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 B1a (1, Scheme 1) is the most potent anthelmintic congener. Ivermectin, a mixture of 22,23-dihydro-avermectins B1a and B1b, 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 B1a (1),7, 8, 9 A1a,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 north17, 18 and south segments, as well as a stereocontrolled total synthesis of avermectin B1a (1).

Results and discussion

Synthesis plan

Our renewed plan for the synthesis of avermectin B1a (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
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 B1a.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 oxidation43, 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 B1a 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 conditions39, 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 B1a.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 B1a (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 B1a (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 B1a (1). The highlights of total synthesis include: (i) a vinylogous Mukaiyama aldol reaction to install the C1–C4 carbons (11+1213); (ii) an intramolecular aldol reaction to form the octahydrobenzofuran (717) (iii) a formation of trityl oxetane acetal to protect a labile β-hydroxy aldehyde (1718); (iv) a regioselective installation of C3–C4 double bond (1920); (v) a highly diastereoselective construction of the north segment via coupling of the C22–C28 alkyne and the C15–C21 lactone (29+364); (vi) an E-selective Wittig reaction using trimethylphosphonium salt to connect the polyfunctionalized north and south segments (6+46c44); (vii) an efficient Shiina macrolactonization using a genuine seco acid (249); (viii) a modified synthesis of the disaccharide and improved glycosylation to afford avermectin B1a (57+581). 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 B1a.

scheme 1

Structure and synthesis plan for avermectin B1a (1).

scheme 2

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%.

scheme 3

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).

scheme 4

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).

scheme 5

Total synthesis of avermectin B1a (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, Jc–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, Jc–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, Jc–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, Jc–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, Rt=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 B1a (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.