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Synlett 2008(11): 1665-1668  
DOI: 10.1055/s-2008-1078486
LETTER
© Georg Thieme Verlag Stuttgart · New York

Studies on the Synthesis of Bitungolides A-E: Synthesis of the C(1)-C(12) Fragment

Yanfen Xua, Xing Huoa, Xinyun Lia, Huaiji Zhenga, Xuegong She*a,b, Xinfu Pana
a Department of Chemistry, State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. of China
Fax: +86(931)8912582; e-Mail: shexg@lzu.edu.cn;
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. of China
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Received 11 March 2008
Publikationsdatum:
11. Juni 2008 (online)

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Abstract

Synthesis of the core segment of bitungolides A-E has been achieved in 17 steps and 4.7% overall yield. Six stereogenic centers of bitungolides A-E were established via allylation, dihydroxylation, Myers alkylation, and Evans aldol reaction

Key words

bitungolide - allylation - dihydroxylation - Myers alkylation - Evans aldol reaction

18

Selected Experimental and Spectroscopic Data
Compound 19
A solution of n-BuLi in hexane (1.85 M, 15.8 mL, 29.23 mmol) was added via cannula to a suspension of LiCl (3.94 g, 92.73 mmol) and diisopropylamine (4.4 mL, 32.86 mmol) in THF (50 mL) at -78 °C. The resulting suspension was warmed to 0 °C briefly and then was cooled to -78 °C, an ice-cooled solution of amide (-)-18 (3.39 g, 15.33 mmol) in THF (50 mL, followed by a 4 mL rinse) was added via cannula. The mixture was stirred at -78 °C for 1 h, at 0 °C for 15 min and at 23 °C for 5 min. The mixture was cooled to 0 °C, and iodide 17 (3.125 g, 7.33 mmol) was added neat to the reaction via cannula. After being stirred for 18 h at 0 °C, the reaction mixture was treated with half-saturated aq NH4Cl solution (40 mL) and the resulting mixture was extracted with EtOAc (4 × 50 mL). The combined organic extracts were dried over Na2SO4 and concentrated. Purification of the residue by flash column chromatography (hexane-EtOAc, 3:1) afforded amide 19 (3.35 g, 88%); [α]D 20 +34.2 (c 7.6, CH2Cl2). IR (KBr): 3385, 2931, 1620, 1465, 1377, 1106, 838 cm-1. 1H NMR (300 MHz, CDCL3): δ = 0.03 (s, 6 H), 0.77 (d, J = 5.7 Hz, 3 H), 0.86 (s, 9 H), 0.96-1.07 (m, 1 H), 0.99 (d, J = 7.2 Hz, 3 H), 1.11 (d, J = 6.9 Hz, 3 H), 1.27 (s, 3 H), 1.28 (s, 3 H), 1.33-1.52 (m, 4 H), 2.71-2.86 (m, 1 H), 2.82 (s, 3 H), 3.42 (dd, J = 15.3, 7.5 Hz, 1 H), 3.51 (dd, J = 10.5, 5.1 Hz, 1 H), 3.59 (dd, J = 10.5, 6.3 Hz, 1 H), 3.73-3.81 (m, 1 H), 4.38 (br s, 1 H), 4.58 (app t, J = 6.9 Hz, 1 H), 4.70 (br s, 1 H), 7.18-7.31 (m, 5 H). 13C NMR (75 MHz, CDCl3): δ = -5.3, -5.2, 14.3, 15.1, 17.1, 18.2, 24.4, 24.6, 25.8, 32.3, 34.6, 35.7, 37.7, 66.1, 67.8, 67.9, 71.2, 76.2, 100.0, 126.2, 126.8, 128.1, 142.5, 179.2. HRMS [M + H]: m/z calcd for C29H52NO5Si: 522.3609; found: 522.3614.
Aldehyde 10
n-Butyllithium (1.72 M in hexanes, 19.95 mmol) was added to diisopropylamine (3.2 mL, 21.95 mmol) in THF (30 mL) at -78 °C. After stirring the mixture at -78 °C for 10 min and at 0 °C for 10 min, BH3·NH3 (0.631 g, 20.44 mmol) was added in one portion. After stirring the mixture at 0 °C for 15 min and then at r.t. for 15 min, (-)-19 (2.597 g, 4.98 mmol) in THF (10 mL) was added via cannula at 0 °C and the mixture was stirred at r.t. for 3 h. The reaction was quenched by slow addition of sat. NH4Cl (20 mL) at 0 °C (gas evolution!). The aqueous phase was extracted with Et2O (4 × 50 mL), and the combined organic layers were washed with brine (25 mL), and dried over Na2SO4. After concentration in vacuo, purification by flash chromato-graphy (hexane-EtOAc, 10:1) provided the alcohol (2.00 g, 92% yield) as a colorless liquid; [α]D 20 -7.7 (c 5.3, CH2Cl2). IR (KBr): 3368, 2931, 1465, 1225, 839 cm-1. 1H NMR (400 MHz, CDCl3): δ = 0.03 (s, 6 H), 0.80-0.92 (m, 15 H), 1.05-1.12 (m, 1 H), 1.30 (s, 3 H), 1.31 (s, 3 H), 1.34-1.40 (m, 1 H), 1.52-1.63 (m, 3 H), 1.70-1.75 (m, 1 H), 2.15 (br, 1 H), 3.39-3.50 (m, 3 H), 3.51-3.54 (m, 1 H), 3.58-3.62 (m, 1 H), 3.79-3.83 (m, 1 H). 13C NMR (75 MHz, CDCl3): δ = -5.3, -5.2, 15.0, 15.9, 18.3, 24.5. 24.7, 25.8, 32.2, 33.1, 34.5, 36.0, 66.2, 67.9, 68.6, 71.2, 100.2.
IBX (1.46g, 5.22 mmol) was added to a solution of alcohol (1.253 g, 3.48 mmol) in DMSO. After 5 h, the reaction mixture was diluted with H2O, filtered, and extracted with Et2O, and the organic layers were dried (Na2SO4) and evaporated. The crude product was purified by flash chromatography (hexane-EtOAc, 20:1) to give 10 (1.15 g, 92%); [α]D 20 -20.7 (c 8.8, CH2Cl2). IR (KBr): 2931, 1728, 1462, 1224, 1138, 839 cm-1. 1H NMR (300 MHz, CDCl3): δ = 0.04 (s, 6 H), 0.83 (d, J = 7.2 Hz, 3 H), 0.87 (s, 9 H), 1.05 (d, J = 7.2 Hz, 3 H), 1.29 (s, 3 H), 1.30 (s, 3 H), 1.39-1.65 (m, 5 H), 2.38-2.47 (m, 1 H), 3.44-3.64 (m, 3 H), 3.77-3.86 (m, 1 H), 9.61 (d, J = 1.8 Hz, 1 H). 13C NMR (75 MHz, CDCl3): δ = -5.3, -5.2, 13.1, 14.5, 18.3, 24.5. 24.7, 25.8, 32.5, 33.3, 35.4, 44.1, 66.2, 67.8, 70.8, 100.2, 205.2.
Compound 21
To a cooled (0 °C) solution of the oxazolidinone 9 (364 mg, 1.47 mmol) in CH2Cl2 (10 mL) was added dropwise TiCl4 (0.18 mL, 1.54 mmol) and the mixture allowed to stir for 5 min. Subsequently, (-)-sparteine (0.862 g, 3.68 mmol) was added to the yellow slurry. The dark red enolate solution was stirred for 20 min at 0 °C before aldehyde 10 (632 mg, 1.76 mmol) was added dropwise and the mixture stirred for 1 h at 0 °C The reaction was quenched with half-saturated NH4Cl solution (6 mL). After separation of the layers, the organic layer was dried (Na2SO4), filtered, and concentrated in vacuo. Purification of the residue by flash chromatography (hexane-EtOAc, 8:1) afforded (0.82 g, 90%) of 21, colorless oil; [α]D 20 +28 (c 2.30, CH2Cl2). IR (KBr): 3448, 2932, 1465, 1110, 1036, 704 cm-1. 1H NMR (400 MHz, CDCl3): δ = 0.04 (s, 6 H), 0.79 (d, J = 6.8 Hz, 3 H), 0.88 (s, 9 H), 0.90 (d, J = 6.4 Hz, 3 H), 0.98 (t, J = 7.2 Hz, 3 H), 1.31 (s, 6 H), 1.35-1.44 (m, 2H), 1.52-1.77 (m, 5 H), 1.89-1.97 (m, 1 H), 2.66 (br, 1 H), 2.70 (dd, J = 13.2, 10.4 Hz, 1 H), 3.37 (dd, J = 13.2, 3.2 Hz, 1 H), 3.47 (dd, J = 15.6, 6.4 Hz, 1 H), 3.54 (dd, J = 10.4, 4.8 Hz, 1 H), 3.61 (dd, J = 10.4, 6.4 Hz, 1 H), 3.78-3.83 (m, 1 H), 4.10-4.15 (m, 1 H), 4.15 (d, J = 4.8 Hz, 2 H), 4.66-4.72 (m, 1 H), 7.22-7.34 (m, 5 H). 13C NMR (100 MHz, CDCl3): δ = -5.3, -5.2, 11.8, 14.7, 15.8, 18.3, 19.3, 24.5. 24.7, 25.8, 32.3, 33.8, 34.8, 35.4, 37.9, 46.7, 55.6, 65.8, 66.2, 67.9, 71.2, 76.4,100.2, 127.3, 128.9, 129.3, 135.2, 153.0, 176.2. HRMS [M + Na]: m/z calcd for C33H55NO7SiNa: 628.3640; found: 628.3652.
Compound 8
To a stirred solution of (Z)-26 (51 mg, 0.094 mmol) in anhyd THF (5 mL) at r.t. was added TBAF (59 mg, 0.188 mmol) in THF (59 mg, 0.188 mmol), and the mixture was stirred for 12 h. Then, H2O (10 mL) was added, the layers were separated, and the aqueous layer was extracted with EtOAc (3 × 20 mL). The combined organic layers were dried (Na2SO4) and concentrated in vacuo. The residue was purified by column chromatography (hexane-EtOAc, 4:1) to give 8 (36 mg, 90%) as a colorless oil; [α]D 20 -9 (c 1.00, CH2Cl2). IR (KBr): 3421, 2926, 1718, 1642, 1458, 1378, 1035cm-1. 1H NMR (300 MHz, CDCl3): δ = 0.70 (d, J = 7.2 Hz, 3 H), 0.75 (t, J = 7.2 Hz, 3 H), 0.89 (d, J = 6.9 Hz, 3 H), 1.24 (t, J = 7.2 Hz, 3 H), 1.30 (s, 3 H), 1.31 (s, 3 H), 1.29-1.39 (m, 2 H), 1.48-1.64 (m, 4 H), 1.71-1.81 (m, 1 H), 2.24-2.28 (m, 1 H), 3.15-3.18 (m, 1 H), 3.36 (s, 3 H), 3.42-3.49 (m, 2 H), 3.52-3.56 (m, 1 H), 3.62-3.72 (m, 1 H), 3.80-3.86 (m, 1 H), 4.11 (q, J = 7.2 Hz, 2 H), 4.60 (s, 2 H), 5.80 (d, J = 11.7 Hz, 1 H), 5.96 (m, 1 H). 13C NMR (100 MHz, CDCl3): δ = 11.5, 13.3, 14.2, 16.4, 22.8, 24.3. 24.8, 30.7, 33.4, 33.5, 34.7, 42.2, 56.0, 59.8, 65.4, 67.8, 70.9, 87.9, 98.5, 100.3, 120.5, 151.4, 166.2. HRMS: [M + Na]: m/z calcd for C23H42O7Na: 453.2823; found: 453.2823.