Introduction

The alkyl citrate family of natural products contain a common 2-alkyl citrate moiety with a wide variety of side chains and oxidation levels.1 These metabolites are inhibitors of squalene synthase, the enzyme responsible for the first pathway-specific step in cholesterol biosynthesis. Some of the simplest examples include CJ-13,982 (1) and CJ-13,981 (2) (Figure 1), which contain an unsubstituted alkyl or alkenyl group, and were isolated from an unidentified fungus CL15036 in 2001.2 A more complex example is the alkyl citrate L-731,120 (3),3 which was isolated from the same fungal species as zaragozic acid A (4),4, 5, 6, 7, 8 a picomolar inhibitor of squalene synthase. Recently, L-731,120 (3) has also been identified as a key intermediate in the biosynthesis zaragozic acid A (4).9 The absolute configuration at C12 in L-731,120 (3) is inferred from its biosynthetic relationship to 4.

Figure 1
figure 1

Alkyl citrate natural products 1–4. A full colour version of this figure is available at the Journal of Antibiotics journal online.

The synthesis of alkyl citrate natural products is challenging due to the high oxidation level and the need for stereocontrol of the two contiguous asymmetric centres.1 To date, only total syntheses of the unnatural enantiomers (+)-CJ-13,982 (ent- 1) and (+)-CJ12,981 (ent- 2) have been reported utilising an anti-Evans aldol reaction followed by a stereoselective Seebach self retention of a stereocentre (SRS) alkylation as the key steps to secure the alkyl citrate fragment.10

Discussion

We envisaged an approach to the unnatural enantiomer of CJ-13,982 (1) starting from the known γ-lactone 5, which was utilised by us for the total synthesis of trachyspic acid11, 12 and the proposed structure for citrafungin A.13 The key step in the synthesis of 5 was the Ireland–Claisen rearrangement of allyl ester 7, synthesised from d-deoxyribose-derived acid 6 (Scheme 1). This reaction proceeds without any observed β-elimination under the standard conditions12 shown to afford the ester 8. The [3,3]-sigmatropic rearrangement occurs to form the new C–C bond on face of the tetrahydrofuran (THF) ring opposite the β-OPMB group in high diastereoselectivity. Ester 8 was then converted into the lactone 5 in a seven-step sequence.11

For the total synthesis of (+)-(2R, 3R)-CJ-13,982 (ent- 1), lactone 5 was reduced with DiBALH to give a mixture of the corresponding lactol and a small amount of diol, which could be oxidised to the lactol with DMP. Wittig extension of the lactol afforded alkene 9 in 46% overall yield. Cross metathesis14 with undecene using Grubbs second-generation catalyst provided the E-alkene 10 as the only stereoisomer. Hydrogenation then afforded the saturated triester 11, which on treatment with trifluoroacetic acid (TFA) gave (+)-CJ-13,982 (ent- 1), [α]D25=+14.1 (c 0.35, acetone) in high yield. This material was identical to that reported by Barrett.10

Whilst the above route could provide the (–)-1 using the known enantiomer of 6,11 it is inefficient especially with respect to constructing the contiguous asymmetric centres. Therefore, we envisaged a more convergent approach which would provide the natural enantiomer (–)-(2S,3S)-CJ-13,982 (1) as well as related alkyl citrate natural products.

As shown in Scheme 2, an alternative approach would be the [3,3]-sigmatropic rearrangement of the Z-silylketene acetal 12 (also derived from d-deoxyribose) with the correct side chain attached via the chair-like transition state shown to afford the alkene 13 with the two new stereocenters introduced in a steroselective manner in a single C–C bond-forming step. This is in analogy with our approach to the bicyclic core of the zaragozic acid C.15 Oxidative cleavage of the alkene and degradation of the THF ring to introduce the final carboxylic acid would then secure the alkyl citrate moiety 14.

This proposal was tested first using the ester 16 derived from a coupling between acid 6 and allylic alcohol 15 as shown in Scheme 3. Exposure of 16 to the standard conditions11, 12 (Scheme 1) gave the alkene 17 as an inseparable ~3:1 mixture of diastereoisomers as judged by 1H NMR integration, presumably differing at the indicated stereocenter. Cleavage of the t-butyldimethylsilyl (TBS) ether induced lactonisation and subsequent PMB group deprotection gave the spirolactones 18 and 19 in a 2.9:1 ratio. These compounds were easily separated by flash chromatography and both were crystalline. X-ray analysis of each revealed the stereochemistry was as predicted. Thus, the rearrangement proceeded via the chair-like transition state as shown in Scheme 2 to afford the desired isomer 18 as the major product. The minor diaseteroisomer 19, with the incorrect stereochemistry at the stereocenter marked (Scheme 3) probably results from [3,3]-sigmatropic rearrangement of the corresponding E-silylketene acetal, which is formed as the minor geometric isomer during the enolisation/silylation reaction.16

The synthesis of the (–)-CJ-13,982 precursor began as shown in Scheme 4. Ester 21 was formed by dicylohexylcarbodiimide-mediated coupling of acid 6 and alcohol 20. Ireland–Claisen rearrangement under the standard conditions11 gave the desired isomer 22 as the major product along with the minor isomer 23 in a 2.5:1 ratio that were separable by flash chromatography. The stereochemistry of the major isomer was assigned in analogy with the initial study (Scheme 3), as well as its subsequent conversion into the target compound.

The total synthesis of (–)-CJ-13,982 (1) is detailed in Scheme 5 and begins with the conversion of acetal 22 into lactone 24 by hydrolysis and oxidation. Removal of the PMB ether and mesylation with concomitant elimination gave the α,β-unsaturated lactone 25 in good yield. DiBAlH reduction afforded the allylic alcohol 26, which was protected on the tertiary alcohol via bis-silylation and desilylation of the primary alcohol to give trimethylsilyl (TMS) ether 27. Alcohol reduction with alkene transposition using the conditions reported by Movassaghi and Ahmad17 with the reagent N-isopropylidene-N′-2-nitrobenzenesulfonyl hydrazine (IPNBSH) gave the diene 28. Protection of the tertiary alcohol was required for this transformation to avoid formation of a cyclic ether in the Mitsunobu reaction. Ozonolysis, oxidation and formation of the t-butyl esters using dicyclohexyl-t-butylisourea18 gave tri-t-butyl ester ent- 11 (the TMS ether was also removed in this sequence) and a final deprotection using TFA afforded (–)-CJ-13,981 (1), [α]D25 –14.1 (c 0.25, acetone), which was identical to the natural product.

Conclusion

We have completed a total synthesis of (+)-CJ-13,981 (ent-1) and the first synthesis of natural (–)-CJ-13,981 (1) from the common acid 6. Each approach utilised an Ireland–Claisen rearrangement in the presence of a β-leaving group. The first involved a sequential formation of the two stereocenters present in the alkyl citrate whilst the second more convergent shorter approach introduced both asymmetric centres in a single C–C bond-forming reaction with good stereocontrol for the desired configuration. This methodology could supply all the simple alkyl citrates in a highly convergent manner.

Experimental procedures

General

1H NMR spectra (600, 500 or 400 MHz) and proton decoupled carbon NMR spectra (13C NMR, 150, 125 or 100 MHz) were obtained in deuterochloroform with residual chloroform as internal standard unless otherwise noted. Chemical shifts are followed by multiplicity, coupling constant(s) (J, Hz), integration and assignments where possible. Flash chromatography was carried out on silica gel 60. Analytical TLC was conducted on aluminium-backed 2 mm-thick silica gel 60 GF254 and chromatograms were visualised with 20% w/w phosphomolybdic acid in ethanol or aq. KMnO4. High-resolution MS were obtained by ionising samples via ESI. Anhydrous THF, Et2O and CH2Cl2 were dried using a solvent cartridge system. Dry methanol was distilled from magnesium methoxide. All other solvents were purified by standard methods. Petrol used refers to petroleum ether 40–60 °C boiling range. All other commercially available reagents were used as received.

Tri-tert-butyl ester 9

A solution of DiBAlH in hexanes (0.55 ml, 1 m, 0.55 mmol) was added to lactone 5 (74 mg, 0.19 mmol) in dry THF (2 ml) at −78 °C and the resulting solution was allowed to stir at −78 °C for 5 h. The reaction was quenched with 10% HCl and water followed by the usual workup with EtOAc. Purification by flash chromatography with 20% EtOAc/petrol yielded lactol (43 mg, 58%) as a pale yellow oil. Further elution with 20% EtOAc/petrol provided diol (12 mg, 16 %) as a pale yellow oil. [α]D23 –2.74 (c 1.20, CH2Cl2); IR (ATR) νmax 3486, 2978, 1730, 1369, 1251, 1153 and 846 cm−1; 1H NMR (500 MHz, CDCl3) δ 1.44 (s, 9H, C(CH3)3), 1.46 (s, 9H, C(CH3)3), 1.48 (s, 9H, C(CH3)3), 1.94 (m, 2H, CH2CHCO2tBu), 2.79 (m, 3H, CHCO2tBu and CH2CO2tBu), 3.62 (m, 1H, OHCHAHB), 3.70 (dd, J=11.25 and 5.61 Hz, 1H, OHCHAHB) and 4.19 (s, 1H, OH); 13C NMR (125 MHz, CDCl3) δ 28.0, 28.2, 28.2, 30.1, 42.0, 50.5, 61.0, 75.4, 81.5, 81.9, 83.1, 169.9, 171.9 and 172.6 ; HRMS (ESI): calculated for C20H36O8Na [M+Na]+ 427.23024; found 427.23015. Diol (12 mg, 0.03 mmol) was dissolved in CH2Cl2 (2 ml) and treated with Dess-Martin periodinane (12.5 mg, 0.03 mmol) for 30 min. Sat. aqueous Na2S2O3 and NaHCO3 were added and the biphasic mixture stirred for 30 min followed by the usual workup with EtO2. Column chromatography with 20% EtOAc/petrol eluent yielded further (10 mg, 83%, total of 53 mg, 72% over two steps) as a pale yellow oil.

tBuOK (62 mg, 0.549 mmol) was added to methyltriphenylphosphonium bromide (209 mg, 0.585 mmol) in dry THF (5 ml) at 0 °C. The resulting solution was stirred at room temperature (RT) for 1 h before cooling to −78 °C, lactol (47 mg, 0.117 mmol) in dry THF (2 ml) was added and the reaction stirred at −78 °C for 1 h. The reaction was warmed to −10 °C and stirred for an additional 2.5 h before quenching with sat. NH4Cl and the usual workup with Et2O. Flash chromatography with 5% EtOAc/petrol as eluent gave alkene 9 (30 mg, 64%) as a yellow oil. [α]D20 –4.80 (c 1.05, CH2Cl2); IR (ATR) νmax 2926, 1731, 1369 and 1152 cm−1; 1H NMR (500 MHz, CDCl3) δ 1.43 (s, 9H, C(CH3)3), 1.46 (s, 9H, C(CH3)3), 1.51 (s, 9H, C(CH3)3), 2.19 (m, 1H, CH2=CHCHAHB), 2.51 (m, 1H, CH2=CHCHAHB), 2.60 (dd J=11.80 and 2.93 Hz, 1H, CH2CHCO2tBu), 2.79 (ABq, J=16.72 Hz, 2H, CH2CO2tBu), 3.93 (s, 1H, OH), 5.01 (apparent d, J=10.04 Hz, 1H, CHAHB=CH), 5.06 (ddd, J=17.06, 3.18 and 1.42 Hz, 1H, CHAHB=CH) and 5.71 (m, 1H, CH2=CH); 13C NMR (125 MHz, CDCl3) δ 28.1, 28.2, 28.3, 31.9, 42.6, 54.0, 75.5, 81.5, 81.6, 83.1, 117.0, 135.4, 170.1, 170.9 and 172.7; HRMS (ESI): calculated for C21H36O7Na [M+Na]+ 423.23532; found 123.23525.

E-Alkene 10

Grubbs second-generation catalyst (3.2 mg, 0.0038 mmol) was added to a solution of triester 9 (15.4 mg, 0.038 mmol) and 1-undecene (78 μl, 0.38 mmol) in CH2Cl2 (2 ml). The resulting solution was refluxed for 16 h before being condensed and purified by flash chromatography with 5% EtOAc/petrol as eluent to yield alkene 10 (15 mg, 74%) as a colourless oil. [α]D25 –5.3 (c 0.27, CH2Cl2); IR (ATR) νmax 3446, 2925, 1738, 1366, 1229, 1217 and 912 cm−1; 1H NMR (500 MHz, CDCl3) δ 0.88 (t, J=7.26 Hz, 3H, CH3C8H16), 1.21–1.34 (m, 14H, CH3C7H14CH2), 1.42 (s, 9H, C(CH3)3), 1.45 (s, 9H, C(CH3)3), 1.50 (s, 9H, C(CH3)3), 1.94 (q, J=6.96 Hz, 2H, C8H17CH2), 2.12 (m, 1H, C=CHCHAHB), 2.43 (m, 1H, C=CHCHAHB), 2.55 (m, 1H, CH2CH), 2.61 (d, J=16.5 Hz, 1H, CHAHBCO2tBu), 2.94 (d, J=16.5 Hz, 1H, CHAHBCO2tBu), 3.90 (s, 1H, OH), 5.29 (m, 1H, CH=CH) and 5.46 (m, 1H, CH=CH); 13C NMR (125 MHz, CDCl3) δ 14.3, 22.8, 28.1, 28.2, 28.3, 29.4, 29.5, 29.7, 30.7, 32.1, 32.7, 42.7, 54.7, 75.6, 81.4, 81.5, 83.0, 126.4, 133.3, 170.1, 171.1 and 172.7; HRMS (ESI): calculated for C30H54O7Na [M+Na]+ 549.37618; found 549.37633.

Tri-tert-butyl ester 11

A suspension of palladium on carbon (10%, 15 mg) and alkene 10 (15 mg, 0.028 mmol) in dry THF (4 ml) under a H2 atmosphere was stirred for 5 h. The suspension was filtered through Filteraid (medium, Ajax Chemicals, Victoria, Australia) and washed with EtOAc and the filtrate was condensed and purified by flash chromatography with 5% EtOAc/petrol as eluent to give alkane 11 (13.8 mg, 92%). [α]D25 +3.2 (c 0.46, CH2Cl2); IR (ATR) νmax 2528, 2926, 1731, 1368 and 1151 cm−1; 1H NMR (500 MHz, CDCl3) δ 0.88 (m, 3H, CH3C11H22), 1.24–1.31 (m, 22H, CH3C11H22), 1.42 (s, 9H, CCH3), 1.47 (s, 9H, CCH3), 1.50 (s, 9H, CCH3), 2.50 (dd, J=11.88 and 2.77 Hz, 1H, CHCO2tBu), 2.77 (ABq, J=16.45 Hz, 2H, CCH2CO2tBu), 3.89 (s, 1H, OH); 13C NMR (125 MHz, CDCl3) δ 14.3, 22.8, 27.4, 27.7, 28.1, 28.2, 28.3, 29.5, 29.5, 29.5, 29.7, 29.8, 29.8, 29.8, 32.1, 42.8, 54.5, 75.8, 81.3, 81.4, 82.9, 170.1, 171.8 and 172.8; HRMS (ESI): calculated for C30H56O7Na [M+Na]+551.39183; found 551.39185

(+)-CJ-13,982 (ent-1)

Triester 11 (9 mg, 0.017 mmol) in CH2Cl2 (3 ml) was cooled to 0 °C and treated with TFA (360 μl). The reaction was stirred at 0 °C for 2 h before warming to RT and stirring for a further 16 h. The volatile organics were removed under reduced pressure to give (+)-CJ-13,982 (ent- 1) (6.1 mg, 99%) as a white amorphous powder. [α]D25 +14.1 (c 0.35, Acetone); lit.10 [α]D25 +16.6 (c 0.5, Acetone); IR (ATR) νmax 3508, 2918, 2850, 1699, 1463, 1415, 1248, 1226 and 1116 cm−1; 1H NMR (500 MHz, CD3OD) δ 0.90 (t, J=6.90 Hz, 3H, CH3C11H22), 1.17–1.39 (m, 20H, CH3C10H20CH2), 1.49 (m, 1H, C11H23CHAHBCH), 1.81 (m, 1H, C11H23CHAHBCH), 2.65 (dd, J=11.90, 2.65 Hz, 1H, CHCO2H), 2.69 (d, J=16.28 Hz, 1H, CHAHBCO2H), 3.03 (d, J=6.44 Hz, 1H, CHAHBCO2H); 13C NMR (125 MHz, CD3OD) δ 14.4, 23.7, 28.3, 28.8, 30.4, 30.5, 30.6, 30.7, 30.7, 30.7, 33.0, 43.1, 54.5, 77.0, 174.5, 177.0 and 177.5; HRMS (ESI): calculated for C18H32O7Na [M+Na]+ 383.20412; found 383.20402.

Ester 16

DMAP (53 mg, 0.43 mmol) and (E)-4-(tert-butyldimethylsilyloxy)but-2-en-1-ol (0.68 g, 3.38 mmol) were added to a solution of acid 6 (0.78 g, 2.75 mmol) in CH2Cl2 (15 ml). The solution was stirred at 0 °C for 5 min before DCC (1.74 g, 8.4 mmol) was added. The resulting suspension was stirred at 0 °C for 2 h before warming to RT for a further 16 h before filtering through Celite. The condensed filtrate was purified by flash chromatography with 15% EtOAc/petrol as eluent yielded ester 16 (0.81 g, 62%) as a pale yellow oil. [α]D24 –43.3 (c 1.10, CH2Cl2); IR (ATR) νmax 2929, 1757, 1730, 1514, 1248, 1060 and 834 cm−1; 1H NMR (500 MHz, CDCl3) δ 0.06 (s, 6H, SiCH3), 0.90 (s, 9H, SiC(CH3)3), 2.07 (m, 1H, CHAHB), 2.23 (m, 1H, CHAHB), 3.38 (s, 3H, OCH3), 3.79 (s, 3H, OCH3), 4.18 (m, 2H, CH=CHCH2OTBS), 4.52 (ABq, J=39.99, 11.42 Hz, 2H, OCH2Ar), 4.54–4.58 (m, 2H, CHOMe and CHOPMB), 4.67 (m, 2H, OCH2CH=CH), 5.15 (dd, J=5.12, 1.61 Hz, 1H, CHCO2), 5.80–5.91 (m, 2H, CH=CH), 6.87 (m, 2H, ArH), 7.26 (m, 2H, ArH); 13C NMR (125 MHz, CDCl3) δ −5.2, 18.5, 26.1, 39.9, 55.4, 55.5, 62.9, 65.4, 71.9, 80.9, 82.2, 106.4, 114.0, 123.1, 129.6, 129.9, 134.9, 159.5 and 171.5; HRMS (ESI): calculated for C24H38O7SiNa [M+Na]+ 489.22790; found 489.22760.

Spirolactones 18 and 19

A solution of nBuLi in hexanes (2.06 ml, 1.88 m, 3.88 mmol) was added dropwise to a solution of iPr2NH (0.5 ml, 3.3 mmol) in THF (6 ml) at −78 °C. The resulting solution was stirred at 0 °C for 10 min before cooling to −78 °C and being added dropwise to a solution of ester 16 (0.8 g, 1.73 mmol) and the supernatant of a centrifuged mixture of freshly distilled TMSCl (1.1 ml, 8.8 mmol) and NEt3 (1.1 ml, 7.7 mmol) in THF/HMPA (15/3 ml) at −100 °C. The resulting solution was stirred at −100 °C for 10 min before warming to RT and stirring for a further 16 h. The reaction was quenched with 1 m NaOH (5 ml) and the resulting layers separated, the aqueous layer was acidified with HCl (10%) and underwent the usual workup with Et2O. The crude acid was dissolved in CH2Cl2 (7 ml) and N,N′-diisopropyl-O-tert-butylisourea18 (2.7 g, 13.3 mmol) was added, the solution was allowed to stir for 18 h at RT. The resulting suspension was filtered through Celite and the filtrate condensed. Flash column chromatography with 10% EtOAc/petrol as eluent gave ester 17 (~1:3 mixture, 594 mg, 65%) as a colourless oil. 1H NMR (500 MHz, CDCl3) δ 0.03 (s, 1.5H, SiCH3), 0.04 (s, 4.5H, SiCH3), 0.88 (s, 9H, SiC(CH3)3), 1.40 (s, 2.25H, C(CH3)3), 1.45 (s, 6.75H, C(CH3)3), 2.07 (m, 1H, CHAHB), 2.18 (m, 1H, CHABB), 2.78 (m, 0.2H, CHCH=CH2), 3.00 (td, J=9.43, 3.49 Hz, 0.8H, CHCH=CH2), 3.37 (s, 1.8H, OCH3), 3.39 (s, 0.6H, OCH3), 3.60 (t, J=9.64 Hz, 0.8H, CHCHAHBOTBS), 3.74 (dd, J=9.97, 3.57 Hz, 0.8H, CHCHAHBOTBS), 3.79 (s, 3H, OCH3), 3.94 (m, 0.2H, CHCHAHBOTBS), 4.12 (dq, J=7.14, 1.08 Hz, 0.2H, CHCHAHBOTBS), 4.37–4.56 (m, 3H, CHOPMB and OCH2Ar), 5.13–5.25 (m, 3H,CHOMe and CH=CH2), 5.65 (dt, J=17.16 and 10.15 Hz, 0.8H, CH=CH2), 5.90 (dt, J=17.02 and 10.52 Hz, 0.2H, CH=CH2), 6.85 (m, 2H, ArH), 7.21 (d, J=8.37 Hz, 2H, ArH).

The ester 17 (~1:3 mixture, 0.29 g, 0.55 mmol) was dissolved in THF (30 ml) and treated with TBAF (175 mg, 0.67 mmol) for 2 days. The usual workup with EtOAc gave the crude lactones, which were dissolved in CH2Cl2/pH 7 buffer (20/2.5 ml), DDQ (0.2 g, 0.88 mmol) was added and the biphasic mixture was stirred for 16 h. The resulting mixture was filtered through Celite and the filtrate was concentrated and purified by flash chromatography with 20% EtOAc/petrol yielded alcohol 19 (15 mg, 13 %) as colourless needle-shaped crystals. m.p. 73–77 °C; [α]D23 –155.0 (c 0.47, CH2Cl2); IR (ATR) νmax 3473, 1771, 1079 and 922 cm−1; 1H NMR (500 MHz, CDCl3) δ 2.12 (brd, J=7.79 Hz, 1H, OH), 2.27 (dd, J=12.56 and 7.04 Hz, 1H, CHAHB), 2.36 (ddd, J=12.60, 9.34 and 5.20 Hz, 1H, CHAHB), 3.34 (s, 3H, OCH3), 4.22 (t, J=9.33 Hz, 1H, CHCHAHBO), 4.47 (dd, J=8.84 and 7.83 Hz, 1H, CHCHAHBO), 4.71 (dd, J=16.74 and 7.64 Hz, 1H, CHOMe), 5.19 (dd, J=5.09 and 0.49 Hz, 1H, CHOH), 5.25 (dt, J=17.35 and 1.21 Hz, 1H, CH=CHAHB), 5.36 (dt, J=10.57 and 1.10 Hz, 1H, CH=CHAHB), 5.88 (ddd, J=17.46, 10.45 and 7.11 Hz, 1H, CH=CH2); 13C NMR (125 MHz, CDCl3) δ 40.8, 47.8, 55.3, 68.3, 71.8, 87.2, 104.6, 120.2, 130.8 and 175.7; HRMS (ESI): calculated for C10H14O5Na [M+Na]+ 237.07334; found 237.07318. Further elution with 20% EtOAc/petrol gave 18 (45 mg, 38 %) as colourless needle-shaped crystals. m.p. 66–69 °C; [α]D24 –156.1° (c 1.20, CH2Cl2); IR (ATR) νmax 3447, 2921, 1773, 1105 and 1056 cm−1; 1H NMR (500 MHz, CDCl3) δ 2.28–2.43 (m, 3H, CHCH2CH and OH), 3.16 (q, J=8.10 Hz, 1H, CHCH=CH2), 3.36 (s, 3H, OCH3), 4.23 (t, J=8.71 Hz, 1H, CHCHAHBO), 4.44 (dd, J=8.88, 7.44 Hz, 1H, CHCHAHBO), 4.49 (m, 1H, CHOMe), 5.12 (d, J=5.32 Hz, 1H, CHOH), 5.26–5.33 (m, 2H, CH=CH2), 5.83 (m, 1H, CH=CH2); 13C NMR (125 MHz, CDCl3) δ 40.5, 47.2, 55.7, 70.4, 73.5, 85.3, 105.1, 120.6, 130.5 and 175.1; HRMS (ESI): calculated for C10H14O5Na [M+Na]+ 237.07334; found 237.07319. Crystallographic data have been deposited with the Cambridge Crystallographic Centre deposit codes: CCDC-1566533 and 1566534.

Allylic alcohol 20

Tridecan-1-ol (2 g, 9.98 mmol) was dissolved in CH2Cl2 (75 ml) and Dess-Martin periodinane (6.39 g, 14.96 mmol) was added to the solution. After stirring at RT for 45 min, sat. NaHCO3, 1.5 m NaS2O3 and CH2Cl2 were added and resulting mixture stirred until both layers became clear. The normal workup with CH2Cl2gave a residue, which was directly used in next step. To a solution of the crude aldehyde in CH2Cl2 (100 ml) was added methyl 2-(triphenylphosphoranylidene)acetate (4.01 g, 12 mmol) and the resultant solution was stirred at RT for 2 h. The normal workup with Et2O followed by purification of the crude product by flash chromatography (5% EtOAc/petrol) to give methyl ester (1.9 g, 8.65 mmol, which was dissolved in THF (86 ml) cooled to −78 °C and treated with DiBAlH (1 m in THF, 37 ml, 37 mmol). After stirring at −78 °C for 2 h, the solution was warmed to 0 °C and 10% HCl was added dropwise to quenched the reaction. The normal workup with EtOAc and flash chromatographic purification of the crude product (10% EtOAc/petrol) gave allylic alcohol 20 (1.52 g, 72%) as white solid. m.p. 27–29 °C IR (ATR) νmax 3314, 2922, 2853, 1466, 1002, 968 and 720 cm−1; 1H NMR (400 MHz, CDCl3) δ 0.88 (t, J=6.7 Hz, 3H, C10H20CH3), 1.19–1.42 (m, 20H, C10H20CH3), 2.04 (dd, J=13.8 and 6.7 Hz, 2H, CH2C11H23), 4.08 (s, 1H, CH2OH), 5.74–5.59 (m, 2H, CH=CH) ; 13C NMR (101 MHz, CDCl3) δ 14.10, 22.67, 29.12, 29.17, 29.34, 29.48, 29.59, 29.65, 31.91, 32.20, 63.87, 128.75 and 133.65.

Ester 21

A solution of alcohol 20 (1.0 g, 4.4 mmol) and acid 6 (1.4 g, 4.8 mmol) in CH2Cl2 (11 ml) was cooled to 0 °C and DMAP (58 mg, 0.48 mmol) was added. The resulting solution was stirred at 0 °C for 10 min then treated with DCC (1.85 g, 8.96 mmol) and the reaction was slowly warmed up to RT and further stirred for 36 h. The suspension was filtered through Celite and evaporation of the solvent followed by purification by flash chromatography (20% Et2O/petrol) gave the ester 21 (1.7 g, 63%) as a colourless oil. [α]D24–39.9 (c 1.00, CH2Cl2); IR (ATR) νmax 2924, 2857, 1758, 1729, 1614, 1514, 1465, 1110, 1065 and 971 cm−1; 1H NMR (600 MHz, CDCl3) δ 0.88 (t, J=7.0 Hz, 3H, C11H22CH3), 1.26 (m, 18H, C9H18CH3), 1.36 (m, 2H, CH2C10H21), 2.07 (m, 3H, CH=CHCH2 and CH3OCHCHAHB), 2.22 (ddd, J=13.27, 6.58 and 1.58 Hz, 1H, CH3OCHCHAHB), 3.39 (s, 3H, OCH3), 3.80 (s, 3H, ArOCH3), 4.48 (d, J=11.45 Hz, 1H, OCHAHBAr), 4.59 (m, 5H, OCHAHBAr, CO2CH2CH=, CHOCH2Ar and CHCO2), 5.15 (dd, J=5.10 and 1.59 Hz, 1H, CHOCH3), 5.58 (dtt, J=5.10, 6.68 and 1.54 Hz, 1H, CH=CHC12H25), 5.80 (dtt, J=5.10, 6.68 and 1.54 Hz, 1H, CH=CHC12H25), 6.87 (m, 2H, ArH), 7.26 (m, 2H, ArH); 13C NMR (150 MHz, CDCl3) 14.1, 22.7, 28.9, 29.2, 29.3, 29.5, 29.6, 29.6, 29.7, 29.7, 31.9, 32.3, 39.7, 55.3, 55.3, 66.0, 71.7, 80.7, 82.1, 106.2, 113.8, 123.8, 123.2, 129.4, 129.7, 137.3, 159.3 and 171.4; HRMS (ESI): calculated for C29H46O6Na [M+Na]+ 513.3180; found 513.3181.

Esters 22 and 23

To a solution of the ester 21 (1.1 g, 2.3 mmol) in THF/HMPA (20 ml, 5:1) was added the supernatant of a centrifuged mixture of freshly distilled TMSCl (1.25 g, 11.24 mmol, 1.47 ml) and NEt3 (0.97 g, 10.39 mmol, 1.45 ml) via cannula. The resulting solution was cooled to 100 °C and a freshly prepared solution of LDA (4.5 mmol, 0.3 m in THF, 15 ml) was added dropwise. After stirring at −100 °C for 10 min, the reaction was slowly warmed to RT over 16 h. The resulting solution was extracted with 1 m NaOH three time and the aqueous layer was acidified with 10% HCl and a normal workup with EtOAc gave the crude acid, which was dissolved in CH2Cl2 and N,N′-diisopropyl-O-tert-butylisourea (5 ml, 21 mmol) was added. After stirring for 24 h, the suspension was filtered through Celite and rinsed with CH2Cl2. Rotary evaporation and flash chromatography (5% EtOAc/petrol) purified the resulting residue and gave ester 22 (670 mg, 53%) as a colourless oil. [α]D24 –61.7 (c 1.19, CH2Cl2); IR (ATR) νmax 2926, 2854, 1735, 1615, 1515, 1466, 1249, 1104 and 1037 cm−1; 1H NMR (600 MHz, CDCl3) δ 0.88 (t, J=6.99 Hz, 3H, C11H22CH3), 1.25 (m, 20H, C10H20CH3), 1.38 (m, 2H, CH2C11H23), 1.45 (s, 9H, C(CH3)3), 2.09 (ddd, J=12.86, 7.10 and 1.64 Hz, 1H, CHACHB), 2.22 (ddd, J=12.86, 8.87 and 5.56 Hz, 1H, CHACHB), 2.71 (td, J=9.69 and 2.76 Hz, 1H, CHCH=CH2), 3.39 (s, 3H, OCH3), 3.80 (s, 3H, ArOCH3), 4.30 (dd, J=8.87 and 7.09 Hz, 1H, CHOCH2Ar), 4.39 (d, J=11.47 Hz,1H, OCHAHBAr), 4.54 (d, J=11.47 Hz,1H, OCHAHBAr), 5.04 (dd, J=17.26 and 2.15 Hz, 1H, CH=CHAHB), 5.16–5.18 (m, 2H, CHOCH3 and CH=CHAHB), 5.56 (dt, J=17.24 and 10.14 Hz, 1H, CH=CH2), 6.85 (m, 2H, ArH) and 7.21 (m, 2H, ArH); 13C NMR (150 MHz, CDCl3) 14.1, 22.7, 27.7, 28.0, 29.3, 29.5, 29.6, 29.6, 29.6, 29.7, 30.7, 31.9, 37.9, 49.3, 55.2, 55.3, 71.9, 80.3, 81.5, 91.2, 104.1, 113.6, 118.4, 129.0, 130.2, 137.9, 159.1 and 171.0; HRMS (ESI): calculated for C33H54O6Na [M+Na]+ 569.38043; found 569.38043. Further elution with 10% EtOAc/petrol gave 23 (265 mg, 21%) as a colourless oil. [α]D25 –29.4 (c 1.21, CH2Cl2); IR (ATR) νmax 2925, 2854, 1735, 1614, 1515, 1466, 1249, 1103 and 1037 cm−1; 1H NMR (600 MHz, CDCl3) δ 0.88 (t, J=6.99 Hz, 3H, C11H22CH3), 1.32–1.19 (m, 20H, C10H20CH3), 1.42 (s, 9H, C(CH3)3), 1.63–1.71 (m, 2H, CH2C11H23), 2.06 (ddd, J=13.27, 6.36 and 2.50 z, 1H, CHACHB), 2.22 (ddd, J=13.27, 6.36 and 2.50 Hz, 1H, CHACHB), 2.34 (td, J=10.36 and 2.56 Hz, 1H, CHCH=CH2), 3.38 (s, 3H, OCH3), 3.80 (s, 3H, ArOCH3), 4.30 (t, J=6.33 Hz, 1H, CHOCH2Ar), 4.42 (ABq, J=14.95 Hz, 2H, OCH2Ar), 5.03 (dd, J=17.17 and 2.22 Hz, 1H, CH=CHAHB), 5.09 (dd, J=10.17 and 2.25 Hz, 1H, CH=CHAHB), 5.22 (dd, J=5.59 and 2.50 Hz, 1H, CHOCH3), 5.91 (dt, J=17.16 and 10.07 Hz, 1H, CH=CH2), 6.84 (m, 2H, ArH), 7.21 (m, 2H, ArH); 13C NMR (125 MHz, CDCl3) 14.1, 22.7, 27.6, 28.1, 29.3, 29.4, 29.6, 29.6, 29.6, 29.7, 30.7, 31.9, 38.2, 52.1, 55.2, 55.4, 71.7, 81.4, 81.4, 91.2, 104.8, 113.6, 117.3, 129.2, 130.1, 139.1, 159.1 and 169.5.

Lactone 24

The solution of ester 22 (600 mg, 1.09 mmol) in THF (22 ml) was treated with 10% HCl (20 ml) at RT and the solution was stirred for 5 days. Sat. NaHCO3 was added followed by a normal workup with EtOAc and purification of the residue with 15% EtOAc/petrol gave the lactols (348 mg, 60%). The mixture lactols was dissolved in dry CH2Cl2 (12 ml) and cooled to 0 °C and 4 Å molecular sieves (370 mg) and PCC (280 mg, 1.30 mmol) were added. The dark brown suspension was stirred at 0 °C for 2 h before warming to RT over 16 h. The solution was filtered through fluorosil, washed with Et2O and concentrated. Purification of the crude product by flash chromatography with 5% EtOAc/petrol yielded lactone 24 (318 mg, 55%) as a pale yellow oil. [α]D25 +0.55 (c 1.59, CH2Cl2); IR (ATR) νmax 2925, 2854, 1794, 1734, 1614, 1515, 1466, 1369, 1247, 1156 and 1035 cm−1; 1H NMR (600 MHz, CDCl3) δ 0.88 (t, J=6.98 Hz, 3H, C11H22CH3), 1.15–1.44 (m, 22H, C11H22CH3), 1.46 (s, 9H, C(CH3)3), 2.59 (dd, J=17.89 and 6.29 Hz, 1H, CHACHB), 2.68 (dd, J=17.89 and 7.83 Hz, 1H, CHACHB), 2.75 (td, J=9.87 and 3.36 Hz, 1H, CHCH=CH2), 3.81 (s, 3H, ArOCH3), 4.29 (t, J=7.82 and 6.29 Hz, 1H, CHOCH2Ar), 4.42 (d, J=11.40 Hz, 1H, OCHAHBAr), 4.51 (d, J=11.58 Hz, 1H, OCHAHBAr), 5.11 (dd, J=17.16 and 1.78 Hz, 1H, CH=CHAHB), 5.22 (dd, J=10.20 and 1.87 Hz, 1H, CH=CHAHB), 5.43 (dd, J=17.1 and 10.1 Hz, 1H, CH=CH2), 6.86 (m, 2H, ArH) and 7.21 (m, 2H, ArH); 13C NMR (101 MHz, CDCl3) δ 14.1, 22.7, 27.3, 27.9, 29.2, 29.3, 29.4, 29.6, 29.6, 29.7, 31.9, 35.1, 49.1, 55.3, 72.3, 83.1, 91.9, 113.8, 120.4, 129.0, 129.4, 135.8, 159.5, 167.5 and 173.9; HRMS (ESI): calculated for C32H50O6Na [M+Na]+ 553.3505; found 553.3503.

α,β-Unsaturated lactone 25

To a biphasic solution of lactone 24 (150 mg, 0.28 mmol) in CH2Cl2 and pH 7 buffer solution (2.7/0.4 ml) was added DDQ (130 mg, 0.57 mmol) and the mixture was allowed to stir for 16 h before being filtered through Celite and the was filtrate concentrated. Flash chromatography (10% EtOAc/petrol) of the resulting residue and gave alcohol (103mg, 90%), which was dissolved in pyridine (1 ml) and cooled to 0 °C and MsCl (60 μl, 0.77 mmol) was added and the solution was stirred at 0 °C for 2 h before warming to RT and stirring for an additional 16 h. Water and Et2O were added and the combined organic extracts were washed with sat. CuSO4, water and dried over MgSO4 and concentrated. Purification by flash chromatography with 10% EtOAc/petrol as eluent gave α,β-unsaturated lactone 25 (86 mg, 78%) as a colourless oil. [α]D25 +50.0 (c 1.45, CH2Cl2); IR (ATR) νmax 2924, 2854, 1789, 1728, 1370, 1252, 1139, 921 and 825 cm−1; 1H NMR (600 MHz, CDCl3) δ 0.88 (t, J=6.96 Hz, 3H, C11H22CH3), 1.24–1.30 (m, 20H, C10H20CH3), 1.45 (m, 2H, CH2C11H23), 1.48 (s, 9H, C(CH3)3), 2.84 (td, J=9.97 and 3.40 Hz, 1H, CHCH=CH2), 5.09 (d, J=17.10 Hz, 1H, CH=CHAHB), 5.15 (d, J=10.19 Hz, 1H, CH=CHAHB), 5.35 (dt, J=17.18 and 9.76 Hz, 1H, CH=CH2), 6.07 (d, J=5.56 Hz, 1H, COCH=CH), 7.29 (d, J=5.59 Hz, 1H, COCH=CH) ; 13C NMR (101 MHz, CDCl3) δ 14.1, 22.7, 27.0, 27.9, 28.9, 29.2, 29.3, 29.4, 29.6, 29.6, 31.9, 48.7, 84.0, 92.4, 120.0, 121.7, 134.1, 155.0, 166.3 and 171.9; HRMS (ESI): calculated for C24H40O4 Na [M+Na]+ 415.2822; found 415.2818.

Allylic alcohol 26

A solution of DiBAlH in CH2Cl2 (0.9 ml, 1 M, 0.9 mmol) was added to a solution of the lactone 25 (70 mg, 0.18 mmol) in dry CH2Cl2 (1 ml) at −78 °C and the resulting solution was allowed to stir at −78 °C for 30 min. The reaction was quenched with 10% HCl and water and the usual workup with EtOAc followed by purification of the crude product by flash chromatography with 15% EtOAc/petrol as eluent yielded diol 26 (50 mg, 70%) as a pale yellow oil. [α]D25 +32.1 (c 0.85, CH2Cl2); IR (ATR) νmax 3500, 2925, 2854, 1718, 1458, 1370, 1254, 1139 and 1032 cm−1; 1H NMR (600 MHz, CDCl3) δ 0.88 (t, J=7.06 Hz, 3H, C11H22CH3), 1.13–1.36 (m, 22H, C11H22CH3), 1.49 (s, 9H, C(CH3)3), 2.33 (td, J=9.29 and 2.43 Hz, 1H, CHCH=CH2), 2.40 (brs, 1H, CH2OH), 3.82 (s, 1H, COH), 4.14 (dt, J=12.83 and 6.08 Hz, 1H, CHAHBOH), 4.31 (dt, J=13.64 and 6.19 Hz, 1H, CHAHBOH), 5.05 (dd, J=17.20 and 1.41 Hz, 1H, CH=CHAHB), 5.17 (dd, J=10.20 and 1.81 Hz, 1H, CH=CHAHB), 5.51 (d, J=12.00 Hz, 1H, CH2CH=CH), 5.59 (dt, J=17.14 and 9.94 Hz, 1H, CH=CH2), 5.75 (dt, J=12.01 and 6.36 Hz, 1H, CH2CH=CH); 13C NMR (101 MHz, CDCl3) δ 14.1, 22.7, 27.2, 27.9, 28.2, 29.3, 29.4, 29.5, 29.6, 29.6, 31.9, 52.4, 58.8, 80.2, 83.3, 118.4, 131.8, 132.2, 136.6 and 174.0; HRMS (ESI): calculated for C24H44O4Na [M+Na]+ 419.3126; found 419.3127.

Diene 28

To a solution of diol 26 (25 mg, 0.063 mmol) in CH2Cl2 (1.6 ml) was added 2,6-lutidine (87 mg, 1.27 mmol) and freshly distilled TMSCl (163 μl, 1.27 mmol) and the resulting suspension was stirred at RT for further 24 h. Sat. NaHCO3 was added to adjust pH then followed by normal workup with Et2O. The crude bis-silyl ether was directly dissolved in anhydrous methanol (0.5 ml) at 0 °C. A catalytic amount of K2CO3 (~10 mg) then was added to the solution and this was stirred for 45 min and a small amount of sat. NH4Cl was added to adjust pH and followed by normal workup with EtOAc. Purification by flash chromatography with 15% EtOAc/petrol as eluent yielded the allylic alcohol 27 (26 mg, 88%) as a colourless oil, which was dissolved in THF/1-hexene (10:1, 2 ml), and PPh3 (57 mg, 0.22 mmol) and IPNBSH (58 mg, 0.22 mmol) were added. The reaction was cooled to 0 °C and diethyl azodicarboxylate (35 μl, 0.22 mmol) was added dropwise to the solution. After stirring at 0 °C for 1 h, a solution of TFE in water (650 μl, 1:1) was added and the reaction was warmed up to RT and stirred further 16 h. The usual workup with Et2O and purification by flash chromatography (100% petrol) yield diene 28 (16 mg, 58%) as a colourless oil. [α]D25 –19.5 (c 0.60, CH2Cl2); IR (ATR) νmax 2925, 2854, 1738, 1458, 1368, 1258, 1155, 1096 and 840 cm−1; 1H NMR (600 MHz, CDCl3) δ 0.12 (s, 9H, Si(CH3)3), 0.87 (t, J=7.1 Hz, 3H, C11H22CH3), 1.30–1.22 (m, 22H, C11H22CH3), 1.44 (s, 9H, C(CH3)), 2.23 (td, J=10.6 and 2.3 Hz, 1H, CHCH=CHAHB), 2.33 (ddd, J=21.6, 14.0 and 7.2 Hz, 2H, CH2CH=CH2), 5.03–4.95 (m, 3H, CH2CH=CH2 and CHCH=CHAHB), 5.11 (dd, J=10.2 and 2.3 Hz, 1H, CHCH=CHAHB), 5.56 (dt, J=17.3 and 10.0 Hz, 1H, CHCH=CH2), 5.77–5.65 (m, 1H, CH2CH=CH2); 13C NMR (101 MHz, CDCl3) δ 2.9, 14.1, 22.7, 27.2, 28.1, 29.3, 29.4, 29.6, 29.6, 29.6, 29.6, 31.9, 44.3, 52.4, 81.1, 82.9, 117.5, 117.5, 133.9, 138.2 and 173.2; HRMS (ESI): calculated for C27H52O3SiNa [M+Na]+ 453.3750; found 453.3757.

Tri-t-butyl ester ent-11

A solution of the diene 28 (40 mg, 0.09 mmol) in CH2Cl2 (2 ml) and MeOH (17 μl) was cooled to −78 °C and ozone was bubbled through the solution until a blue colour persisted. Me2S (105 mg, 1.7 mmol) was added and the solution allowed to warm to RT for 30 min and then concentrated under reduced pressure. The crude dialdehyde was dissolved in tBuOH (2.1 ml) and 2-methyl-2-butene (420 μl) and a solution of 80% NaClO2 (150 mg, 1.65 mmol) and NaH2PO4 (91 mg, 0.75 mmol) in water (0.7 ml) was added and the biphasic solution was stirred for 16 h. Water and EtOAc were added and the phases were separated and the aqueous phase was adjusted to pH 2 with HCl and further extracted with EtOAc. The combined organic extracts were washed with brine, dried and concentrated and the crude diacid was dissolved in CH2Cl2 (1.5 ml) and treated with N,N′-diisopropyl-O-tert-butylisourea (400 mg, 0.5 ml) for 16 h. The resulting suspension was filtered through a pad of Celite and the filtrate was concentrated and the crude product was purified by flash chromatography (5% EtOAc/petrol) to give triester ent-11 (13 mg, 27%) as a colourless oil. [α]D25 –3.2 (c 0.42, CH2Cl2); the IR and NMR spectra were identical to those for the (+)-enantiomer; HRMS (ESI): calculated for C30H56O7Na [M+Na]+ 551.39183; found 551.39178.

(–)-CJ-13,982 (1)

A solution of triester ent-11 (8 mg, 0.015 mmol) in CH2Cl2 (3 ml) was cooled to 0 °C and treated with TFA (340 μl) then was stirred at 0 °C for 2 h before warming to RT and stirring for a further 16 h. The volatile organics were removed under reduced pressure gave (–)-CJ-13,982 (1) (6 mg, 80%) as a white amorphous powder. [α]D25 –14.1 (c 0.25, acetone); lit.2 [α]D25 –18.3 (c 2.6, acetone). The IR and NMR spectra were identical to those for the (+)-enantiomer. HRMS (ESI): calculated for C30H56O7Na [M+Na]+ 383.2041; found 383.2041.

scheme 1

Synthesis of (+)-CJ-13,982 (ent- 1). A full colour version of this scheme is available at the Journal of Antibiotics journal online.

scheme 2

Convergent approach to simple alkyl citrates. A full colour version of this scheme is available at the Journal of Antibiotics journal online.

scheme 3

Ireland–Claisen rearrangement of ester 16. A full colour version of this scheme is available at the Journal of Antibiotics journal online.

scheme 4

Ireland–Claisen rearrangement of ester 21. A full colour version of this scheme is available at the Journal of Antibiotics journal online.

scheme 5

Synthesis of (–)-CJ-13,982 (1). A full colour version of this scheme is available at the Journal of Antibiotics journal online.