To a solution of 1α-(tert-butyldimethylsilyloxy)-3β-hydroxy-20(S)-phenoxycarbonylthiopregna-5,7,16-triene (81.6 mg, 0.140 mmol) and 4-bromo-2-methyl-2-butanol (117 mg, 0.700 mmol) in tetrahydrofuran (1 ml) was added 1M KOH solution in methanol (1 ml), and the mixed solution was stirred at room temperature for 30 minutes, then concentrated under reduced pressure. The residue was diluted with hexane, washed with brine and dried over magnesium sulfate, and then the solvent was distilled off under reduced pressure. The resulting residue was purified by preparative thin layer chromatography (0.5 mm×2, dichloromethane:ethyl acetate=8:1, developed once) to give the title compound as a colorless oil (60.3 mg, 79%).
