On stereochemical preference in the SE2′ reaction

Abstract

Attempts are described to detect stereochemical preference during an S_E2′ reaction in a synthetic model system. 7β-Deuterio-4,5-secocholest-5-en-4-al ethylene acetal (20d) cyclised with Lewis acid to give 7-deuterio-4α-(2-hydroxyethoxy)-5β-chloest-6-ene (23c) with complete retention of deuterium. By contrast, 7α-deuterio-4,5-secocholest-5-en-4-yl p-bromobenzenesulphonate (29c) cyclised in 2,2,2-trifluoroethanol to 5β-cholest-6-ene (31) with total loss of deuterium. In both cases there is a syn-relationship between the newly formed C–C bond and the allylic hydrogen atom that is lost. The implications of these findings for the S_E2′ reaction are discussed.