Intramolecular catalysis. Part II. Electrophilic anchimeric assistance by a hydroxy-group in the opening of steroidal epoxides by azide anions

Abstract

5α,6α-Epoxycholestane and its 3-substituted derivatives and 4α,5α-epoxycholestane and its 7-substituted derivatives have been prepared and their structures established. The stereochemistry of epoxidation of 7-substituted cholest-4-enes and 3-substituted cholest-5-enes with m-chloroperbenzoic acid is discussed in detail. The reactions of the 4α,5α- and 5α, 6α-epoxides with sodium azide in refluxing acetone–water (2 : 1) gave the corresponding trans-diaxial hydroxy-azides. The presence of a 7α-hydroxy-group in a 4α,5α-epoxycholestane, and of a 3α-hydroxy-group in a 5α,6α-epoxycholestane, strongly accelerated the opening of the epoxide ring by the nucleophile. Evidence is given for an intramolecular electrophilically assisted reaction, and various factors which may affect the mechanisms of these reactions are discussed.