A solution of 1.0 g of 11β-acetyloxy-9-fluoro-17-(phenylthio)androsta-1,4,16-trien-3-one, 12 ml of dry dichloromethane and 0.5 ml of ethanethiol is cooled to -10° C. under nitrogen. Boron trifluoride etherate is then added. The solution is stirred at -10° C. under nitrogen for 2.5 hours. The resulting solution is diluted with dichloromethane, washed with saturated sodium bicarbonate and water, dried over anhydrous Na2SO4 and evaporated in vacuo to give a foamy solid. This is dissolved in chloroform and chromatographed on two precoated silica gel TLC plates (E. Merck, 20 cm×20 cm×2 mm, 1:4 ethyl acetate-chloroform for development) to give 800 mg of slightly impure title compound. This is crystallized from ethyl acetate-hexane to give 500 mg of title compound, melting point 145°-148° C., with consistent spectral data.
