Oxidation of secondary allylic acetates with chromium(VI) reagents

Abstract

Steroidal allylic acetates which can form strongly resonance stabilized allylic rations are oxidized smoothly by chromic acid in acidic medium to the corresponding αβ-unsaturated ketones. Axial acetates are more reactive than the equatorial counterparts. Oxidation of the quasi axial 3α-acetoxycholest-4-ene is 2.7 times faster than that of the quasi equatorial 3β-acetoxycholest-4-ene. The difference is very sharp when the allylic system is in ring B which has less conformational flexibilitythan ring A. Thus, 3β,6β-diacetoxy-5α-cholest-7-ene is oxidized smoothly to the corresponding 7-en-6-one, whereas the stereoisomeric 3β,6α-diacetoxy-5α-cholest-7-ene remains unchanged. A similar difference was observed with α-substituted cyclopropanes such as 6β-acetoxy-3α-5-cyclo-5α-cholestane and 6α-acetoxy-3α,5-cyclo-5α-cholestane.