Neighbouring group effects in the acid-catalysed opening of steroidal epoxides

Abstract

The hydrobromic-acid-induced opening of the epoxide ring in the stereoisomeric 1-hydroxy(or -acetoxy)-2,3-epoxy- and 3-hydroxy(or -acetoxy)-1,2-epoxy-5α-cholestanes was investigated. The results are evaluated with regard to the tendency to ‘diaxial opening’ of the epoxide ring and the tendency of the nucleophile to attack at the carbon atom β with respect to the neighbouring group. The 2α,3α-epoxides give mixtures of diaxial and di-equatorial products, whereas 2β,3β-epoxides give only diaxial products. All products obtained from 1,2-epoxides are diaxial. In several instances, the epoxide ring is displaced by an appropriately oriented acetoxy-group.