Steroidal sulphur compounds. Part VIII. Pummerer reactions of steroidal sulphoxides induced by acetic anhydride

Abstract

Regioselection in the acetic anhydride-induced Pummerer reactions of steroidal sulphoxides was influenced by steric factors associated with the axial or equatorial orientation of the sulphoxide groups and the degree of steric compression in the axial orientation. The axial 6β-alkylsulphinyl-5α-cholestanes gave only the corresponding 6-alkylthiocholest-5-enes (and in one case the Δ⁶-isomer), providing the first examples of predominant oxidation of the more substituted α-carbon atom in acetic anhydride-induced Pummerer reactions of sulphoxides, whereas the equatorial 6α-alkylsulphinyl-5α-cholestanes gave products derived mainly from oxidation of the alkyl groups. Oxidation at C-3 occurred to a larger extent for the axial 3α-methylsulphinyl-5α-cholestane than for its equatorial 3β-isomer, although both isomers gave predominantly the corresponding 3-acetoxymethylthio-5α-cholestane with retention of configuration at C-3. The reactions were not influenced by the configuration at the sulphur atom. The mechanistic implications of these observations are discussed.