A solution of 1.01 g (2.5 mmole) of 11β-acetyloxy-17-(ethylthio)-9-fluoroandrosta-1,4,16-trien-3-one in 15 ml of dry dichloromethane and 3.4 ml of a solution of methyl mercaptan in dichloromethane (1.82 g in 10 ml of dry dichloromethane) is cooled to about -40° C. (acetonitrile-dry ice bath) under nitrogen; boron trifluoride etherate (0.7 ml) is then added. The solution is stirred at approximately -40° C. under nitrogen for 3 hours, quenched with a saturated sodium bicarbonate solution at -40° C. under vigorous stirring, diluted with chloroform, washed with water, dried over anhydrous Na2SO4 and evaporated in vacuo to give 1.0 g of the title compound, melting point 185°-186° C., with consistent spectral data.
