HETEROCYCLES

Paper | Special issue | Vol. 82, No. 1, 2010, pp. 761-774
Received, 19th June, 2010, Accepted, 26th July, 2010, Published online, 27th July, 2010.
DOI: 10.3987/COM-10-S(E)61

■ Stereoselective Synthesis of Caribbean Ciguatoxin M-Ring Using [2+2] Photocyclization

Shuji Yamashita,* Naoya Iijima, Takahiro Shida, and Masahiro Hirama*

Department of Chemistry, Graduate School of Science, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan

Abstract

Ciguatoxins, the principal causative toxins of ciguatera seafood poisoning, are potent toxic polycyclic ethers. We report herein a stereoselective synthesis of the seven-membered M-ring moiety of Caribbean ciguatoxin C-CTX-1. The key features of the synthesis are the photo-induced [2+2] electrocyclization and enzymatic asymmetric hydrolysis to construct the congested oxepane ring.

INTRODUCTION
Ciguatera seafood poisoning, an important medical issue in tropical and subtropical regions, causes gastrointestinal, cardiovascular and neurological disorders that may last for weeks or even years.1 Ciguatoxins (CTXs), the principal causative toxins of ciguatera, are ladder-like polycyclic ethers.2 To date, more than 20 ciguatoxin congeners have been structurally identified.3 Caribbean ciguatoxin C-CTX-1 (1, Figure 1) was isolated by Lewis and co-workers from the carnivorous fish horse-eye jack (Caranx latus) as the main toxin of ciguatera in the Caribbean Sea.4 In contrast to the typical Pacific ciguatoxin CTX3C (2, Figure 1), 1 possesses 14 ether rings with dissimilar functional group patterns. These structural differences are believed to cause the distinct ciguatera symptoms.
The very limited supply of ciguatoxins from natural sources has prevented chemical and biological studies of ciguatera. Thus, we have successfully synthesized three Pacific ciguatoxins5,6 and developed a sandwich enzyme-linked immunosorbent assay (ELISA) for their detection.7 Furthermore, we have reported the syntheses of ABCDE-8 and LMN-ring9 fragments of C-CTX-1 (1). Unfortunately, the large scale preparation of LMN-ring moiety was hampered, due to the low yield of the sterically congested seven-membered M-ring.10 This prompted us to improve the synthetic route to the M-ring moiety of 1.

RESULTS AND DISCUSSION
Our new synthesis commenced with commercially available 2,5-dimethylfuran 3 and maleic anhydride 4 (Scheme 1). Diels-Alder reaction between 3 and 4 afforded the thermodynamically favored exo-adduct 5,11 which was successively reduced to the corresponding diol 6 in 77% overall yield. Treatment of 6 with methanesulfonyl chloride and triethylamine gave bis-mesylate 7.12 Ozonolysis of 7 followed by NaBH4 reduction produced oxolane diol 8 in 68% yield from 6. After the protection of 8 as its bis-TBS ether, β-elimination of the bis-mesylate was realized by treatment with DBU under reflux in toluene in the presence of tetrabutylammonium iodide (TBAI), leading to the required diene 10a (74% from 8). Following the same procedure, bis-acetate 10b was prepared in 70% yield from 8.
Next, we examined the [2+2] photocyclization of diene 10 (Scheme 2). Exposure of 10a and 10b to a low pressure mercury lamp in a quartz vessel led to formation of oxa-bicyclo[3.2.0]heptene 11a and 11b, respectively, in good yields.13 Ozonolysis of the internal olefin of 11a afforded the desired seven-membered diketone 12 in 62% yield, which has a bis-trisubstituted alkyl ether structure. With the requisite oxepane 12 in hand, stereoselective reduction of the diketone was pursued. After a considerable number of attempts, it was found that treatment of 12 with LiBHEt3 at low temperature accomplished the desired β-face reduction, although the reaction stopped at the reduction of a single ketone to form hemiacetal 13 in 90% yield. Due to the stability of the hemiacetal structure of 13, further reduction to the corresponding diol was not realized even at higher temperature. Other reducing reagents [NaBH4, LiAlH4, DIBAL, Al(Oi-Pr)3, and SmI2] did not provide 13 as a major product. Fortunately, we found that the acetylation of 13 occurred at the secondary alcohol formed via the hemiacetal-ring opening, and the resulting ketone 15 could be reduced by LiBHEt3 stereoselectively. Acetylation of alcohol 16 furnished meso-M-ring moiety 17 in 14% overall yield (12 steps) from the starting material 4.

In order to prepare the chiral M-ring, we focused on the desymmetrization of meso-intermediates 12 and 17. Attempts at enantioselective reduction of diketone 12 and enzymatic asymmetric hydrolysis of bis-acetate 17 were unsuccessful. After examining various enzymes (Table 1), we eventually found that treatment of bis-acetate 11b with lipoprotein lipase in MeCN/pH 8.0 phosphate buffer at 45 °C gave rise to mono-acetate (-)-18 (85% ee)14 in 72% yield along with a small amount of diol 19 (5%) and recovered 11b (16%) (Entry 4).

The M-ring triol 22 was synthesized from the obtained mono-acetate (-)-18 (Scheme 3). TBS-protection of (-)-18, followed by oxidative cleavage of cyclobutene (+)-20 afforded diketone (+)-21 in 74% overall yield. Stereoselective reduction of (+)-21 was achieved using LiBHEt3 in the presence of 12-crown-4 to give triol (-)-22 in 80% yield.15 The absolute configuration of (-)-22 was confirmed by X-ray crystallographic analysis.16, 17
In conclusion, we have developed a secure and stereocontrolled synthetic route to the optically active M-ring of 1. The key intermediate (-)-22 was prepared in 11 steps from the commercially available compounds. The synthesis features (1) photo-induced [2+2] cyclization to construct the cyclobutene structure; (2) enzymatic desymmetrization of bis-acetate 11b; and (3) stereoselective reduction of diketone 21. The strategy described herein will accelerate our synthetic study of Caribbean C-CTX-1, which is being actively investigated in our laboratory.

EXPERIMENTAL
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. THF was distilled from sodium/benzophenone. Toluene, dichloromethane (CH2Cl2), triethylamine (Et3N), acetonitrile (MeCN) and hexane were distilled from calcium hydride. DMF was distilled under reduced pressure from calcium hydride. All other reagents were used as supplied unless otherwise stated.
Analytical thin-layer chromatography (TLC) was performed using E. Merck Silica gel 60 F254 pre-coated plates. Column chromatography was performed using 100-210 μm Silica Gel 60N (Kanto Chemical Co., Inc.), and for flash column chromatography 40-50 μm Silica Gel 60N (Kanto Chemical Co., Inc.) was used.
1H- and 13C-NMR spectra were recorded on Varian 400 (400 MHz), or Varian INOVA 500 (500 MHz) spectrometers. Chemical shifts are reported in δ (ppm) down field from tetramethylsilane (TMS) with reference to solvent signals [1H-NMR: CHCl3 (7.26); 13C-NMR: CDCl3 (77.0)]. Signal patterns are indicated as s, singlet; d, doublet; m, multiplet; br, broad peak. IR spectra were recorded on Perkin Elmer Spectrum BX FT-IR spectrometer. High resolution ESI-FT mass spectra were measured on Thermo Fisher Scientific Orbitrap Discovery (ESI LTQ Orbitrap). Optical rotations were recorded on JASCO P-2200 polarimeter. The carbon numbers of all compounds are corresponding with C-CTXs.
Diol 6. 2,5-Dimethylfuran (6.5 mL, 61 mmol) and maleic anhydride (5.1 g, 52 mmol) were dissolved with THF (10 mL) and stirred for 21 h at room temperature. The resulting crude mixture of adduct 5 was used for the next step without any purification.
To a suspension of LiAlH4 (3.0 g, 79 mmol) in THF (100 mL) at 0 °C was added dropwise the above mixture in THF (60 mL). After being stirred for 2 h at room temperature, the mixture was warmed to reflux and stirred for 20 h. The reaction mixture was quenched at 0 °C by slow addition of saturated aqueous Na2SO4. The resulting inorganic salts were filtered and washed with EtOAc, then the filtrate was concentrated in vacuo. Purification by column chromatography on silica gel (hexane/EtOAc 1:1-0:1) gave diol 6 (7.29 g, 39.6 mmol) in 77% yield over 2 steps: colorless oil; IR (film) ν 3443, 2932, 1731, 1714, 1650, 1454, 1383, 1033, 865 cm-1; 1H-NMR (400 MHz, CDCl3) δ 1.52 (6H, s, Me61, Me62), 2.06 (2H, m, H49, H52), 3.86 (2H, m, H50, H51), 3.94 (2H, dd, J = 11.4, 3.5 Hz, H50, H51), 6.16 (2H, s, H47, H54); 13C-NMR (100 MHz, CDCl3) δ 16.2 (Me61, Me62), 46.3 (C49, C52), 60.8 (C50, C51), 86.7(C48, C53), 140.2(C47, C54); HRMS (ESI), calcd for C10H16O3Na 207.0992 (M+Na+), found 207.0994.
Diol 8. To a solution of 6 (7.03 g, 38.2 mmol) and Et3N (17.5 mL, 126 mmol) in CH2Cl2 (80 mL) was added MsCl (6.5 mL, 84 mmol) at 0 °C. After being stirred for 1 h at room temperature, the mixture was quenched with saturated aqueous NaHCO3 at 0 °C. The organic layer was washed with brine and the aqueous layer was extracted with EtOAc. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. The residue was passed through a short pad of silica gel (hexane/EtOAc 1:1-0:1), and the resulting bis-mesylate 7 was used for the next step without further purification. [Caution!] Bis-mesylate 7 underwent decomposition in high concentration.
To a solution of 7 in CH2Cl2 (300 mL) and MeOH (75 mL) was bubbled O3 at -78 °C for 70 min. After the color of solution turned to blue, the excess O3 was removed with a stream of air for 10 min. NaBH4 (1.58 g, 41.8 mmol) was added to the resulting mixture at -78 °C, and then allowed to warm to room temperature. After being stirred for 3 h, the mixture was quenched with saturated aqueous NaHCO3 at 0 °C. The organic layer was washed with brine and the aqueous layer was extracted with EtOAc. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by column chromatography on silica gel (EtOAc/MeOH 1:0-10:1) gave diol 8 (9.84 g, 26.1 mmol) in 68% yield over 2 steps: colorless solid; mp 116-117 °C; IR (film) ν 3389, 2939, 1351, 1172, 1048, 954, 848, 773 cm-1; 1H-NMR (400 MHz, CDCl3) δ 1.21 (6Η, s, Me61, Me62), 3.08 (6H, s, Ms x 2), 3.11 (2H, m, H49, H52), 3.46 (2H, d, J = 11.4 Hz, H47, H54), 3.56 (2H, d, J = 11.4 Hz, H47, H54), 4.33 (2H, ddd, J = 10.2, 4.9, 2.0 Hz, H50, H51), 4.44 (2H, ddd, J = 10.2, 5.1, 1.5 Hz, H50, H51); 13C-NMR (100 MHz, CDCl3) δ 21.1 (Me61, Me62), 37.6 (Ms x 2), 43.7 (C49, C52), 66.5 (C50, C51), 69.4 (C47, C54), 84.0 (C48, C53); HRMS (ESI), calcd for C12H24O9S2Na 399.0754 (M+Na+), found 399.0757.
Bis-TBS ether 9a. To a solution of 8 (129 mg, 343 µmol) in DMF (3.4 mL) were added imidazole (98.0 mg, 1.44 mmol) and TBSCl (170 mg, 1.13 mmol). After being stirred for 6 h at room temperature, the mixture was quenched with saturated aqueous NaHCO3. The organic layer was washed with brine and the aqueous layer was extracted with Et2O. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by flash column chromatography on silica gel (hexane/EtOAc 4:1-3:1) gave bis-TBS ether 9a (197 mg, 326 µmol) in 95% yield: colorless solid; mp 74-75 °C; IR (film) ν 2930, 2857, 1471, 1359, 1255, 1176, 1095, 953, 836, 777 cm-1; 1H-NMR (400 MHz, CDCl3) δ 0.06 (12H, s, TBS x 2), 0.90 (18H, s, TBS x 2), 1.20 (6H, s, Me61, Me62), 2.96 (2H, m, H49, H52), 3.05 (6H, s, Ms x 2), 3.34 (2H, d, J = 9.8 Hz, H47, H54), 3.49 (2H, d, J = 9.8 Hz, H47, H54), 4.35 (2H, ddd, J = 10.2, 5.3, 2.2 Hz, H50, H51), 4.43 (2H, ddd, J = 10.2, 4.9, 1.0 Hz, H50, H51); 13C-NMR (100 MHz, CDCl3) δ -5.6 (TBS x 2), -5.4 (TBS x 2), 18.2 (TBS x 2), 21.3 (Me61, Me62), 25.9 (TBS x 2), 37.3 (Ms x 2), 44.5 (C49, C52), 66.9 (C50, C51), 70.7(C47, C54), 83.4 (C48, C53); HRMS (ESI), calcd for C24H52O9S2Si2Na 627.2483(M+Na+), found 627.2485.
Diene 10a. To a solution of 9a (10.8 g, 17.9 mmol) in toluene (75 mL) were added TBAI (3.45 g, 9.34 mmol) and DBU (11.0 mL, 73.2 mmol) at room temperature, then warmed to reflux. After being stirred for 20 h, the mixture was quenched with saturated aqueous NaHCO3 at 0 °C. The organic layer was washed with brine and the aqueous layer was extracted with EtOAc. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by column chromatography on silica gel (hexane/EtOAc 50:1-20:1) gave diene 10a (5.76 g, 14.0 mmol) in 78% yield: colorless oil; IR (film) ν 2927, 2855, 1471, 1255, 1092, 836, 775 cm-1; 1H-NMR (400 MHz, CDCl3) δ 0.02 (6H, s, TBS x 2), 0.03 (6H, s, TBS x 2), 0.88 (18H, s, TBS x 2), 1.31 (6H, s, Me61, Me62), 3.47 (2H, d, J = 9.8 Hz, H47, H54), 3.50 (2H, d, J = 9.8 Hz, H47, H54), 4.91 (2H, s, H50, H51), 5.40 (2H, s, H50, H51); 13C-NMR (100 MHz, CDCl3) δ -5.5 (TBS x 2), -5.4 (TBS x 2), 18.3 (TBS x 2), 25.85 (Me61, Me62), 25.92 (TBS x 2), 70.0 (C47, C54), 83.9 (C48, C53), 104.0 (C50, C51), 150.9 (C49, C52); HRMS (ESI), calcd for C22H44O3Si2Na 435.2721 (M+Na+), found 435.2719.
Bis-acetate 9b. To a mixture of 8 (9.71 g, 25.8 mmol), Et3N (18.0 mL, 129 mmol) and DMAP (1.54 g, 12.6 mmol) in CH2Cl2 (200 mL) was added Ac2O (7.5 mL, 79.3 mmol). After being stirred for 14 h at room temperature, the mixture was quenched with saturated aqueous NaHCO3 at 0 °C. The organic layer was washed with brine and the aqueous layer was extracted with EtOAc. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by column chromatography on silica gel (hexane/EtOAc 2:1-0:1) gave bis-acetate 9b (11.5 g, 25.0 mmol) in 97% yield: colorless solid; mp 73-74 °C; IR (film) ν 2941, 1731, 1359, 1242, 1173, 1045, 956, 824, 734 cm-1; 1H-NMR (400 MHz, CDCl3) δ 1.29 (6H, s, Me61, Me62), 2.12 (6H, s, Ac x 2), 2.88 (2H, m, H49, H52), 3.09 (6H, s, Ms x 2), 3.89 (2H, d, J = 11.3 Hz, H47, H54), 4.08 (2H, d, J = 11.3 Hz, H47, H54), 4.36 (2H, ddd, J = 10.4, 4.5, 1.8 Hz, H50, H51), 4.41 (2H, ddd, J = 10.4, 5.1, 2.1 Hz, H50, H51); 13C-NMR (100 MHz, CDCl3) δ 20.9 (Ac x 2), 21.2 (Me61, Me62), 37.6 (Ms x 2), 44.5 (C49, C52), 65.5 (C50, C51), 69.6 (C47, C54), 82.2 (C48, C53), 170.6 (Ac x 2); HRMS (ESI), calcd for C16H28O11S2Na 483.0965 (M+Na+), found 483.0965.
Diene 10b. To a solution of 9b (11.5 g, 25.0 mmol) in toluene (125 mL) were added TBAI (2.52 g, 6.82 mmol) and DBU (11.5 mL, 76.5 mmol) at room temperature, then warmed to reflux. After being stirred for 20 h, the mixture was quenched with saturated aqueous NaHCO3 at 0 °C. The organic layer was washed with brine and the aqueous layer was extracted with EtOAc. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by column chromatography on silica gel (hexane/EtOAc 8:1-2:1) gave diene 10b (4.81 g, 17.9 mmol) in 72% yield: colorless oil; IR (film) ν 2979, 1743, 1372, 1243, 1103, 1044, 996, 901 cm-1; 1H-NMR (400 MHz, CDCl3) δ 1.38 (6H, s, Me61, Me62), 2.08 (6H, s, Ac x 2), 4.03 (2H, d, J = 11.3 Hz, H47, H54), 4.08 (2H, d, J = 11.3 Hz, H47, H54), 4.90 (2H, s, H50, H51), 5.51 (2H, s, H50, H51); 13C-NMR (100 MHz, CDCl3) δ 20.9 (Ac x 2), 25.8 (Me61, Me62), 69.9 (C47, C54), 83.0 (C48, C53), 105.5 (C50, C51), 149.0 (C49, C52), 170.9 (Ac x 2); HRMS (ESI), calcd for C14H20O5Na 291.1203 (M+Na+), found 291.1205.
Cyclobutene 11a. A solution of 10a (5.76 g, 14.0 mmol) in hexane (700 mL) in quartz vessel was irradiated by low pressure mercury lamps (20 W x 4) for 60 h at room temperature. The resulting solution was concentrated in vacuo, purified by column chromatography on silica gel (hexane/EtOAc 50:1) to give cyclobutene 11a (4.28 g, 10.4 mmol) in 74% yield: colorless oil; IR (film) ν 2956, 2928, 2857, 1472, 1255, 1096, 836, 775 cm-1; 1H-NMR (400 MHz, CDCl3) δ 0.02 (12H, s, TBS x 2), 0.87 (18H, s, TBS x 2), 1.27 (6H, s, Me61, Me62), 2.61 (4H, s, H50 x 2, H51 x 2), 3.50 (2H, d, J = 9.6 Hz, H47, H54), 3.52 (2H, d, J = 9.6 Hz, H47, H54), 13C-NMR (100 MHz, CDCl3) δ -5.5 (TBS), -5.4 (TBS), 18.2 (TBS), 23.5 (Me61, Me62), 25.8 (TBS), 26.5 (C50, C51), 69.7 (C47, C54), 87.9 (C48, C53), 150.0 (C49, C52); HRMS (ESI), calcd for C22H44O3Si2Na 435.2721 (M+Na+), found 435.2719.
Cyclobutene 11b. A solution of 10b (4.81 g, 17.9 mmol) in hexane (700 mL) in quartz vessel was irradiated by low pressure mercury lamps (20 W x 4) for 13 h at room temperature. The resulting solution was concentrated in vacuo, purified by column chromatography on silica gel (hexane/EtOAc 8:1-1:1) to give cyclobutene 11b (3.75 g, 14.0 mmol) in 78% yield: colorless oil; IR (film) ν 2930 1745, 1454, 1371, 1233, 1148, 1039, 906, 858 cm-1; 1H-NMR (400 MHz, CDCl3) δ 1.33 (6H, s, Me61, Me62), 2.07 (6H, s, Ac x 2), 2.57-2.66 (4H, m, H50 x 2, H51 x 2), 4.05 (2H, d, J = 11.3 Hz, H47, H54), 4.07 (2H, d, J = 11.3 Hz, H47, H54); 13C-NMR (100 MHz, CDCl3) δ 20.9 (Ac x 2), 23.3 (Me61, Me62), 26.2 (C50, C51), 68.9 (C47, C54), 86.4 (C48, C53), 149.6 (C49, C52), 170.8 (Ac x 2); HRMS (ESI), calcd for C14H20O5Na 291.1203 (M+Na+), found 291.1202.
Diketone 12. To a solution of 11a (2.23 g, 5.40 mmol) in CH2Cl2 (40 mL) and MeOH (10 mL) was bubbled O3 at -78 °C for 20 min. After the color of solution turned to blue, the excess O3 was removed with a stream of air for 10 min. Me2S (600 µl, 8.10 mmol) was added to the resulting mixture at -78 °C, and then allowed to warm to room temperature. After being stirred for 1 h, the mixture was concentrated in vacuo. Purification by column chromatography on silica gel (hexane/EtOAc 50:1-20:1) gave diketone 12 (1.48 g, 3.33 mmol) in 62% yield: colorless oil; IR (film) ν 2954, 2929, 2857, 1724, 1471, 1256, 1112, 838, 777 cm-1; 1H-NMR (500 MHz, CDCl3) δ 0.04 (6H, s, TBS x 2), 0.06 (6H, s, TBS x 2), 0.88 (18H, s, TBS x 2), 1.28 (6H, s, Me61, Me62), 2.45 (2H, m, H50, H51), 3.27 (2H, m, H50, H51), 3.67 (2H, d, J = 10.1 Hz, H47, H54), 3.78 (2H, d, J = 10.1 Hz, H47, H54); 13C-NMR (100 MHz, CDCl3) δ -5.6 (TBS x 2), -5.5 (TBS x 2), 18.3 (TBS x 2), 21.3 (Me61, Me62), 25.8 (TBS x 2), 36.8 (C50, C51), 70.6 (C47, C54), 86.8 (C48, C53), 213.3 (C49, C52); HRMS (ESI), calcd for C22H44O5Si2Na 467.2619 (M+Na+), found 467.2615.
Hemiacetal 13. To a solution of 12 (430 mg, 967 µmol) in THF (10 mL) was added LiBHEt3 (1.0 M in THF, 4.84 mL, 4.84 mmol) at -78 °C. After being stirred for 30 min at the same temperature, the mixture was quenched with 2N NaOH and 35% aqueous H2O2. After being stirred for 1 h at room temperature, the excess H2O2 was quenched with aqueous Na2S2O3. The organic layer was washed with brine and the aqueous layer was extracted with EtOAc. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by flash column chromatography on silica gel (hexane/EtOAc 60:1-40:1) gave hemiacetal 13 (390 mg, 875 µmol) in 90% yield: colorless solid; mp 80-81 °C; IR (film) ν 3406, 2954, 2929, 2884, 2857, 1471, 1463, 1255, 1106, 837, 776 cm-1; 1H-NMR (400 MHz, CDCl3) δ 0.00 (3H, s, TBS), 0.01 (3H, s, TBS), 0.07 (6H, s, TBS x 2), 0.85 (9H, s, TBS), 0.89 (9H, s, TBS), 1.44 (3H, s, Me61), 1.48 (3H, s, Me62), 1.66 (1H, ddd, J = 12.9, 12.9, 4.5 Hz, H51), 1.88 (1H, ddd, J = 13.5, 9.8, 4.5 Hz, H50), 2.00 (1H, m, H50), 2.28 (1H, ddd, J = 12.9, 9.8, 5.7 Hz, H51), 3.26 (1H, d, J = 9.4 Hz, H47), 3.34 (1H, d, J = 9.6 Hz, H54), 3.38 (1H, d, J = 9.4 Hz, H47), 3.57 (1H, d, J = 9.6 Hz, H54), 3.87 (1H, s, OH), 4.14 (1H, d, J = 7.0 Hz, H49); 13C-NMR (100 MHz, CDCl3) δ -5.72 (TBS), -5.69 (TBS x 2), -5.64 (TBS), 18.0 (TBS), 18.1 (TBS), 19.9 (Me62), 23.1 (C50), 23.2 (Me61), 25.7 (TBS), 25.8 (TBS), 30.8 (C51), 68.3 (C47), 69.1 (C54), 75.1 (C48), 76.7 (C53), 80.0 (C49), 105.3 (C52); HRMS (ESI), calcd for C22H46O5Si2Na 469.2776 (M+Na+), found 469.2779.
Acetate 15. To a mixture of 13 (768 mg, 1.72 mmol), Et3N (1.2 mL, 8.6 mmol) and DMAP (215 mg, 1.76 mmol) in CH2Cl2 (17 mL) was added Ac2O (500 µL, 5.29 mmol). After being stirred for 17 h at room temperature, the mixture was quenched with saturated aqueous NaHCO3. The organic layer was washed with brine and the aqueous layer was extracted with EtOAc. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by flash column chromatography on silica gel (hexane/EtOAc 50:1-25:1) gave acetate 15 (826 mg, 1.69 mmol) in 98% yield: colorless oil; IR (film) ν 2954, 2930, 2857, 1747, 1721, 1472, 1373, 1234, 1115, 1033, 838, 777 cm-1; 1H-NMR (400 MHz, CDCl3) δ 0.00 (3H, s, TBS), 0.01 (3H, s, TBS), 0.03 (3H, s, TBS), 0.04 (3H, s, TBS), 0.85 (9H, s, TBS), 0.88 (9H, s, TBS), 1.16 (3H, s, Me61), 1.37 (3H, s, Me62), 1.73 (1H, dddd, J = 13.8, 11.7, 8.5, 5.1 Hz, H50), 2.00 (3H, s, Ac), 2.08 (1H, m, H50), 2.29 (1H, ddd, J = 11.7, 5.1, 5.1 Hz, H51), 3.07 (1H, ddd, J = 11.7, 11.7, 6.1 Hz, H51), 3.43 (1H, d, J = 10.0 Hz, H54), 3.45 (1H, d, J = 10.0 Hz, H47), 3.50 (1H, d, J = 10.0 Hz, H47), 3.60 (1H, d, J = 10.0 Hz, H54), 5.02 (1H, dd, J = 8.5, 4.0 Hz, H49); 13C-NMR (100 MHz, CDCl3) δ -5.6 (TBS), -5.5 (TBS), -5.44 (TBS), -5.41 (TBS), 17.9 (Me61), 18.20 (TBS), 18.22 (TBS), 21.19 (Ac), 21.25 (Me62), 24.4 (C50), 25.7 (TBS), 25.8 (TBS), 36.4 (C51), 69.2 (C47), 71.0 (C54), 74.6 (C49), 81.0 (C48), 85.0 (C53), 169.6 (Ac), 214.1 (C52); HRMS (ESI), calcd for C24H48O6Si2Na 511.2882 (M+Na+), found 511.2881.
Alcohol 16. To a solution of 15 (728 mg, 1.49 mmol) in THF (15 mL) was added LiBHEt3 (1.0 M in THF, 2.3 mL, 2.3 mmol) at -90 °C. After being stirred for 20 min at the same temperature, the mixture was quenched with saturated aqueous NaHCO3. The organic layer was washed with brine and the aqueous layer was extracted with EtOAc. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by flash column chromatography on silica gel (hexane/EtOAc 50:1-20:1) gave alcohol 16 (673 mg, 1.37 mmol) in 92% yield: colorless solid; mp 93-94 °C; IR (film) ν 3499, 2929, 2857, 1745, 1472, 1363, 1250, 1105, 837, 776 cm-1; 1H-NMR (400 MHz, CDCl3) δ 0.02 (3H, s TBS), 0.03 (3H, s, TBS), 0.08 (6H, s, TBS x 2), 0.88 (9H, s, TBS), 0.89 (9H, s, TBS), 1.17 (3H, s, Me61), 1.46 (3H, s, Me62), 1.61-1.81 (3H, m, H50, H51 x 2), 1.86 (1H, m, H50), 2.10 (3H, s, Ac), 3.17 (1H, d, J = 9.7 Hz, H47), 3.30 (1H, d, J = 9.7 Hz, H47), 3.40 (1H, d, J = 9.4 Hz, H54), 3.57 (1H, d, J = 9.4 Hz, H54), 3.65 (1H, dd, J = 10.5, 3.8 Hz, H52), 4.06 (1H, brs, OH), 5.29 (1H, d, J = 5.9 Hz, H49); 13C-NMR (100 MHz, CDCl3) δ -5.7 (TBS), -5.5 (TBS), 15.4 (Me62), 18.0 (TBS), 18.1 (TBS), 21.1 (Ac), 21.5 (Me61), 23.2 (C50), 25.7 (TBS), 25.8 (TBS), 26.4 (C51), 69.0 (C47), 73.8 (C49), 74.7 (C54), 78.0 (C53), 78.8 (C52), 80.8 (C48), 169.7 (Ac); HRMS (ESI), calcd for C24H50O6Si2Na 513.3038 (M+Na+), found 513.3036.
Meso-M-ring 17. To a mixture of 16 (124 mg, 253 µmol), Et3N (250 µL, 1.79 mmol) and DMAP (60 mg, 491 µmol) in CH2Cl2 (3.0 mL) was added Ac2O (125 µL, 1.32 mmol). After being stirred for 16 h at room temperature, the mixture was quenched with saturated aqueous NaHCO3. The organic layer was washed with brine and the aqueous layer was extracted with Et2O. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by flash column chromatography on silica gel (hexane/EtOAc 30:1-20:1) gave meso-M-ring 17 (127 mg, 238 µmol) in 94% yield: colorless oil; IR (film) ν 2930, 2857, 1746, 1366, 1237, 1110, 1037, 837, 776 cm-1; 1H-NMR (400 MHz, CDCl3) δ 0.01 (6H, s, TBS x 2), 0.02 (6H, s, TBS x 2), 0.87 (18H, s, TBS x 2), 1.27 (6H, s, Me61, Me62), 1.70-1.89 (4H, m, H50 x 2, H51 x 2), 2.05 (6H, s, Ac x 2), 3.30 (2H, d, J = 10.0 Hz, H47, H54), 3.33 (2H, d, J = 10.0 Hz, H47, H54), 5.04 (2H, d, J = 7.6 Hz, H49, H52); 13C-NMR (100 MHz, CDCl3) δ -5.5 (TBS x 2), 18.2 (Me61, Me62), 18.3 (TBS x 2), 21.2 (Ac x 2), 23.2 (C50, C51), 25.8 (TBS x 2), 69.7 (C47, C54), 74.4 (C49, C52), 80.2 (C48, C53), 169.6 (Ac x 2); HRMS (ESI), calcd for C26H52O7Si2Na 555.3144 (M+Na+), found 555.3148.
Mono-acetate (-)-18. To a solution of 11b (75.4 mg, 281 μmol) in MeCN (500 μL) and 0.1 M phosphate buffer (pH 8.0, 4.5 mL) was added lipoprotein lipase (TOYOBO Co., Ltd., 12.0 mg), then warmed to 45 °C. After being stirred for 16 h, the mixture was extracted with EtOAc and the organic layer was washed with brine. The organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by flash column chromatography on silica gel (hexane/EtOAc 10:1-0:1) gave mono-acetate (-)-18 (45.7 mg, 202 µmol, 85% ee) in 72% yield, diol 19 (2.6 mg, 14.1 µmol) in 5% yield, and starting material 11b in 16%. Enantiomeric excess of (-)-18 was determined by 1H-NMR analysis of the corresponding MTPA ester. (-)-18: colorless oil; [α]D21 -61.3 (c 0.85 CHCl3, 85% ee); IR (film) ν 3484, 2971, 2929, 1744, 1370, 1235, 1042, 901 cm-1; 1H-NMR (500 MHz, CDCl3) δ 1.26 (3H, s, Me61), 1.33 (3H, s, Me62), 2.07 (3H, s, Ac), 2.59-2.71 (4H, m, H50 x 2, H51 x 2), 3.08 (1H, dd, J = 9.5, 2.4 Hz, OH), 3.50 (1H, dd, J = 11.5, 9.5 Hz, H47), 3.58 (1H, dd, J = 11.5, 2.4 Hz, H47), 3.85 (1H, d, J = 11.5 Hz, H54), 4.47 (1H, d, J = 11.5 Hz, H54); 13C-NMR (100 MHz, CDCl3) δ 20.8 (Ac), 23.1 (Me61), 23.8 (Me62), 25.9 (C50 or C51), 26.1 (C50 or C51), 67.3 (C47), 68.2 (C54), 86.2 (C53), 89.4 (C48), 148.9 (C52), 150.7 (C49), 170.6 (Ac); HRMS (ESI), calcd for C12H18O4Na 249.1097 (M+Na+), found 249.1096.
TBS ether (+)-20. To a solution of (-)-18 (27.1 mg, 120 µmol) in DMF (2.5 mL) were added imidazole (30 mg, 440 µmol) and TBSCl (38 mg, 250 µmol). After being stirred for 80 min at room temperature, the mixture was quenched with saturated aqueous NaHCO3. The organic layer was washed with brine and the aqueous layer was extracted with Et2O. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by flash column chromatography on silica gel (hexane/EtOAc 50:1) gave TBS ether (+)-20 (40.1 mg, 118 µmol) in 98% yield: colorless oil; [α]D21 3.5 (c 1.04 CHCl3, 85% ee); IR (film) ν 2929, 2857, 1746, 1368, 1249, 1098, 1040, 907, 837, 776 cm-1; 1H-NMR (400 MHz, CDCl3) δ 0.015 (3H, s, TBS), 0.020 (3H, s, TBS), 0.87 (9H, s, TBS), 1.29 (3H, s, Me61), 1.32 (3H, s, Me62), 2.06 (3H, s, Ac), 2.54-2.64 (4H, m, H50 x 2, H51 x 2), 3.51 (1H, d, J = 9.6 Hz, H47), 3.53 (1H, d, J = 9.6 Hz, H47), 4.05 (2H, s, H54 x 2); 13C-NMR (100 MHz, CDCl3) δ -5.6 (TBS), -5.5 (TBS), 18.2 (TBS), 20.9 (Ac), 23.2 (Me61), 23.3 (Me62), 25.8 (TBS), 26.2 (C50 or C51), 26.4 (C50 or C51), 69.4 (C47), 69.5 (C54), 85.9 (C53), 88.6 (C48), 147.9 (C52), 151.7 (C49), 170.8 (Ac); HRMS (ESI), calcd for C18H32O4SiNa 363.1962 (M+Na+), found 363.1962.
Diketone (+)-21. To a solution of (+)-20 (40.1 mg, 118 µmol) in CH2Cl2 (4.0 mL) and MeOH (1.0 mL) was bubbled O3 at -78 °C for 20 min. After the color of solution turned to blue, the excess O3 was removed with a stream of air for 10 min. Me2S (26 µL, 350 µmol) was added to the resulting mixture at -78 °C, and then allowed to warm to room temperature. After being stirred for 3 h, the mixture was concentrated in vacuo. Purification by flash column chromatography on silica gel (hexane/EtOAc 30:1-5:1) gave diketone (+)-21 (32.8 mg, 88.0 µmol) in 75% yield: colorless oil; [α]D21 1.6 (c 1.00 CHCl3, 85% ee); IR (film) ν 2930, 2857, 1747, 1742, 1471, 1374, 1235, 1182, 1114, 1047, 839, 781 cm-1; 1H-NMR (400 MHz, CDCl3) δ 0.03 (3H, s, TBS), 0.05 (3H, s, TBS), 0.86 (9H, s, TBS), 1.26 (3H, s, Me61), 1.37 (3H, s, Me62), 2.06 (3H, s, Ac), 2.51 (1H, ddd, J = 14.9, 6.0, 5.8 Hz, H51), 2.72 (1H, ddd, J = 14.9, 10.5, 5.8 Hz, H50), 2.96 (1H, ddd, J = 14.9, 6.8, 6.0 Hz, H50), 3.30 (1H, ddd, J = 14.9, 10.5, 6.8 Hz, H51), 3.69 (1H, d, J = 10.4 Hz, H47), 3.80 (1H, d, J = 10.4 Hz, H47), 4.24 (1H, d, J = 11.3 Hz, H54), 4.29 (1H, d, J = 11.3 Hz, H54); 13C-NMR (100 MHz, CDCl3) δ -5.7 (TBS), -5.6 (TBS), 18.3 (TBS), 20.8 (Ac), 21.98 (Me61), 22.04 (Me62), 25.8 (TBS), 35.9 (C51), 36.7 (C50), 69.2 (C54), 70.3 (C47), 84.6 (C53), 87.3 (C48), 170.4 (Ac), 211.5 (C52), 212.8 (C49); HRMS (ESI), calcd for C18H32O6SiNa 395.1860 (M+Na+), found 395.1863.
Triol (-)-22. To a solution of (+)-21 (16.0 mg, 42.9 µmol) in THF (1.5 mL) were added 12-crown-4 (70 µL, 430 µmol) and LiBHEt3 (1.0 M in THF, 440 µL, 440 µmol) at -78 °C. The reaction mixture was allowed to warm to room temperature over 16 h. The mixture was quenched with 2N NaOH and aqueous H2O2 at 0 °C. After being stirred for 1 h at room temperature, the excess H2O2 was quenched with aqueous Na2S2O3. The organic layer was washed with brine and the aqueous layer was extracted with EtOAc. The combined organic layer was dried over Na2SO4, filtered and concentrated in vacuo. Purification by flash column chromatography on silica gel (hexane/EtOAc 4:1-1:1) gave triol (-)-22 (11.4 mg, 34.1 µmol) in 80% yield: colorless solid; mp 78-79 °C; [α]D23 -8.8 (c 0.93 CHCl3, 85% ee); IR (film) ν 3411, 2929, 2858, 1471, 1254, 1103, 1055, 837, 778 cm-1; 1H-NMR (400 MHz, CDCl3) δ 0.08 (6H, s, TBS), 0.90 (9H, s, TBS), 1.33 (6H, s, Me61, Me62), 1.63-1.75 (2H, m, H50, H51), 1.84 (1H, brs, OH49), 1.89-2.01 (2H, m, H50, H51), 2.60 (1H, brdd, J = 6.3, 5.3 Hz, OH54), 2.67 (1H, brs, OH52), 3.36 (1H, dd, J = 11.2, 6.3 Hz, H54), 3.48 (2H, s, H47 x 2), 3.51 (1H, dd, J = 11.2, 5.3 Hz, H54), 3.75 (1H, brd, J = 8.0 Hz, H52), 3.88 (1H, brd, J = 8.4 Hz, H49); 13C-NMR (100 MHz, CDCl3) δ -5.60 (TBS), -5.58 (TBS), 18.2 (TBS), 18.6 (Me62), 19.5 (Me61), 25.8 (TBS), 26.3 (C50), 26.8 (C51), 70.2 (C54), 71.1 (C47), 73.0 (C52), 73.7 (C49), 80.7 (C48), 81.4 (C53); HRMS (ESI), calcd for C16H34O5SiNa 357.2068 (M+Na+), found 357.2069.
X-Ray crystallographic analysis of (-)-2217
Single crystals of triol (-)-22 suitable for X-ray crystallographic analysis was obtained by recrystallization from diethyl ether / hexane at room temperature. The single crystal coated by apiezon grease was mounted on the grass fiber and transferred to the cold gas stream of the diffractometer. X-ray data were collected on Bruker AXS APEXII diffractometer with graphite monochromated Mo-Kα radiation (λ 0.71073 Å). The data were corrected for Lorentz and polarization effects. An empirical absorption correction based on the multiple measurement of equivalent reflections was applied using the program SADABS.18 The structures were solved by direct methods and refined by full-matrix least squares against F2 using all data (SHEXL-97).19 The absolute structure was deduced based on Flack parameter, 0.01(13).16
Crystal data (100 K): C16H34O5Si; Fw 334.52; monoclinic; space group P21, a = 7.4994(16) Å, b = 12.153(3) Å, c = 10.466(2) Å, β = 97.057(3)°, V = 946,7(4) Å3, Z = 2, Dcalcd = 1.174 Mg/m3, R = 0.0364 (I>2σ(I)), wR2 = 0.0785 (all data), GOF = 1.044.

ACKNOWLEDGEMENTS
This work was supported financially by a Grant-in-Aid for Specially Promoted Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan. We are grateful to Prof. T. Iwamoto and Prof. S. Ishida at Graduate School of Science, Tohoku University for their assistance of X-ray crystallographic analysis and helpful discussions.

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