Isopregnanolone → 5alpha-Pregn-2-en-20-one

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To a solution of 5α-pregnan-3β-ol-20-one (3.75 g, 11.8 mmol) in anhydrous dichloromethane (200 mL) at −78° C. under argon atmosphere was added diethylaminosulfur trifluoride (DAST) (2.30 mL, 17.7 mmol) and the resulting solution stirred over a period of 1 h at −78° C. The solution was then directly evaporated with silica gel and purified by FCC with EtOAc/hexanes (2:98) as eluant to give the desired compound 11 (880 mg, 25%) with a ratio of C2-C3 alkene vs. C3-C4 alkene isomer of 96:4. 1H NMR (acetone-d6) δ: 0.60 (s, 18-CH3), 0.78 (s, 19-CH3), 0.75-2.40 (residual CH and CH2), 2.07 (s, 21-CH3), 2.60 (t, J=9.1 Hz, 17α-CH), 5.57 (m, 2-CH and 3-CH). 13C NMR (CDCl3) δ: 11.7, 13.4, 20.9, 22.7, 24.4, 28.6, 30.2, 31.6, 31.7, 34.6, 35.6, 39.1, 39.7, 41.4, 44.2, 53.9, 56.7, 63.9, 125.8, 125.9, 209.8. LRMS for C21H32O [M+H]+: 301.1 m/z.

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