Base-catalysed fragmentation of 5,6β-epoxy-3β-hydroxy-5β-cholestan-19-al

Abstract

Under basic conditions, 5,6β-epoxy-3β-hydroxy-5β-cholestan-19-al (1) underwent fragmentative elimination of the 10β-function, whereas the isomeric 5α,6α-epoxide (9) underwent diaxial opening of the epoxy-group. The ¹H n.m.r. spectrum of the 5β,6β-epoxide (1) and the reactivity of its 3β-methoxy-derivative (4) indicate that the fragmentation involves chelation between an intramolecular 3,19-hemiacetal (22) and the epoxide oxygen, with a chair-like transition state. The products obtained from the acid-catalysed and thermal reactions of the 5β,6β-epoxide (1) similarly implicate the intramolecular hemiacetal (22).