Anomalous Grignard reactions on steroidal epoxides

Abstract

The reactions of epimeric 6β-acetoxy-4,5-epoxy- and 4β-acetoxy-5,6-epoxy-cholestanes with methylmagnesium iodide afforded 6α-methyl-4,6β- and 4α-methyl-4β,6-diols, respectively. The products were characterised by various transformations including degradation to β-methyl ketones, which were also obtained by an independent route. A mechanism of Grignard attack is postulated which entails prior migration to the 5-position of the proton attached to the acetate-bearing carbon atom, giving directly the appropriate β-ketol, which then undergoes nucleo-philic alkylation by the reagent.