, purity 92%, 75%) as a colorless oil. 35: [α]_D²⁴=40.4 (c 1.01, CHCl₃, 76% ee); IR (film) ν: 3435, 2926, 1732, 1455, 1364, 1252, 1096 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.37–7.27 (5H, m, Bn), 4.52 (1H, d, J=11.8 Hz, Bn), 4.48 (1H, d, J=11.8 Hz, Bn), 4.41 (1H, dddd, J=11.7, 7.8, 4.5, 3.1 Hz, H17), 4.22 (1H, dddd, J=9.0, 7.5, 5.9, 5.7 Hz, H19), 3.68 (1H, dt, J=9.6, 4.5 Hz, H15), 3.61 (1H, dt, J=9.6, 5.3 Hz, H15), 2.86 (1H, ddd, J=17.0, 5.9, 1.4 Hz, H20), 2.44 (1H, dd, J=17.0, 7.5 Hz, H20), 2.37 (1H, brs, OH), 2.24 (1H, dddd, J=13.7, 5.7, 3.1, 1.4 Hz, H18), 2.03–1.88 (2H, m, H16), 1.59 (1H, ddd, J=13.7, 11.7, 9.0 Hz, H18); ¹³C NMR (100 MHz, CDCl₃) δ: 170.65 (C, C21), 138.04 (C, Bn), 128.42 (CH, Bn), 127.73 (CH, Bn), 74.31 (CH, C17), 73.20 (CH₂, Bn), 65.58 (CH₂, C15), 63.75 (CH, C19), 39.41 (CH₂, C20), 37.85 (CH₂, C18), 35.76 (CH₂, C16); HRMS (ESI) m/z calcd for C₁₄H₁₈O₄Na [M+Na]⁺ 273.1097, found 273.1104.

TBS ether 36

To a solution of alcohol 35 (1.65 g, 6.6 mmol) in DMF (6.6 ml) was added imidazole (0.67 g, 9.9 mmol), followed by TBSCl (1.19 g, 7.9 mmol) at 40 °C. The reaction mixture was stirred at this temperature overnight, then quenched with EtOAc and saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=1/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=5/1) to yield TBS ether 36 (1.97 g, 82%) as a colorless oil. 36: [α]_D²⁶21.1 (c 1.02, CHCl₃, 76% ee); IR (film) ν: 2954, 2929, 2857, 1740, 1458, 1388, 1362, 1252, 1101 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.37–7.27 (5H, m, Bn), 4.53 (1H, d, J=11.7 Hz, Bn), 4.48 (1H, d, J=11.7 Hz, Bn), 4.41 (1H, dddd, J=11.7, 7.6, 4.3, 3.1 Hz, H17), 4.15 (1H, dddd, J=9.2, 7.8, 5.7, 5.7 Hz, H19), 3.69 (1H, dt, J=8.4, 4.9 Hz, H15), 3.61 (1H, dt, J=8.4, 5.3 Hz, H15), 2.81 (1H, ddd, J=17.2, 5.7, 1.4 Hz, H20), 2.42 (1H, dd, J=17.2, 7.8 Hz, H20), 2.14 (1H, dddd, J=13.7, 5.7, 3.1, 1.4 Hz, H18), 2.02–1.86 (2H, m, H16), 1.60 (1H, ddd, J=13.7, 11.7, 9.2 Hz, H18), 0.87 (9H, s, TBS), 0.07 (6H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 170.64 (C, C21), 138.01 (C, Bn), 128.42 (CH, Bn), 127.74 (CH, Bn), 127.72 (CH, Bn), 74.11 (CH, C17), 73.23 (CH₂, Bn), 65.64 (CH₂, C15), 64.50 (CH, C19), 40.16 (CH₂, C20), 38.72 (CH₂, C18), 35.96 (CH₂, C16), 25.65 (CH₃, TBS), 17.92 (C, TBS), −4.77 (CH₃, TBS); HRMS (ESI) m/z calcd for C₂₀H₃₂O₄SiNa [M+Na]⁺ 387.1962, found 387.1959.

Ynone S17

To a solution of alkyne 29 (102 mg, 0.48 mmol) in THF (8 ml) was added n-BuLi (0.31 ml, 1.56 m in hexane, 0.48 mmol) at −78 °C. After the reaction mixture was stirred at this temperature for 20 min, lactone 36 (144 mg, 0.40 mmol) was added dropwise. The reaction mixture was stirred at this temperature for 1 h and then quenched with saturated aqueous NH₄Cl. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=20/1–15/1) to yield ynone S17 (175 mg, purity 97%, 74%) as an inseparable mixture with hemiacetal. S17: yellowish oil; [α]_D²⁷=5.8 (c 1.12, CHCl₃); IR (film) ν: 3528, 29257, 2857, 2210, 1671, 1458, 1362, 1251, 1076 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.36–7.27 (5H, m, Bn), 4.55 (1H, m, H19), 4.52 (1H, d, J=12.0 Hz, Bn), 4.49 (1H, d, J=12.0 Hz, Bn), 4.04 (1H, m, H17), 3.66 (1H, ddd, J=9.4, 6.5, 5.5 Hz, H15), 3.62 (1H, ddd, J=9.4, 6.5, 5.5 Hz, H15), 3.48 (1H, dd, J=5.9, 4.3 Hz, H25), 3.33 (1H, brs, OH), 2.86 (1H, dd, J=15.7, 6.1 Hz, H20), 2.80–2.73 (1H, m, H24), 2.77 (1H, dd, J=15.7, 6.6 Hz, H20), 1.80–1.66 (3H, m, H16x2, H18), 1.61 (1H, m, H18), 1.54 (1H, m, H26), 1.41 (1H, m, H27), 1.22 (1H, m, H27), 1.19 (3H, d, J=7.2 Hz, Me24a), 0.90 (3H, t, J=7.0 Hz, Me28), 0.88 (9H, s, TBS), 0.86 (3H, d, J=6.7 Hz, Me26a), 0.14 (9H, s, TMS), 0.12 (3H, s, TBS), 0.09 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 185.49 (C, C21), 138.21 (C, Bn), 128.39 (CH, Bn), 127.65 (CH, Bn), 127.60 (CH, Bn), 97.92 (C, C22), 82.42 (C, C23), 78.49 (CH, C25), 73.19 (CH₂, Bn), 68.07 (CH₂, C15), 67.21 (CH, C19), 66.83 (CH, C17), 52.83 (CH₂, C20), 43.38 (CH₂, C18), 38.13 (CH, C26), 37.44 (CH₂, C16), 31.66 (CH, C24), 26.67 (CH₂, C27), 25.80 (CH₃, TBS), 17.93 (C, TBS), 17.18 (CH₃, C24a), 13.59 (CH₃, C26a), 11.80 (CH₃, C28), 0.80 (CH₃, TMS), −4.75 (CH₃, TBS), −4.78 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₂H₅₆O₅Si₂Na [M+Na]⁺ 599.3558, found 599.3557.

Methyl acetal 37

To a solution of ketone S17 (2.46 g, 4.3 mmol) in MeOH (85 ml) was added citric acid (1.23 g, 6.4 mmol) at room temperature. The reaction mixture was stirred overnight and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=15/1) to yield methyl acetal 37 (1.84 g, 83%, dr=7:1) as a colorless oil. 37: [α]_D²⁷26.7 (c 0.98, CHCl₃); IR (film) ν: 3503, 2956, 2856, 2239, 1458, 1381, 1255, 1186, 1147, 1115 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.36–7.27 (5H, m, Bn), 4.49 (2H, s, Bn), 4.03 (1H, dddd, J=10.9, 10.9, 4.8, 4.8 Hz, H19), 3.76 (1H, dddd, J=12.2, 8.6, 3.9, 1.8 Hz, H17), 3.66 (1H, ddd, J=9.2, 9.2, 5.1 Hz, H15), 3.57 (1H, dt, J=9.2, 5.1 Hz, H15), 3.48 (1H, s, OH), 3.32–3.22 (4H, m, H25, OMe), 2.74 (1H, dq, J=7.0, 6.8 Hz, H24), 2.20 (1H, ddd, J=12.9, 4.8, 1.6 Hz, H20), 1.88–1.73 (3H, m, H16x2, H18), 1.69 (1H, dd, J=12.9, 10.9 Hz, H20), 1.57–1.43 (2H, m, H26, H27), 1.32–1.23 (2H, m, H27, H18), 1.21 (3H, d, J=6.8 Hz, H24a), 0.92 (3H, d, J=6.7 Hz, Me26a), 0.91 (3H, t, J=7.4 Hz, Me28), 0.87 (9H, s, TBS), 0.05 (3H, s, TBS), 0.04 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 138.35 (C, Bn), 128.36 (CH, Bn), 127.73 (CH, Bn), 127.58 (CH, Bn), 95.91 (C, C21), 85.33 (C, C22), 81.06 (C, C23), 77.32 (CH, C25), 73.11 (CH₂, Bn), 66.57 (CH₂, C15), 65.99 (CH, C17), 64.63 (CH, C19), 49.99 (CH₃, OMe), 45.82 (CH₂, C20), 40.85 (CH₂, C18), 37.65 (CH, C26), 35.69 (CH₂, C16), 30.74 (CH, C24), 26.45 (CH₂, C27), 25.81 (CH₃, TBS), 18.04 (C, TBS), 17.56 (CH₃, C24a), 13.20 (CH₃, C26a), 11.57 (CH₃, C28), −4.63 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₀H₅₀O₅SiNa [M+Na]⁺ 541.3320, found 541.3320.

Cis-olefin S18

To a solution of methyl acetal 37 (419 mg, 0.83 mmol) in MeOH (17 ml) was added Lindlar catalyst (166 mg) at room temperature. The reaction mixture was stirred under H₂ atmosphere for 3 h and then filtered through Celite and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=30/1–25/1) to yield cis-olefin S18 (0.43 g, purity 92%, 92%, dr=7:1) as a colorless oil. S18: [α]_D²⁴44.2 (c 1.34, CHCl₃); IR (film) ν: 3567, 3032, 2957, 2856, 1654, 1461, 1382, 1255, 1076 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.35–7.30 (5H, m, Bn), 5.70 (1H, dd, J=9.8, 1.8 Hz H23), 5.55 (1H, dd, J=9.8, 2.5 Hz, H22), 4.50 (1H, d, J=11.7 Hz, Bn), 4.47 (1H, d, J=11.7 Hz, Bn), 4.12 (1H, dddd, J=11.0, 11.0, 4.5, 4.5 Hz, H19), 3.91 (1H, dddd, J=11.5, 7.8, 4.5, 2.0 Hz, H17), 3.63–3.52 (2H, m, H15x2), 3.48 (3H, s, OMe), 3.33 (1H, dd, J=9.8, 1.8 Hz, H25), 2.22 (1H, dqdd, J=9.8, 7.2, 2.5, 1.8 Hz, H24), 1.90–1.71 (4H, m, H16x2, H18, H20), 1.52 (1H, m, H26), 1.46–1.30 (3H, m, H20, H27x2), 1.29–1.18 (1H, m, H18), 0.91 (3H, d, J=7.2 Hz, Me24a), 0.88 (9H, s, TBS), 0.88 (3H, m, Me28), 0.85 (3H, d, J=7.2 Hz, Me26a), 0.06 (3H, s, TBS), 0.05 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 138.48 (C, Bn), 135.05 (CH, C23), 128.58 (CH, C22), 128.29 (CH, Bn), 127.66 (CH, Bn), 127.45 (CH, Bn), 95.38 (C, C21), 75.71 (CH, C25), 73.03 (CH₂, Bn), 67.21 (CH₂, C15), 66.43 (CH, C17), 65.46 (CH, C19), 50.85 (CH₃, OMe), 44.72 (CH₂, C20), 41.32 (CH₂, C18), 36.13 (CH₂, C16), 35.49 (CH, C26), 30.55 (CH, C24), 27.90 (CH₂, C27), 25.89 (CH₃, TBS), 18.21 (C, TBS), 16.69 (CH₃, C24a), 12.38 (CH₃, C28), 12.23 (CH₃, C26a), −4.68 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₀H₅₂O₅SiNa [M+Na]⁺ 543.3476, found 543.3472.

Spiroacetal S19

To a solution of cis-alkene S18 (0.34 g, 0.65 mmol) in Et₂O (22 ml) was added PPTS (16.3 mg, 65 μmol) at room temperature. The reaction mixture was stirred for 3 h and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=50/1) to yield spiroacetal S19 (0.27 g, 86%, dr=5:1) as a colorless oil. S19: [α]_D²⁴=62.6 (c 0.98, CHCl₃); IR (film) ν: 3032, 2957, 2928, 2856, 1654, 1460, 1381, 1255, 1117, 1076 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.36–7.27 (5H, m, Bn), 5.70 (1H, dd, J=9.8, 2.5 Hz, H23), 5.55 (1H, dd, J=9.8, 2.5 Hz, H22), 4.50 (1H, d, J=11.9 Hz, Bn), 4.46 (1H, d, J=11.9 Hz, Bn), 4.12 (1H, dddd, J=11.2, 11.2, 4.7, 4.7 Hz, H19), 3.92 (1H, dddd, J=11.9, 7.8, 4.5, 2.0 Hz, H17), 3.59 (1H, dd, J=6.3, 3.1 Hz, H15), 3.58 (1H, dd, J=6.3, 2.2 Hz, H15), 3.34 (1H, dd, J=10.0, 1.8 Hz, H25), 2.23 (1H, dqdd, J=10.0, 7.0, 2.5, 2.5 Hz, H24), 1.91–1.71 (4H, m, H16x2, H18, H20), 1.55 (1H, m, H26), 1.46–1.29 (2H, m, H27x2), 1.41 (1H, dd, J=12.3, 11.2 Hz, H20), 1.23 (1H, ddd, J=11.9, 11.9, 11.2 Hz, H18), 0.91 (3H, d, J=7.0 Hz, Me24a), 0.88 (9H, s, TBS), 0.88 (3H, m, H28), 0.85 (3H, d, J=7.4 Hz, Me26a), 0.06 (3H, s, TBS), 0.05 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 138.51 (C, Bn), 135.06 (CH, C23), 128.59 (CH, C22), 128.30 (CH, Bn), 127.67 (CH, Bn), 127.46 (CH, Bn), 95.38 (C, C21), 75.72 (CH, C25), 73.04 (CH₂, Bn), 67.22 (CH₂, C15), 66.44 (CH, C17), 65.46 (CH, C19), 44.74 (CH₂, C20), 41.34 (CH₂, C18), 36.15 (CH₂, C16), 35.50 (CH, C26), 30.57 (CH, C24), 27.92 (CH₂, C27), 25.90 (CH₃, TBS), 18.23 (C, TBS), 16.70 (CH₃, C24a), 12.40 (CH₃, C28), 12.24 (CH₃, C26a), −4.67 (CH₃, TBS), −4.65 (CH₃, TBS); HRMS (ESI) m/z calcd for C₂₉H₄₈O₄SiNa [M+Na]⁺ 511.3214, found 511.3214.

Alcohol 38

To a solution of spiroacetal S19 (761 mg, 1.6 mmol) in THF (60 ml) and NH₃ (ca. 60 ml) was added sodium in small pieces at −60 °C. The reaction mixture was stirred at this temperature for 30 min and then quenched with THF and NH₄Cl. After excess NH₃ was removed at room temperature over 3 h, H₂O was added. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=20/1) to yield alcohol 38 (505 mg, purity 93%, 75%) as a yellowish oil. 38: [α]_D²⁴85.8 (c 0.91, CHCl₃); IR (film) ν: 3467, 2959, 1654, 1459, 1382, 1255, 1117, 1076 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 5.69 (1H, dd, J=9.5, 1.5 Hz, H23), 5.53 (1H, dd, J=9.5, 2.5 Hz, H22), 4.13 (1H, dddd, J=11.3, 11.3, 4.5, 4.5 Hz, H19), 4.00 (1H, dddd, J=11.3, 9.2, 2.6, 2.6 Hz, H17), 3.80–3.74 (2H, m, H15x2), 3.34 (1H, dd, J=10.0, 1.8 Hz, H25), 2.85 (1H, brs, OH), 2.23 (1H, dqdd, J=10.0, 7.0, 2.5, 1.5 Hz, H24), 1.88 (1H, ddd, J=12.3, 4.5, 2.0 Hz, H20), 1.84–1.75 (2H, m, H16, H18), 1.70 (1H, m, H16), 1.57 (1H, m, H26), 1.49–1.38 (2H, m, H27x2), 1.42 (1H, dd, J=12.3, 11.3 Hz, H20), 1.33 (1H, ddd, J=11.3, 11.3, 11.3 Hz, H18), 0.96 (3H, t, J=7.0 Hz, Me28), 0.90 (3H, d, J=7.0 Hz, Me24a), 0.88 (9H, s, TBS), 0.87 (3H, d, J=7.0 Hz, Me26a), 0.06 (3H, s, TBS), 0.06 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 135.30 (CH, C23), 128.21 (CH, C22), 95.68 (C, C21), 75.96 (CH, C25), 70.32 (CH, C17), 65.08 (CH, C19), 61.92 (CH₂, C15), 44.59 (CH₂, C20), 41.25 (CH₂, C18), 37.64 (CH₂, C16), 35.43 (CH, C26), 30.49 (CH, C24), 28.07 (CH₂, C27), 25.88 (CH₃, TBS), 18.20 (C, TBS), 16.63 (CH₃, C24a), 12.38 (CH₃, C26a, C28), −4.67 (CH₃, TBS); HRMS (ESI) m/z calcd for C₂₂H₄₂O₄SiNa [M+Na]⁺ 421.2745, found 421.2748.

Ester 39

To a solution of alcohol 38 (913 mg, 2.3 mmol) in CH₂Cl₂ (46 ml) was added NaHCO₃ (1.93 g, 23 mmol), followed by DMP (2.86 g, 6.9 mmol) at 0 °C. The reaction mixture was stirred at this temperature for 30 min and then allowed to warm to room temperature. After being stirred for 1.5 h, the mixture was quenched with saturated aqueous Na₂S₂O₃. The phases were separated and the aqueous phase was extracted with CH₂Cl₂. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated to provide aldehyde S20, which was used in the next reaction.

To a solution of aldehyde S20 in THF (46 ml) was added Ph₃P=C(Me)CO₂Et (1.67 g, 4.6 mmol). The reaction mixture was stirred overnight under reflux conditions and then quenched with saturated aqueous NH₄Cl. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=5/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=200/1–10/1) to yield ester 39 (965 mg, 87%, 2 steps) as a colorless oil. 39: [α]_D²³=43.6 (c 0.91, CHCl₃); IR (film) ν: 2958, 1712, 1654, 1459, 1381, 1256, 1076 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 6.82 (1H, td, J=7.0, 1.2 Hz, H15), 5.71 (1H, dd, J=10.0, 1.8 Hz, H23), 5.55 (1H, dd, J=10.0, 2.7 Hz, H22), 4.17 (2H, q, J=7.1 Hz, Et), 4.12 (1H, dddd, J=11.0, 11.0, 4.5, 4.5 Hz, H19), 3.84 (1H, m, H17), 3.39 (1H, dd, J=10.0, 1.7 Hz, H25), 2.40 (1H, ddd, J=15.5, 7.0, 7.0 Hz, H16), 2.33 (1H, ddd, J=15.5, 7.0, 7.0 Hz, H16), 2.23 (1H, dqdd, J=10.0, 7.1, 2.7, 1.8 Hz, H24), 1.88 (1H, ddd, J=12.5, 4.5, 1.8 Hz, H20), 1.82 (3H, d, J=1.2 Hz, Me14a), 1.81 (1H, m, H18), 1.58 (1H, m, H26), 1.46–1.30 (3H, m, H20, H27x2), 1.27 (3H, t, J=7.1 Hz, Et), 1.28 (1H, m, H18), 0.92 (3H, t, J=7.4 Hz, Me28), 0.91 (3H, d, J=7.1 Hz, Me24a), 0.88 (9H, s, TBS), 0.87 (3H, d, J=6.9 Hz, Me26a), 0.06 (3H, s, TBS), 0.06 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 168.01 (C, C13), 138.10 (CH, C15), 135.28 (CH, C23), 129.24 (C, C14), 128.42 (CH, C22), 95.62 (C, C21), 75.16 (CH, C25), 68.29 (CH, C17), 65.37 (CH, C19), 60.34 (CH₂, Et), 44.63 (CH₂, C20), 40.81 (CH₂, C18), 35.41 (CH, C26), 35.11 (CH₂, C16), 30.55 (CH, C24), 27.81 (CH₂, C27), 25.88 (CH₃, TBS), 18.18 (C, TBS), 16.59 (CH₃, C24a), 14.25 (CH₃, Et), 12.67 (CH₃, C28), 12.59 (CH₃, C14a), 12.24 (CH₃, C26a), −4.65 (CH₃, TBS); HRMS (ESI) m/z calcd for C₂₇H₄₈O₅SiNa [M+Na]⁺ 503.3163, found 503.3159.

Alcohol S21

To a solution of ester 39 (965 mg, 1.8 mmol) in CH₂Cl₂ (90 ml) was added DIBAL (14.3 ml, 1.01 m in toluene, 14.4 mmol) at −60 °C. The reaction mixture was stirred at this temperature for 5 min and then quenched with saturated aqueous Rochelle salt. After being stirred for 2 h, the phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=20/1–5/1) to yield alcohol S21 (867 mg, 98%) as a colorless oil. S21: [α]_D²²62.4 (c 1.11, CHCl₃); IR (film) ν: 3434, 2928, 2857, 1734, 1654, 1461, 1381, 1255, 1117, 1071 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 5.70 (1H, dd, J=10.0, 1.8 Hz, H23), 5.55 (1H, dd, J=10.0, 2.6 Hz, H22), 5.46 (1H, t, J=7.0 Hz, H15), 4.10 (1H, dddd, J=11.7, 11.7, 4.5, 4.5 Hz, H19), 4.00 (2H, brs, H13), 3.76 (1H, m, H17), 3.54 (1H, d, J=5.1 Hz, OH), 3.39 (1H, dd, J=9.8, 1.6 Hz, H25), 2.29 (1H, ddd, J=14.7, 7.0, 7.0 Hz, H16), 2.25–2.15 (2H, m, H16, H24), 1.87 (1H, ddd, J=12.3, 4.5, 1.8 Hz, H20), 1.81 (1H, dddd, J=11.7, 4.5, 1.8, 1.8 Hz, H18), 1.66 (3H, s, Me14a), 1.55 (1H, qtd, J=6.9, 6.9, 1.6 Hz, H26), 1.47–1.23 (3H, m, H20, H27x2), 1.20 (1H, ddd, J=11.7, 11.7, 11.7 Hz, H18), 0.91 (3H, t, J=7.4 Hz, Me28), 0.90 (3H, d, J=7.2 Hz, Me24a), 0.88 (9H, s, TBS), 0.85 (3H, d, J=6.9 Hz, Me26a), 0.06 (3H, s, TBS), 0.05 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 136.62 (C, C14), 135.22 (CH, C23), 128.56 (CH, C22), 121.84 (CH, C15), 95.57 (C, C21), 74.92 (CH, C25), 68.98 (CH, C17), 68.87 (CH₂, C13), 65.49 (CH, C19), 44.73 (CH₂, C20), 40.61 (CH₂, C18), 35.33 (CH, C26), 34.11 (CH₂, C16), 30.55 (CH, C24), 27.74 (CH₂, C27), 25.88 (CH₃, TBS), 18.17 (C, TBS), 16.54 (CH₃, C24a), 13.87 (CH₃, C14a), 12.70 (CH₃, C26a), 12.22 (CH₃, C28), −4.64 (CH₃, TBS); HRMS (ESI) m/z calcd for C₂₅H₄₆O₄SiNa [M+Na]⁺ 461.3058, found 461.3065.

Allylic alcohol 41

To a solution of alcohol S21 (865 mg, 2.0 mmol) in CH₂Cl₂ (20 ml) was added NaHCO₃ (1.70 g, 20 mmol), followed by DMP (2.50 g, 5.9 mmol) at 0 °C. The reaction mixture was stirred at this temperature for 30 min and then allowed to warm to room temperature. After being stirred for 2.5 h, the mixture was quenched with saturated aqueous Na₂S₂O₃. The phases were separated and the aqueous phase was extracted with CH₂Cl₂. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated to provide aldehyde S22, which was used in the next reaction.

To a solution of aldehyde S22 in toluene (99 ml) was added MS4A (2 g), followed by (E)-crotyl boronate 40 (6.6 ml, 0.9 m in toluene, 5.9 mmol) at −80 °C. The reaction mixture was stirred this temperature for 1 h and then (E)-crotyl boronate 40 (2.2 ml, 0.9 m in toluene, 2.0 mmol) was added. The reaction mixture was stirred at this temperature for 30 min and then quenched with 2 n NaOH. After being stirred for 1 h, the phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=40/1–10/1) to yield allylic alcohol 41 (839 mg, 86%, 2 steps) as a colorless oil. 41: [α]_D²³=39.6 (c 0.89, CHCl₃); IR (film) ν: 3462, 2958, 2857, 1733, 1636, 1458, 1380, 1255, 1117, 1074 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 5.74 (1H, ddd, J=17.2, 10.2, 8.3 Hz, H11), 5.68 (1H, dd, J=10.0, 1.8 Hz, H23), 5.55 (1H, dd, J=10.0, 2.5 Hz, H22), 5.43 (1H, t, J=6.7 Hz, H15), 5.15 (1H, ddd, J=17.2, 1.9, 1.0 Hz, H10), 5.13 (1H, ddd, J=10.2, 1.9, 0.6 Hz, H10), 4.11 (1H, dddd, J=11.4, 11.4, 4.5, 4.5 Hz, H19), 3.76 (1H, dddd, J=11.4, 6.5, 6.5, 1.0 Hz, H17), 3.67 (1H, d, J=8.4 Hz, H13), 3.39 (1H, dd, J=10.0, 1.8 Hz, H25), 2.38–2.27 (2H, m, H12, H16), 2.25–2.13 (2H, m, H16, H24), 1.86 (1H, ddd, J=12.8, 4.5, 1.8 Hz, H20), 1.83 (1H, dddd, J=11.4, 6.5, 4.5, 1.8 Hz, H18), 1.62 (3H, s, Me14a), 1.56 (1H, m, H26), 1.49–1.29 (2H, m, H27x2), 1.41 (1H, dd, J =12.8, 11.4 Hz, H20), 1.20 (1H, ddd, J=11.4, 11.4, 11.4 Hz, H18), 0.94 (3H, t, J=7.2 Hz, Me28), 0.90 (3H, d, J=8.6 Hz, Me24a), 0.88 (3H, d, J=8.4 Hz, Me12a), 0.88 (9H, s, TBS), 0.86 (3H, d, J=8.4 Hz, Me26a), 0.06 (3H, s, TBS), 0.05 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 141.25 (C, C14), 136.84 (CH, C11), 135.07 (CH, C23), 128.59 (CH, C22), 124.62 (CH, C15), 116.37 (CH₂, C10), 95.58 (C, C21), 81.36 (CH, C13), 75.01 (CH, C25), 68.95 (CH, C17), 65.52 (CH, C19), 44.76 (CH₂, C20), 42.15 (CH, C12), 40.81 (CH₂, C18), 35.35 (CH, C26), 34.24 (CH₂, C16), 30.55 (CH, C24), 27.84 (CH₂, C27), 25.89 (CH₃, TBS), 18.22 (C, TBS), 16.90 (CH₃, C12a), 16.52 (CH₃, C24a), 12.61 (CH₃, C26a), 12.31 (CH₃, C28), 11.22 (CH₃, C14a), −4.66 (CH₃, TBS); HRMS (ESI) m/z calcd for C₂₉H₅₂O₄SiNa [M+Na]⁺ 515.3527, found 515.3522.

The north segment 4

To a solution of alcohol 41 (0.206 g, 0.418 mmol) in DMF (4.2 ml) were added imidazole (0.171 g, 2.51 mmol) and TBSCl (0.189 g, 1.25 mmol) at room temperature. The reaction mixture was heated to 60 °C, stirred for 2 h at this temperature and then quenched with saturated aqueous NaHCO₃. The phases were separated and aqueous phase was extracted with hexane/EtOAc=2/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=100/1) to yield the north segment 4 (0.254 g, quant.) as a colorless oil. 4: [α]_D²⁵=48.0 (c 0.96, CHCl₃); IR (film) ν: 2958, 2928, 2856, 1458, 1254, 1066, 997, 873, 835, 774 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 5.84 (1H, ddd, J=17.5, 10.5, 7.4 Hz, H11), 5.70 (1H, dd, J=10.0, 1.7 Hz, H23), 5.56 (1H, dd, J=10.0, 2.6 Hz, H22), 5.26 (1H, t, J=7.1 Hz, H15), 4.95–4.99 (2H, m, H10x2), 4.10 (1H, dddd, J=11.1, 11.1, 4.6, 4.6 Hz, H19), 3.75 (1H, m, H17), 3.66 (1H, d, J=7.7 Hz, H13), 3.41 (1H, dd, J=9.7, 1.4 Hz, H25), 2.12–2.36 (4H, m, H12, H16x2, H24), 1.84–1.89 (2H, m, H18, H20), 1.55 (4H, m, H26, Me14a), 1.33–1.47 (3H, m, H20, H27x2), 1.16 (1H, ddd, J=11.7, 11.7, 11.1 Hz, H18), 0.93 (3H, t, J=7.4 Hz, Me28), 0.90 (3H, d, J=7.2 Hz, Me24a), 0.88 (9H, s, TBS), 0.87 (9H, s, TBS), 0.86 (3H, d, J=6.9 Hz, Me12a), 0.83 (3H, d, J=7.1 Hz, Me26a), 0.05 (6H, s, TBS), 0.00 (3H, s, TBS), −0.06 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 142.30 (CH, C11), 138.56 (C, C14), 135.23 (CH, C23), 128.73 (CH, C22), 122.51 (CH, C15), 113.79 (CH₂, C10), 95.73 (C, C21), 82.99 (CH, C13), 75.12 (CH, C25), 69.19 (CH, C17), 65.70 (CH, C19), 44.89 (CH₂, C20), 42.25 (CH, C12), 40.84 (CH₂, C18), 35.47 (CH, C26), 34.32 (CH₂, C16), 30.71 (CH, C24), 27.99 (CH₂, C27), 26.05 (CH₃, TBS), 26.02 (CH₃, TBS), 18.39 (C × 2, TBS), 16.83 (CH₃, C12a), 16.67 (CH₃, C24a), 12.74 (CH₃, C26a), 12.48 (CH₃, C28), 11.80 (CH₃, C14a), −4.30 (CH₃, TBS), −4.50 (CH₃, TBS), −4.54 (CH₃, TBS), −4.83 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₅H₆₆O₄Si₂Na [M+Na]⁺ 629.4392, found 629.4397.

Aldehyde 42

To a solution of 4 (227 mg, 375 μmol) in t-BuOH (5.4 ml)/THF (1.6 ml)/H₂O (0.54 ml) were added NMO (65.8 mg, 562 μmol) and OsO₄ (0.37 ml, 50 mM in H2O, 18.7 μmol) at room temperature. After being stirred for 2 h at this temperature, the reaction mixture was quenched with saturated aqueous Na₂S₂O₃. The phases were separated and aqueous phase was extracted with ethyl acetate. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=10/1) to yield diol S23 (192 mg, 88%, dr=7: 3) as a white amorphous.

To a solution of diol S23 (190 g, 299 μmol) in MeOH (9.6 ml)/H₂O (2.4 ml) were added KH₂PO₄ (1.20 g, 8.82 mmol) and NaIO₄ (192 mg, 896 μmol) at room temperature. The reaction mixture was stirred for 3 h at this temperature and then quenched with water. The phases were separated and aqueous phase was extracted with ethyl acetate. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=100/1) to yield aldehyde 42 (162 mg, 89%) as a colorless oil: [α]_D²⁴=36.6 (c 1.00, CHCl₃); IR (film) ν: 2957, 2929, 2856, 1731, 1462, 1381, 1252, 1066, 997, 836, 775 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 9.57 (1H, d, J=2.9 Hz, H11), 5.69 (1H, dd, J=10.0, 1.7 Hz, H23), 5.54 (1H, dd, J=10.0, 2.6 Hz, H22), 5.38 (1H, t, J=7.0 Hz, H15), 4.11 (1H, dddd, J=11.7, 11.1, 4.6, 4.6 Hz, H19), 4.09 (1H, d, J=8.6 Hz, H13), 3.77 (1H, dddd, J=11.7, 7.0, 7.0, 2.3 Hz, H17), 3.38 (1H, dd, J=10.3, 2.0 Hz, H25), 2.55 (1H, dqd, J=8.6, 6.0, 2.9 Hz, H12), 2.33 (1H, ddd, J=14.0, 7.0, 7.0 Hz, H16), 2.15–2.25 (2H, m, H16, H24), 1.80–1.89 (2H, m, H18eq, H20eq), 1.58 (4H, m, H26, H14a), 1.33–1.46 (3H, m, H20ax, H27x2), 1.18 (1H, ddd, J=11.7, 11.7, 11.7 Hz, H18ax), 0.93 (3H, t, J=7.1 Hz, H28), 0.89 (3H, d, J=7.4 Hz, H24a), 0.85–0.88 (24H, m, TBS × 2(0.88, 0.85), H12a, H26a), 0.052 (3H, s, TBS), 0.049 (3H, s, TBS), 0.02 (3H, s, TBS), −0.03 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ:205.54 (CH, C11), 136.63 (C, C14), 135.20 (CH, C23), 128.61 (CH, C22), 124.64 (CH, C15), 95.74 (C, C21), 80.55 (CH, C13), 75.25 (CH, C25), 68.85 (CH, C17), 65.56 (CH, C19), 50.32 (CH, C12), 44.85 (CH₂, C20), 40.94 (CH₂, C18), 35.44 (CH, C26), 34.36 (CH₂, C16), 30.66 (CH, C24), 28.03 (CH₂, C27), 26.02 (CH₃, TBS), 25.83 (CH₃, TBS), 18.36 (C × 2, TBS), 18.18 (CH₃, C12a), 16.65 (CH₃, C24a), 12.66 (CH₃, C26a), 12.48 (CH₃, C28), 11.29 (CH₃, C14a), −4.27 (CH₃, TBS), −4.53 (CH₃, TBS), −4.57 (CH₃, TBS), −5.19 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₄H₆₄O₅Si₂Na [M+Na]⁺ 631.4184, found 631.4190.

Allylic alcohol S24

To a solution of aldehyde 42 (136 mg, 222 μmol) in THF (4.5 ml) was added ethyl (triphenylphosphoranylidene)acetate (233 mg, 669 μmol) at room temperature. After being stirred for 12 h at this temperature, the reaction mixture quenched with water. The phases were separated and aqueous phase was extracted with ethyl acetate. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated.

The crude product was dissolved in ether (4.5 ml) and added DIBAL (1.8 ml, 1.0 m in toluene, 1.78 mmol) at −78 °C. The reaction mixture was stirred for 20 min at this temperature, then quenched with MeOH and saturated aqueous Rochelle salt. After being stirred for 2 h, the phases were separated and aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated in vacuo to give the crude products. The residue was purified by open column chromatography (hexane/EtOAc=15/1) to yield allylic alcohol S24 (106 mg, 75%) as a colorless oil. S24: [α]_D²⁸41.3 (c 1.22, CHCl₃); IR (film) ν: 3415, 2957, 2929, 2857, 1462, 1380, 1254, 1064, 998, 873, 835, 774 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 5.70 (1H, dd, J=10.0, 1.7 Hz, H23), 5.68 (1H, dd, J=15.1, 7.1 Hz, H11), 5.60 (1H, dt, J=15.1, 5.4 Hz, H10), 5.55 (1H, dd, J=10.0, 2.6 Hz, H22), 5.25 (1H, t, J=7.1 Hz, H15), 4.09 (3H, m, H9x2, H19), 3.75 (1H, dddd, J=12.0, 7.1, 7.1, 1.7 Hz, H17), 3.64 (1H, d, J=8.3 Hz, H13), 3.40 (1H, dd, J=10.3, 2.3 Hz, H25), 2.25–2.35 (2H, m, H12, H16), 2.23 (1H, dqdd, J=10.3, 7.1, 2.6, 1.7 Hz, H24), 2.15 (1H, ddd, J=14.2, 7.1, 7.1 Hz, H16), 1.81–1.88 (2H, m, H18, H20), 1.54 (4H, m, H26, Me14a), 1.35–1.45 (3H, m, H20, H27x2), 1.17 (1H, ddd, J=12.0, 12.0, 12.0 Hz, H18), 0.93 (3H, t, :J=7.4 Hz, Me28), 0.90 (3H, d, J=7.1 Hz, Me24a), 0.88–0.86 (21H, m, TBS × 2 (0.88, 0.86), Me12a), 0.83 (3H, d, J=6.9 Hz, Me26a), 0.053(3H, s, TBS), 0.049 (3H, s, TBS), −0.01 (3H, s, TBS), −0.06 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 138.40 (C, C14), 136.85 (CH, C11), 135.23 (CH, C23), 128.70 (CH, C22), 128.52 (CH, C10), 122.82 (CH, C15), 95.72 (C, C21), 83.10 (CH, C13), 75.12 (CH, C25), 69.10 (CH, C17), 65.70 (CH, C19), 64.21 (CH₂, C9), 44.86 (CH₂, C20), 40.98 (CH, C12), 40.78 (CH₂, C18), 35.45 (CH, C26), 34.23 (CH₂, C16), 30.68 (CH, C24), 27.99 (CH₂, C27), 26.03 (CH₃, TBS), 25.95 (CH₃, TBS), 18.37 (C, TBS),18.33 (C, TBS), 17.01 (CH₃, C12a), 16.65 (CH₃, C24a), 12.72 (CH₃, C26a), 12.46 (CH₃, C28), 11.64 (CH₃, C14a), −4.33 (CH₃, TBS), −4.54 (CH₃ × 2, TBS), −4.86 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₆H₆₈O₅Si₂Na [M+Na]⁺ 659.4497, found 659.4503.

Allyl bromide 45

To a solution of allylic alcohol S24 (88.2 mg, 138 μmol) in THF (3.5 ml) was added LiBr (180 mg, 2.08 mmol), Et₃N (77.2 μl, 554 μmol) and MsCl (21.4 μl, 277 μmol) at 0 °C. After being stirred for stirred for 5 h at room temperature, the reaction mixture was quenched with saturated aqueous NaHCO₃. The phases were separated and aqueous phase was extracted with ethyl acetate. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=50/1) to yield allyl bromide 45 (59.9 mg, 62%) as a colorless oil. 45: [α]_D²⁵=34.7 (c 0.90, CHCl₃); IR (film) ν: 2957, 2928, 2856, 2363, 1458, 1380, 1253, 1067, 997, 872, 835, 774 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 5.78 (1H, dd, J=15.4, 7.4 Hz, H11), 5.69 (1H, dd, J=10.0, 1.7 Hz, H23), 5.64 (1H, dt, J=15.4, 7.7 Hz, H10), 5.55 (1H, dd, J=10.0, 2.6 Hz, H22), 5.26 (1H, t, J=7.0 Hz, H15), 4.10 (1H, dddd, J=11.7, 11.7, 4.3, 4.3 Hz, H19), 3.96 (1H, dd, J=12.0, 7.7 Hz, H9), 3.94 (1H, dd, J=12.0, 7.7 Hz, H9), 3.75 (1H, dddd, J=11.7, 7.0, 7.0, 1.7 Hz, H17), 3.64 (1H, d, J=8.3 Hz, H13), 3.40 (1H, dd, J=9.7, 1.7 Hz, H25), 2.35–2.27 (2H, m, H12, H16), 2.23 (1H, dqdd, J=9.7, 7.1, 2.6, 1.7 Hz, H24), 2.16 (1H, ddd, J=14.0, 7.0, 7.0 Hz, H16), 1.88–1.81 (2H, m, H18, H20), 1.59–1.54 (4H, m, H26, Me14a), 1.47–1.35 (3H, m, H20, H27x2), 1.17 (1H, ddd, J=11.7, 11.7, 11.7 Hz, H18), 0.93 (3H, t, J=7.1 Hz, Me28), 0.90 (3H, d, J=7.1 Hz, Me24a), 0.88 (9H, s, TBS), 0.86 (9H, s, TBS), 0.86 (3H, d, J=6.6 Hz, Me12a), 0.83 (3H, d, J=6.9 Hz, Me26a), 0.052 (3H, s, TBS), 0.048 (3H, s, TBS), 0.00 (3H, s, TBS), −0.06 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 140.01 (CH, C11), 138.27 (C, C14), 135.22 (CH, C23), 128.71 (CH, C22), 125.70 (CH, C10), 123.02 (CH, C15), 95.73 (C, C21), 82.99 (CH, C13), 75.15 (CH, C25), 69.08 (CH, C17), 65.65 (CH, C19), 44.88 (CH₂, C20), 40.92 (CH, C12), 40.86 (CH₂, C18), 35.47 (CH, C26), 34.31 (CH₂, C16), 33.99 (CH₂, C9), 30.69 (CH, C24), 27.99 (CH₂, C27), 26.04 (CH₃, TBS), 26.00 (CH₃, TBS), 18.37 (C, TBS),18.33 (C, TBS), 16.77 (CH₃, C12a), 16.67 (CH₃, C24a), 12.72 (CH₃, C26a), 12.48 (CH₃, C28), 11.64 (CH₃, C14a), −4.32 (CH₃, TBS), −4.49 (CH₃, TBS), −4.54 (CH₃, TBS), −4.85 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₆H₆₇BrO₄Si₂Na [M+Na]⁺ 721.3653, found 721.3657.

Phosphonium salt 46c

To a solution of bromide 45 (45.8 mg, 65.4 μmol) in MeCN (1.3 ml) was added PMe₃ (13 μl, 1.0 m in toluene, 131 μmol) at room temperature. The reaction mixture was stirred for 1 h at this temperature, and then concentrated in vacuo to give the phosphonium salt 46c (50.7 mg, quant.) as a colorless solid. 46c: [α]_D²⁵29.3 (c 1.15, CHCl₃); IR (film) ν: 2928, 2958, 2856, 1462, 1253, 1071, 985, 873, 835, 774 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ:5.82 (1H, ddd, J=15.1, 7.5, 7.5 Hz, H11), 5.67 (1H, dd, J=10.0, 1.7 Hz, H23), 5.51 (1H, dd, J=10.0, 2.3 Hz, H22), 5.30–5.21 (2H, m, H10, H15), 4.08 (1H, dddd, J=12.0, 12.0, 4.6, 4.6 Hz, H19), 3.74 (1H, dddd, J=12.0, 7.1, 7.1, 2.0 Hz, H17), 3.68 (1H, d, J=7.4 Hz, H13), 3.38 (1H, ddd, J=15.1, 15.1, 7.5 Hz, H9), 3.36 (1H, brd, J=8.6 Hz, H25), 3.17 (1H, ddd, J=15.1, 15.1, 7.5 Hz, H9), 2.33–2.27 (2H, m, H12, H16), 2.23–2.06 (2H, m, H16, H24), 2.15 (9H, m, PMe₃), 1.86–1.78 (2H, m, H18, H20), 1.57–1.50 (4H, m, H26, Me14a), 1.42–1.33 (3H, m, H20, H27x2), 1.16 (1H, ddd, J=12.0, 12.0, 12.0 Hz, H18), 0.90 (3H, t, J=7.4 Hz, Me28), 0.86–0.83 (9H, m, Me12a, Me24a, Me26a), 0.85 (9H, s, TBS), 0.83 (9H, s, TBS), 0.03 (6H, s, TBS × 2), −0.03 (3H, s, TBS), −0.07 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 144.42 (CH, d, J_c–p=13.0 Hz, C11), 137.93 (C, C14), 135.21 (CH, C23), 128.62 (CH, C22), 122.78 (CH, C15), 114.03 (CH, d, J_c–p=10.7 Hz, C10), 95.69 (C, C21), 82.16 (CH, C13), 75.15 (CH, C25), 68.84 (CH, C17), 65.50 (CH, C19), 44.88 (CH₂, C20), 41.50 (CH, C12), 40.76 (CH₂, C18), 35.38 (CH, C26), 34.21 (CH₂, C16), 30.62 (CH, C24), 28.27 (CH₂, d, J_c–p=51.1 Hz, C9), 27.94 (CH₂, C27), 25.99 (CH₃ × 2, TBS), 18.29 (C × 2, TBS), 17.55 (CH₃, C12a), 16.67 (CH₃, C24a), 12.65 (CH₃, C26a), 12.45 (CH₃, C28), 12.11 (_CH3, C14a), 8.48 (CH₃x3, d, J_c–p=54.9 Hz, PMe₃), −4.29 (CH₃, TBS), −4.53 (CH₃, TBS), −4.58 (CH₃, TBS), −4.77 (CH₃, TBS); HRMS (ESI) m/z calcd for C₃₉H₇₆O₄PSi₂ [M-Br]⁺ 695.5014, found 695.5014.

Triol 47

To a solution of phosphonium salt 46c (19.1 mg, 24.6 μmol) in hexamethylphosphoric triamide (15.0 μl)/THF (0.2 ml) was slowly added n-BuLi (15.8 μl, 1.56 n in hexane, 24.6 μmol) at −78 °C. The reaction mixture was stirred for 15 min and gradually warmed to −60 °C. Later, a THF solution of ketone 6 (8.16 mg, 14.4 μmol) was added dropwise and the mixture was stirred for 3 h at −60 to −40 °C and 45 min at 0 °C. Afterwards, the mixture was quenched with saturated aqueous NaHCO₃. The phases were separated and aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was filtered through a florisil short column, and the resulting coupling product 44 was used in next reaction without further purification.

To a solution of crude product 44 in THF (0.83 ml) was added TBAF (0.33 ml, 1.0 m in THF, 0.33 mmol) at room temperature. The reaction mixture was heated to 45 °C and stirred for 4 h at this temperature. In this term, further TBAF (0.15 × 3 ml) was added every 1 h. Then the mixture was quenched with pH 7 phosphate buffer. The phases were separated and aqueous phase was extracted with ethyl acetate. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=1/2, 1% Et₃N) to yield triol 47 (6.5 mg, 56%) as a colorless solid. 47: [α]_D²⁶=19.8 (c 0.38, CH₂Cl₂); IR (film) ν: 3416, 2960, 2926, 1651, 1448, 1378, 1072, 983, 735, 702 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.47–7.45 (6H, m, Tr), 7.34–7.24 (9H, m, Tr), 6.71 (1H, dt, J=11.4, 2.3 Hz, H9), 6.04 (1H, dd, J=15.0, 11.4 Hz, H10), 5.80 (1H, dd, J=15.0, 8.9 Hz, H11), 5.72 (1H, dd, J=10.0, 1.7 Hz, H23), 5.56 (1H, dd, J=10.0, 2.3 Hz, H22), 5.45 (1H, t, J=7.1 Hz, H15), 5.03 (1H, d, J=3.4 Hz, H1), 4.59 (2H, d, J=2.3 Hz, H8ax2), 4.43 (1H, dd, J=5.1, 1.4 Hz, H3), 4.15–4.09 (2H, m, H5, H19), 4.07 (1H, d, J=5.4 Hz, H6), 3.78 (1H, m, H17), 3.75 (1H, d, J=8.6 Hz, H13), 3.41 (1H, dd, J=8.3, 1.7 Hz, H25), 3.01 (1H, brs, H2), 2.45–2.34 (2H, m, H12, H16), 2.26–2.19 (2H, m, H16, H24), 2.00–1.97 (2H, m, H18, H20), 1.68 (3H, brs, Me4a), 1.63 (3H, s, Me14a), 1.61–1.55 (1H, m, H26), 1.45–1.35 (3H, m, H20, H27x2), 1.13 (1H, ddd, J=12.0, 12.0, 12.0 Hz, H18), 0.93 (3H, t, J=7.4 Hz, Me28), 0.91–0.86 (9H, m, Me12a, Me24a, Me26a); ¹³C NMR (100 MHz, CDCl₃) δ: 144.33 (C, Tr), 141.74 (C, C8), 139.53 (CH, C11), 137.19 (C, C14), 136.67 (C, C4), 135.65 (CH, C23), 128.63 (CH, Tr), 128.50 (CH, C22), 128.19 (CH, Tr), 128.14 (CH, C10), 127. 45 (CH, Tr), 124.65 (CH, C15), 123.34 (CH, C9), 118.87 (CH, C3), 102.09 (CH, C1), 95.75 (C, C21), 88.15 (CH, C13), 81.64 (C, C7), 81.42 (CH, C6), 75.26 (CH, C25), 69.13 (CH₂, C8a), 68.84 (CH × 2, C5, C17), 64.97 (CH, C19), 50.61 (CH, C2), 44.39 (CH₂, C20), 41.65 (CH, C12), 40.25 (CH₂, C18), 35.40 (CH, C26), 34.34 (CH₂, C16), 30.70 (CH, C24), 27.83 (CH₂, C27), 20.26 (CH₃, C4a), 17.61 (CH₃, C12a), 16.62 (CH₃, C24a), 12.85 (CH₃, C26a), 12.42 (CH₃, C28), 11.45 (CH₃, C14a); HRMS (ESI) m/z calcd for C₅₃H₆₄O₈Na [M+Na]⁺ 851.4493, found 851.4499.

Avermectin aglycon 49

To a degassed solution of 47 (6.5 mg, 7.84 μmol) in 2-methyl-2-butene (0.35 ml) and t-BuOH (0.35 ml) was added HCOOH (35.5 μl, 0.941 mmol) under argon at room temperature. After being stirred for 45 min, the reaction mixture was slowly added to a degassed aqueous solution of NaClO₂ (28.3 mg, 0.313 mmol) and NaH₂PO₄ · 2H₂O (48.8 mg, 0.313 mmol) at room temperature. After being stirred for 1 h, the mixture was diluted with brine and extracted with Et₂O and dried over Na₂SO₄. Toluene (1 ml) was added and the mixture was concentrated in vacuo. After this procedure was repeated again, the volatiles were removed completely in vacuo to give yellow residue. To a solution of 2-methyl-6-nitrobenzoic anhydride (8.1 mg, 23.5 μmol) and DMAP (5.7 mg, 47.0 μmol) in CH₂Cl₂ (5.2 ml) at room temperature was added a solution of crude seco acid 2 in dry CH₂Cl₂ (2.6 ml) over 15 min. After being stirred for 1 h, the mixture was cooled to 0 °C and then quenched with saturated aqueous NaHCO₃. The phases were separated and aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=2/1) to yield avermectin aglycon 49 (2.5 mg, 54%) as a colorless solid: [α]_D²⁸61.9 (c 0.20, CHCl₃), (lit. [α]_D=65.5 (c 0.595, CHCl₃)³⁸); IR (film) ν: 3466, 2961, 2929, 1718, 1458, 1376, 1159, 1117, 1035, 994 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 5.83 (1H, dt, J=11.1, 2.3 Hz, H9), 5.80–5.68 (3H, m, H10, H11, H23), 5.54 (1H, dd, J=10.0, 2.9 Hz, H22), 5.42 (1H, brs, H3), 5.40–5.31 (2H, m, H15, H19), 4.71 (1H, dd, J=14.3, 2.3 Hz, H8a), 4.66 (1H, dd, J=14.3, 2.3 Hz, H8a), 4.29 (1H, dd, J=6.3, 6.3 Hz, H5), 4.01 (1H, brs, H13), 3.98 (1H, s, OH), 3.97 (1H, d, J=6.3 Hz, H6), 3.87 (1H, m, H17), 3.46 (1H, dd, J=10.0, 1.7 Hz, H25), 3.27 (1H, q, J=2.3 Hz, H2), 2.53 (1H, m, H12), 2.34–2.24 (4H, m, H16x2, H24, OH), 2.01 (1H, ddd, J=12.0, 4.8, 2.0 Hz, H20), 1.87 (3H, t, J=2.3 Hz, Me4a), 1.77 (1H, dddd, J=12.0, 4.6, 2.0, 2.0 Hz, H18), 1.61–1.57 (1H, m, H26), 1.52 (3H, s, Me14a), 1.51–1.44 (3H, m, H20, H27x2), 1.18 (3H, d, J=6.9 Hz, Me12a), 0.96 (3H, t, J=7.4 Hz, Me28), 0.92 (6H, m, Me24a, Me26a), 0.87 (1H, ddd, J=12.0, 12.0, 12.0 Hz, H18); ¹³C NMR (175 MHz, CDCl₃) δ: 173.64 (C, C1), 139.95 (C, C8), 138.82 (C, C14), 137.97 (C, C4), 137.16 (CH, C11), 136.40 (CH, C23), 127.96 (CH, C22), 124.95 (CH, C10), 120.53 (CH, C9), 118.27 (CH, C3), 117.28 (CH, C15), 95.87 (C, C21), 80.41 (C, C7) 79.28 (CH, C6), 77.77 (CH, C13), 75.37 (CH, C25), 68.63 (C17 or C8a or C19), 68.51 (C17 or C8a or C19), 68.42 (C17 or C8a or C19), 67.87 (CH, C5), 45.84 (CH, C2), 40.77 (CH₂, C20), 40.25 (CH, C12), 36.66 (CH₂, C18), 35.40 (CH, C26), 34.56 (CH₂, C16), 30.72 (CH, C24), 27.76 (CH₂, C27), 20.10 (CH₃, C4a), 19.30 (CH₃, C12a), 16.55 (CH₃, C24a), 14.75 (CH₃, C14a), 12.97 (CH₃, C26a), 12.30 (CH₃, C28); HRMS (ESI) m/z calcd for C₃₄H₄₈O₈Na [M+Na]⁺ 607.3241, found 607.3245.

Bis-acetate 51

To a suspension of l-rhamnose monohydrate 50 (10.0 g, 54.9 mmol) in acetic anhydride (31 ml, 329 mmol) was added HBr (1 ml, 33% in AcOH, 5.5 mmol) at 0 °C. After being stirred for 1 h at room temperature, the additional HBr (51 ml, 33% in AcOH, 281mmol) was added and the reaction mixture was stirred 2 h. Anhydrous sodium acetate (25 g) was then added to neutralized the excess HBr, and the reaction mixture was poured into a suspension of pulverized CuSO₄·5H₂O (2.5 g) and zinc powder (100 g) in a solution of water (100 ml) and acetic acid (50 ml) containing sodium acetate (75 g) at 0 °C. The reaction mixture was stirred vigorously at room temperature for 2 h. The mixture was filtered and the collected solid was washed with ethyl acetate and water. The filtered phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with water, saturated aqueous NaHCO₃ and brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=7/1–5/1) to yield rhamnal 51 (9.04 g, 77%) as a colorless oil. 51: [α]_D²³=52.4 (c 1.10, CHCl₃); IR (film) ν: 2987, 1738, 1648, 1375, 1225, 1051 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 6.43 (1H, dd, J=6.1, 1.4 Hz, H1′), 5.33 (1H, dddd, J=6.1, 2.9, 1.4, 0.6 Hz, H3′), 5.02 (1H, dd, J=8.2, 6.1 Hz, H4′), 4.77 (1H, dd, J=6.1, 2.9 Hz, H2′), 4.10 (1H, dq, J=8.2, 6.5 Hz, H5′), 2.08 (3H, s, Ac), 2.04 (3H, s, Ac), 1.31 (3H, d, J=6.5 Hz, H6′); ¹³C NMR (100 MHz, CDCl₃) δ: 170.69 (C, Ac), 169.94 (C, Ac), 145.96 (CH, C1′), 98.73 (CH, C2′), 72.48 (CH, C5′), 71.79 (CH, C4′), 68.28 (CH, C3′), 21.07 (CH₃, Ac), 20.89 (CH₃, Ac), 16.52 (CH₃, C6′); HRMS (ESI) m/z calcd for C₁₀H₁₄O₅Na [M+Na]⁺ 237.0733, found 237.0733.

Rhamnal 52

To a solution of bis-acetate 51 (9.04 g, 42 mmol) in MeOH (48 ml) was added K₂CO₃ (2.9 g, 21 mmol) at room temperature. The reaction mixture was stirred overnight and then quenched with EtOAc and H₂O. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was dried over Na₂SO₄ and concentrated. The residue was purified by recrystallization (hexane) to yield diol 52 (4.79 g, 87%) as a white solid. 52: m.p.=69 °C; [α]_D²³=23.8 (c 1.13, CHCl₃); IR (film) ν: 3270, 2931, 1645, 1448, 1227, 1149, 1046 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 6.31 (1H, dd, J=6.0, 1.6 Hz, H1′), 4.70 (1H, dd, J=6.0, 2.2 Hz, H2′), 4.21 (1H, m, H3′), 3.85 (1H, dq, J=9.6, 6.5 Hz, H5′), 3.51–3.37 (2H, m, OH, H4′), 2.94 (1H, m, OH), 1.38 (3H, d, J=6.5 Hz, Me6′); ¹³C NMR (100 MHz, CDCl₃) δ: 144.82 (CH, C1′), 102.66 (CH, C2′), 75.34 (CH, C4′), 74.46 (CH, C5′), 70.32 (CH, C3′), 17.17 (CH₃, C6′); HRMS (ESI) m/z calcd for C₆H₁₀O₃Na [M+Na]⁺ 153.0522, found 153.0522.

TBS ether S26

To a solution of rhamnal 52 (4.79 g, 37 mmol) in CH₂Cl₂ (60 ml) was added pyridine (8.9 ml, 110 mmol), followed by BzCl (4.3 ml, 37 mmol) at 0 °C. The reaction mixture was stirred for 1 h and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated to give benzoate S25, which was used directly in the next reaction.

To a solution of crude benzoate S25 in DMF (37 ml) was added imidazole (5.0 g, 74 mmol), followed by TBSCl (8.3 g, 55 mmol) at 40 °C. The reaction mixture was stirred at this temperature overnight, then quenched with EtOAc and saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=3/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=50/1) to yield TBS ether S26 (9.34 g, 73%, 2 steps) as a colorless oil. S26: [α]_D²⁴=166.2 (c 0.99, CHCl₃); IR (film) ν: 2930, 2857, 1719, 1648, 1451, 1272, 1105 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 8.07–8.03 (2H, m, Bz), 7.57 (1H, m, Bz), 7.48–7.42 (2H, m, Bz), 6.42 (1H, dd, J=6.0, 1.4 Hz, H1′), 5.50 (1H, ddd, J=6.5, 2.5, 1.4 Hz, H3′), 4.80 (1H, dd, J=6.0, 2.5 Hz, H2′), 3.96 (1H, dq, J=9.0, 6.3 Hz, H5′), 3.88 (1H, dd, J=9.0, 6.5 Hz, H4′), 1.40 (3H, d, J=6.3 Hz, Me6′), 0.82 (9H, s, TBS), 0.12 (3H, s, TBS), −0.02 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 166.30 (C, Bz), 146.07 (CH, C1′), 133.00 (C, Bz), 130.26 (C, Bz), 129.66 (CH, Bz), 128.34 (CH, Bz), 99.59 (CH, C2′), 75.51 (CH, C5′), 73.76 (CH, C3′), 72.43 (CH, C4′), 25.69 (CH₃, TBS), 18.00 (C, TBS), 17.76 (CH₃, C6′), −4.15 (CH₃, TBS), −4.56 (CH₃, TBS); HRMS (ESI) m/z calcd for C₁₉H₂₈O₄SiNa [M+Na]⁺ 371.1649, found 371.1651.

Methyl ether 53

To a solution of S26 (1.00 g, 2.9 mmol) in THF (30 ml) was added MeLi (9.0 ml, 1.06 m in Et₂O, 8.5 mmol) at −80 °C. The reaction mixture was stirred for 30 min and then MeOTf (830 μl, 7.3 mmol) was added at −90 °C. The reaction mixture gradually warmed to −55 °C, stirred 1 h and then added with iPr₂NH (3 ml). The reaction mixture was gradually warmed to room temperature, stirred overnight and quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=10/1. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=50/1) to yield methyl ether 53 (636 mg, 85%) as a colorless oil. 53: [α]_D²³=22.7 (c 1.06, CHCl₃); IR (film) ν: 2930, 2888, 2857, 1646, 1472, 1254, 1125 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 6.34 (1H, dd, J=6.1, 1.4 Hz, H1′), 4.83 (1H, dd, J=6.1, 2.2 Hz, H2′), 3.79 (1H, dq, J=9.4, 6.3 Hz, H5′), 3.73 (1H, ddd, J=6.9, 2.2, 1.4 Hz, H3′), 3.48 (1H, dd, J=9.4, 6.9 Hz, H4′), 3.32 (3H, s, OMe), 1.32 (3H, d, J=6.3 Hz, Me6′), 0.90 (9H, s, TBS), 0.12 (3H, s, TBS), 0.10 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 144.85 (CH, C1′), 99.33 (CH, C2′), 79.49 (CH, C3′), 75.38 (CH, C5′), 73.66 (CH, C4′), 55.54 (CH₃, Me), 25.91 (CH₃, TBS), 18.20 (C, TBS), 17.85 (CH₃, C6′), −4.14 (CH₃, TBS), −4.89 (CH₃, TBS); HRMS (ESI) m/z calcd for C₁₃H₂₆O₃SiNa [M+Na]⁺ 281.1543, found 281.1543.

Hemiacetal 54

To a solution of methyl ether 53 (3.6 g, 12 mmol) in MeCN-H₂O (95:5, 95 ml) was added N-iodosuccinimide (2.9 g, 13 mmol) at 0 °C. The reaction mixture was allowed to warm to room temperature and stirred for 25 min. The solvent was then removed in vacuo. The residue was dissolved in DMF (60 ml) and H₂O (119 ml). To the solution were added NaHCO₃ (10.0 g, 119 mmol) and Na₂S₂O₄ (8.3 g, 48 mmol) at room temperature. The reaction mixture was stirred for 4 h and then quenched with EtOAc and H₂O. The phases were separated and the aqueous phase was extracted with hexane/EtOAc=1/1. The combined organic layer was washed with H₂O and brine, dried over Na₂SO₄ and concentrated. The residue was purified by open column chromatography (hexane/EtOAc=7/1) to yield hemiacetal 54 (3.01 g, 92%, α/β=7/3) as a colorless oil. 54: IR (film) ν: 3400, 2930, 1463, 1252, 1109 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 5.34 (7/10H, m, H1′α), 4.80 (3/10H, ddd, J=9.8, 6.7, 2.0 Hz, H1′β), 3.87 (7/10H, dq, J=8.8, 6.3 Hz, H5′α), 3.45 (7/10H, ddd, J=11.4, 8.8, 4.9 Hz, H3′α), 3.32 (3H, s, OMe), 3.31 (3/10H, m, H5′β), 3.20–3.12 (1H, m, H4′), 3.12 (3/10H, m, H3′β), 2.59 (7/10H, d, J=2.2 Hz, OHα), 2.43 (3/10H, ddd, J=12.2, 4.5, 2.0 Hz, H2′β), 2.30 (7/10H, ddd, J=13.1, 4.9, 1.2 Hz, H2′α), 1.47 (7/10H, dddd, J=13.1, 11.4, 3.5, 2.2 Hz, H2′α), 1.34 (3/10H, m, H2′β), 1.28 (9/10H, d, J=6.3 Hz, Me6′β), 1.23 (21/10H, d, J=6.3 Hz, Me6′α), 0.89 (63/10H, s, TBSα), 0.88 (27/10H, s, TBSβ), 0.09 (21/10H, s, TBSα), 0.09 (9/10H, s, TBSβ), 0.07 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 93.97 (CH, C1′β), 92.05 (CH, C1′α), 80.77 (CH, C3′β), 78.09 (CH, C3′α), 77.19 (CH, C4′α), 76.37 (CH, C4′β), 72.88 (CH, C5′β), 68.61 (CH, C5′α), 56.35 (CH₃, OMeα), 56.18 (CH₃, OMeβ), 37.06 (CH₂, C2′β), 34.35 (CH₂, C2′α), 25.99 (CH₃, TBS), 18.53 (CH₃, C6′α), 18.49 (CH₃, C6′β), 18.29 (C, TBSα), 18.27 (C, TBSβ), −3.98 (CH₃, TBS), −4.79 (CH₃, TBS); HRMS (ESI) m/z calcd for C₁₃H₂₈O₄SiNa [M+Na]⁺ 299.1649, found 299.1651.

Alcohol 55

To a solution of hemiacetal 54 (1.5 g, 5.4 mmol) in CH₂Cl₂ (54 ml) was added PhSH (463 μl, 5.4 mmol), followed by BF₃·Et₂O (0.48 ml, 2.7 mmol) at 0 °C. The reaction mixture was stirred for 10 min at this temperature and then additional BF₃·Et₂O (0.48 ml, 2.7 mmol) was added. The reaction mixture was stirred for 5 min and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated to provide thioglycoside S27, which was used directly in the next reaction.

To a solution of crude thioglycoside S27 in THF (108 ml) was added TBAF (10.8 ml, 1.0 m in THF, 10.8 mmol) at 40 °C. The reaction mixture was stirred for overnight and then quenched with saturated aqueous NH₄Cl. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=α: 7/1–6/1; β: 6/1–5/1) to yield alcohol 55 (α: 418 mg; β: 362 mg, total 56%, 2 steps) as yellow oil. 55α: [α]_D²³=275.2 (c 1.14, CHCl₃); IR (film) ν: 3447, 2931, 1582, 1478, 1438, 1082 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.48–7.44 (2H, m, SPh), 7.33–7.28 (2H, m, SPh), 7.25 (1H, m, SPh), 5.62 (1H, dd, J=5.7, 1.0 Hz, H1′), 4.19 (1H, dq, J=9.0, 6.3 Hz, H5′), 3.52 (1H, ddd, J=11.7, 9.0, 4.7 Hz, H3′), 3.43 (3H, s, OMe), 3.20 (1H, dd, J=9.0, 9.0 Hz, H4′), 2.56 (1H, s, OH), 2.49 (1H, ddd, J=13.3, 4.7, 1.0 Hz, H2′), 1.93 (1H, ddd, J=13.3, 11.7, 5.7 Hz, H2′), 1.31 (3H, d, J=6.3 Hz, Me6′); ¹³C NMR (100 MHz, CDCl₃) δ: 135.11 (C, SPh), 131.14 (CH, SPh), 128,93 (CH, SPh), 127.09 (CH, SPh), 83.95 (CH, C1′), 78.86 (CH, C3′), 76.38 (CH, C4′), 68.50 (CH, C5′), 56.61 (CH₃, OMe), 34.84 (CH₂, C2′), 17.69 (CH₃, C6′); HRMS (ESI) calcd for C₁₃H₁₈O₃SNa (M+Na)⁺ 277.0869, found 277.0874. 55β: [α]_D²³=578.0 (c 1.08, CHCl₃); IR (film) ν: 3436, 2932, 1584, 1479, 1439, 1292, 1078 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.51–7.48 (2H, m, SPh), 7.33–7.27 (3H, m, SPh), 4.78 (1H, dd, J=11.7, 2.0 Hz, H1′), 3.40 (3H, s, OMe), 3.38 (1H, m, H5′), 3.27–3,14 (2H, m, H3′, H4′), 2.47 (1H, ddd, J=12.5, 4.5, 2.0 Hz, H2′), 1.63 (1H, ddd, J=12.5, 11.7, 10.8 Hz, H2′), 1.37 (3H, d, J=6.3 Hz, Me6′); ¹³C NMR (100 MHz, CDCl₃) δ: 133.40 (C, SPh), 131.34 (CH, SPh), 128.84 (CH, SPh), 127.38 (CH, Ph), 81.96 (CH, C3′), 81.92 (CH, C1′), 75.79 (CH, C5′), 75.28 (CH, C4′), 56.48 (CH₃, OMe), 35.33 (CH₂, C2′), 18.16 (CH₃, C6′); HRMS (ESI) m/z calcd for C₁₃H₁₈O₃SNa [M+Na]⁺ 277.0869, found 277.0871.

Fluoroglycoside 56

To a solution of hemiacetal 54 (750 mg, 2.7 mmol) in THF (27 ml) was added DAST (538 μl, 4.1 mmol) at −40 °C. The reaction mixture was stirred at this temperature for 1 h and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with CH₂Cl₂. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by NH column chromatography (hexane/EtOAc=50/1) to yield fluoride 56, which was used in the next reaction without further purification.

Disaccharide 3

To a solution of fluoride 56 (10.5 mg, 0.038 mmol) in Et₂O (0.38 ml) and alcohol 55α (9.6 mg, 0.038 mmol) was added MS4A (3.8 mg), followed by SnCl₂ (14 mg, 0.076 mmol) and AgOTf (19 mg, 0.076 mmol) at −30 °C. The reaction mixture was allowed to warm to 0 °C and stirred for 3 h. The mixture was warmed to room temperature, stirred for 2.5 h and then quenched with saturated aqueous NaHCO₃. The phases were separated and the aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=30/1) to yield disaccharide 3 (8.7 mg, 46%) as a colorless oil and its β-glycoside (1.7 mg, 9%). 3: [α]_D¹⁹−258.1 (c 0.29, CHCl₃); IR (film) ν: 3436, 2932, 1584, 1479, 1439, 1292, 1078 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 7.47–7.43 (2H, m, SPh), 7.32–7.22 (3H, m, SPh), 5.59 (1H, dd, J=5.7, 1.6 Hz, H1′), 5.30 (1H, dd, J=4.1, 1.4 Hz, H1″), 4.16 (1H, dq, J=9.0, 6.3 Hz, H5′), 3.70 (1H, dq, J=8.8, 6.3 Hz, H5″), 3.58 (1H, ddd, J=11.7, 9.0, 4.9 Hz, H3′), 3.39 (3H, s, OMe), 3.34 (1H, m, H3″), 3.34 (3H, s, OMe), 3.25 (1H, dd, J=9.0, 9.0 Hz, H4′), 3.14 (1H, dd, J=8.8, 8.8 Hz, H4″), 2.45 (1H, ddd, J=13.5, 4.9, 1.6 Hz, H2′), 2.31 (1H, ddd, J=13.1, 4.9, 1.4 Hz, H2″), 1.95 (1H, ddd, J=13.5, 11.7, 5.7 Hz, H2′), 1.50 (1H, ddd, J=13.1, 11.3, 4.1 Hz, H2″), 1.30 (3H, d, J=6.3 Hz, Me6′), 1.22 (3H, d, J=6.3 Hz, Me6″), 0.90 (9H, s, TBS), 0.10 (3H, s, TBS), 0.08 (3H, s, TBS); ¹³C NMR (100 MHz, CDCl₃) δ: 135.12 (C, SPh), 131.18 (CH, SPh), 128.92 (CH, SPh), 127.08 (CH, SPh), 98.71 (CH, C1″), 83.70 (CH, C1′), 81.12 (CH, C4′), 79.78 (CH, C3′), 78.56 (CH, C3″), 77.07 (CH, C4″), 69.13 (CH, C5″), 67.82 (CH, C5′), 56.64 (CH₃, OMe), 56.35 (CH₃, OMe), 35.26 (CH₂, C2′), 34.69 (CH₂, C2″), 26.02 (CH₃, TBS), 18.35 (CH₃, C6′), 18.31 (C, TBS), 18.29 (CH₃, C6″), −3.98 (CH₃, TBS), −4.74 (CH₃, TBS); HRMS (ESI) m/z calcd for C₂₆H₄₄O₆SSiNa [M+Na]⁺ 535.2520, found 535.2526.

Fluoride 57

To a solution of thio glycoside 3 (40.0 mg, 78.0 μmol) in CH₂Cl₂ (1.56 ml) was added DAST (15.5 μl, 117 μmol) and NBS (18.0 mg, 101 μmol) at −30 °C. The reaction mixture was stirred for 25 min at this temperature, and then poured into saturated aqueous NaHCO₃. The phases were separated and aqueous phase was extracted with Et₂O. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by NH column chromatography (hexane/EtOAc=100/1) to yield fluoride 57 (~28 mg, ca. 85%), which was used in the next reaction without further purification.

TBS ether 58

To a solution of alcohol 49 (3.3 mg, 5.64 μmol) in DMF (0.5 ml) were added imidazole (4.6 mg, 67.8 μmol) and TBSCl (5.2 mg, 33.8 μmol) at room temperature. After being stirred for 3.5 h, the reaction mixture was quenched with saturated aqueous NaHCO₃. The phases were separated and aqueous phase was extracted with EtOAc. The combined organic layer was washed with brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=5/1) to yield TBS ether 58 (3.1 mg, 78%) as a colorless solid: [α]_D²⁶55.3 (c 0.15, CHCl₃); IR (film) ν: 3462, 2958, 2928, 2856, 1715, 1456, 1376, 1250, 1160, 1123, 1081, 995, 866, 836 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ: 5.81–5.65 (4H, m, H9, H10, H11, H23), 5.54 (1H, dd, J=9.7, 2.6 Hz, H22), 5.36–5.28 (3H, m, H3, H15, H19), 4.68 (1H, d, J=14.0 Hz, H8a), 4.58 (1H, d, J=14.0 Hz, H8a), 4.43 (1H, m, H5), 4.01 (2H, m, H13, OH), 3.86 (1H, m, H17), 3.82 (1H, d, J=5.7 Hz, H6),, 3.46 (1H, dd, J=10.0, 1.1 Hz, H25), 3.36 (1H, q, J=2.3 Hz, H2), 2.52 (1H, m, H12), 2.37–2.24 (3H, m, H16x2, H24), 2.03 (1H, ddd, J=11.9, 4.9, 2.1 Hz, H20), 1.79 (3H, d, J=2.3 Hz, Me4a), 1.76 (1H, m, H18), 1.65–1.43 (7H, m, Me14a, H20, H26, H27x2), 1.17 (3H, d, J=7.1 Hz, Me12a), 0.93 (9H, s, TBS), 0.98–0.85 (10H, m, H18, Me24a, Me26a, Me28), 0.13 (6H, s, TBS); ¹³C NMR (175 MHz, CDCl₃) δ: 173.90 (C, C1), 140.52 (C, C8), 138.90 (C, C14), 137.56 (C, C4), 136.63 (CH, C11), 136.33 (CH, C23), 128.00 (CH, C22), 125.03 (CH, C10), 119.47 (CH, C9), 117.49 (CH, C3), 117.25 (CH, C15), 95.88 (C, C21), 80.30 (CH, C6; C, C7), 77.81 (CH, C13), 75.35 (CH, C25), 69.60 (CH, C5), 68.53 (CH, C17; CH, C19), 68.10 (CH₂, C8a), 45.91 (CH, C2), 40.77 (CH₂, C20), 40.14 (CH, C12), 36.59 (CH₂, C18), 35.41 (CH, C26), 34.60 (CH₂, C16), 30.71 (CH, C24), 27.75 (CH₂, C27), 26.03 (CH₃, TBS), 20.18 (CH₃, C4a), 19.39 (CH₃, C12a), 18.59 (C, TBS), 16.56 (CH₃, C24a), 14.79 (CH₃, C14a), 12.96 (CH₃, C26a), 12.30 (CH₃, C28), −4.50 (CH₃, TMS), −4.83 (CH₃, TMS); HRMS (ESI) m/z calcd for C₄₀H₆₂O₈SiNa [M+Na]⁺ 721.4106, found 721.4111.

Glycoside 59α

To a mixture of alcohol 58 (5.7 mg, 8.11 μmol), MS4A (30 mg) in MeCN (0.3 ml) were added SnCl₂ (3.4 mg, 17.8 μmol) and AgOTf (4.6 mg, 17.8 μmol) at room temperature and stirred 5 min. After the mixture was cooled to −40 °C, MeCN solution of DTBMP (12.0 mg, 48.6 μmol) and fluoride 57 (6.9 mg, 16.2 μmol) was added at this temperature. The reaction mixture was allowed to warm up to 0 °C over 1 h and then stirred for 30 min at room temperature. Later, the precipitates were filtered and the filtrate was extracted with ether. The combined organic layer was washed with saturated aqueous NaHCO₃ and brine, dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (hexane/EtOAc=10/1) to yield 59 as a 2.3:1 mixture of α and β isomers. The mixture was purified by HPLC (YMC-Pack SIL-06, hexane/EtOAc=9/1, 1.0 ml min⁻¹, R_t=15 min (α isomer), 16.3 min (β isomer)) to yield 59α (4.0 mg, 45%) as a colorless solid. 59α: [α]_D²⁶39.3 (c 0.33, CHCl₃); IR (film) ν: 2959, 2930, 2856, 1717, 1459, 1386, 1250, 1159, 1123, 1102, 987, 870, 836, 776 cm⁻¹; ¹H NMR (700 MHz, CDCl₃) δ: 5.82 (1H, m, H9), 5.76 (1H, dd, J=9.9, 1.6 Hz, H23), 5.51–5.73 (2H, m, H10, H11), 5.55 (1H, dd, J=9.9, 2.7 Hz, H22), 5.37 (1H, dddd, J=11.5, 11.5, 4.8, 4.8 Hz, H19), 5.32 (2H, m, H3, H1″), 4.98 (1H, m, H15), 4.76 (1H, brd, J=3.2 Hz, H1′), 4.68 (1H, dd, J=14.4, 2.4 Hz, H8a), 4.59 (1H, d, J=14.4, 2.4 Hz, H8a), 4.43 (1H, m, H5), 4.12 (1H, s, OH), 3.93 (1H, brs, H13), 3.88–3.82 (3H, m, H6, H17, H5′), 3.69 (1H, dq, J=8.8, 6.4 Hz, H5″), 3.61 (1H, ddd, J=11.2, 8.8, 4.8 Hz, H3′), 3.48 (1H, dd, J=9.9, 1.6 Hz, H25), 3.44 (3H, s, OMe), 3.39 (1H, q, J=2.1 Hz, H2), 3.35 (1H, m, H3″), 3.34 (3H, s, OMe), 3.21 (1H, dd, J=8.8, 8.8 Hz, H4′), 3.14 (1H, dd, J=8.8, 8.8 Hz, H4″), 2.51 (1H, m, H12), 2.33 (1H, ddd, J=13.1, 4.8, 1.1 Hz, H2″), 2.30–2.25 (3H, m, H16x2, H24), 2.21 (1H, ddd, J=12.8, 4.8, 1.0 Hz, H2′), 2.02 (1H, ddd, J=12.0, 4.8, 1.9 Hz, H20), 1.79 (3H, s, Me4a), 1.76 (1H, m, H18), 1.62–1.45 (6H, m, H20, H26, H27x2, H2′, H2″), 1.50 (3H, s, Me14a), 1.26 (3H, d, J=6.1 Hz, Me6′), 1.22 (3H, d, J=6.4 Hz, Me6″), 1.16 (3H, d, J=7.0 Hz, Me12a), 0.93 (9H, s, TBS), 0.89 (9H, s, TBS), 0.94–0.88 (10H, m, H18, Me24a, Me26a, Mr28), 0.13 (6H, s, TBS), 0.09 (3H, s, TBS), 0.08 (3H, s, TBS); ¹³C NMR (175 MHz, CDCl₃) δ: 174.27, 140.38, 137.75, 137.73, 136.39, 135.37, 127.94, 124.96, 119.46, 118.42, 117.35, 98.77, 95.92, 95.13, 82.15, 80.98, 80.40, 80.19, 79.43, 78.71, 77.29, 75.01, 69.65, 69.20, 68.60, 68.50, 68.10, 67.47, 56.79, 56.51, 45.92, 40.60, 39.83, 36.72, 35.32, 34.81, 34.70, 34.45, 30.72, 27.67, 26.18, 26.04, 20.46, 20.21, 18.59, 18.53, 18.49, 18.45, 16.53, 15.30, 13.13, 12.20, −3.81, −4.43, −4.59, −4.71; HRMS (ESI) m/z calcd for C₆₀H₁₀₀O₁₄Si₂Na [M+Na]⁺ 1123.6544, found 1123.6550.

Avermectin B_1a (1)

To a solution of 59α (3.0 mg, 2.72 μmol) in MeCN (0.15 ml)/pyridine (0.15 ml) was added pyridinium hydrofluoride (0.11 ml) at room temperature. After being stirred for 2 h, an additional pyridinium hydrofluoride (0.11 ml) was added to the reaction mixture. After being stirred for 12 h, an additional pyridinium hydrofluoride (0.05 ml) was introduced. After being stirred for further 1 h, the mixture was diluted with Et₂O and quenched with saturated aqueous NaHCO₃. The mixture was stirred for 30 min and the phases were separated. The aqueous phase was extracted with ether and the combined organic layer was washed with 5% HCl, saturated aqueous NaHCO₃, water and brine. The resultant organic layer was dried over Na₂SO₄ and concentrated in vacuo to give the crude products. The residue was purified by flash column chromatography (hexane/EtOAc=1/2) to yield 1 (2.1 mg, 88%) as a colorless solid. 1: [α]_D²⁴=63.3 (c 0.22, CHCl₃), (lit. [α]_D²⁷55.7 (c 1.06, CHCl₃);² lit. [α]_D²⁰=63.7 (c 0.98, CHCl₃);⁸ lit. [α]_D²³=62.2 (c 0.78, CHCl₃)⁹); IR (film) ν: 3467, 2966, 2931, 1718, 1457, 1376, 1340, 1159, 1117, 1050, 985, 732 cm⁻¹; ¹H NMR (700 MHz, CDCl₃) δ: 5.86 (1H, ddd, J=10.4, 2.4, 2.4 Hz, H9), 5.77 (1H, dd, J=9.9, 1.6 Hz, H23), 5.75–5.72 (2H, m, H10, H11), 5.55 (1H, dd, J=9.9, 2.7 Hz, H22), 5.42 (1H, brs, H3), 5.40 (1H, d, J=4.0 Hz, H1″), 5.39 (1H, m, H19), 4.98 (1H, m, H15), 4.77 (1H, brd, J=3.2 Hz, H1′), 4.70 (1H, dd, J=14.2, 2.4 Hz, H8a), 4.67 (1H, d, J=14.2, 2.4 Hz, H8a), 4.30 (1H, dd, J=6.5, 6.5 Hz, H5), 4.04 (1H, s, OH), 3.97 (1H, d, J=6.5 Hz, H6), 3.94 (1H, brs, H13), 3.87 (1H, m, H17), 3.84 (1H, dq, J=9.1, 6.1 Hz, H5′), 3.77 (1H, dq, J=9.1, 6.1 Hz, H5″), 3.63 (1H, ddd, J=11.2, 9.1, 4.8 Hz, H3′), 3.50–3.47 (2H, m, H25, H3″), 3.44 (3H, s, OMe), 3.43 (3H, s, OMe), 3.30 (1H, dd, J=4.5, 2.1 Hz, H2), 3.25 (1H, dd, J=9.1, 9.1 Hz, H4′), 3.17 (1H, dd, J=9.1, 9.1 Hz, H4″), 2.52 (1H, m, H12), 2.35–2.25 (4H, m, H16x2, H24, H2″), 2.22 (1H, ddd, J=12.8, 4.8, 1.3 Hz, H2′), 2.01 (1H, ddd, J=12.0, 4.8, 2.0 Hz, H20), 1.88 (3H, brs, Me4a), 1.78 (1H, dddd, J=12.3, 4.8, 2.0, 2.0 Hz, H18), 1.44–1.62 (6H, m, H20, H26, H27x2, H2′, H2″), 1.49 (3H, s, Me14a), 1.28 (3H, d, J=6.1 Hz, Me6′), 1.26 (3H, d, J=6.1 Hz, Me6″), 1.17 (3H, d, J=7.2 Hz, Me12a), 0.94 (3H, t, J=7.5 Hz, Me28), 0.92 (3H, d, J=6.7 Hz, Me24a), 0.92 (3H, d, J=7.5 Hz, Me26a), 0.88 (1H, ddd, J=12.3, 12.3, 12.3 Hz, H18); ¹³C NMR (175 MHz, CDCl₃) δ: 173.98 (C, C1), 139.79 (C, C8), 138.24 (CH, C11), 138.19 (C, C4), 136.46 (CH, C22), 135.31 (C, C14), 127.90 (CH, C23), 124.89 (CH, C10), 120.56 (CH, C9), 118.43 (CH, C3), 118.17 (CH, C15), 98.67 (CH, C1″), 95.92 (CH₂, C21), 95.09 (CH, C1′), 82.06 (CH, C13), 80.54 × 2 (C, C7, CH, C4′), 79.52 (CH, C3′), 79.20 (CH, C6), 78.31 (CH, C3″), 76.27 (CH, C4″), 75.06 (CH, C25), 68.64 (C8a or C17 or C19 or C5″), 68.50 × 2 (C8a or C17 or C19 or C5″), 68.25 (C8a or C17 or C19 or C5″), 67.87 (CH, C5), 67.41 (CH, C5′), 56.69 (CH₃, OMe), 56.56 (CH₃, OMe), 45.88 (CH, C2), 40.61 (CH₂, C20), 39.92 (CH, C12), 36.80 (CH₂, C18), 35.32 (CH, C26), 34.65 (C16 or C2′ or C2″), 34.41 (C16 or C2′ or C2″), 34.32 (C16 or C2′ or C2″), 30.73 (CH, C24), 27.67 (CH₂, C27), 20.36 (CH₃, C12a), 20.14 (CH₃, C4a), 18.56 (CH₃, C6″), 17.83 (CH₃, C6′), 16.53 (CH₃, C24a), 15.28 (CH₃, C14a), 13.12 (CH₃, C26a), 12.20 (CH₃, C28); HRMS (ESI) m/z calcd for C₄₈H₇₂O₁₄Na [M+Na]⁺ 895.4814, found 895.4818.