To a solution of anhydrous hydrogen fluoride (6 g) in tetrahydrofuran (11.8 g) and chloroform (6 ml) contained in a screw-capped polyethylene bottle chilled to about -60° C. was added a solution of 4-acetamino-1,2α-epoxy-androsta-4,6-diene-3,17-dione (3.554 g, 10 mmol) in chloroform (30 ml) likewise chilled to about -60° C. The hydrogen fluoride-tetrahydrofuran reagent was immersed in an acetone-dry ice bath while the steroid was being added. Additional chloroform (6 ml) was used to aid the transfer of the epoxide. Subsequently the reaction mixture was maintained at -30° C. for 4 h and then added gradually to a well agitated mixture of an aqueous solution of potassium carbonate, chloroform and ice. The organic layer was separated, the alkaline aqueous layer back-extracted twice with chloroform and the combined organic layers were evaporated to dryness. The residue was submitted to flash chromatography with ethyl acetate-ethanol to give pure 4-acetylamino-2-fluoroandrosta-1,4,6-triene-3,17-dione in 70% yield (2.502 g).
