Stereochemistry of epoxidation of allylic and homoallylic cyclohexene alcohols

Abstract

The reactivity of cyclohexene-type allylic alcohols towards epoxidation reagents (peroxy acids and Bu^tO₂H with transition metal catalysts) is largely dependent on the magnitude of steric hindrance in the substrate molecules. With unhindered [2(R = H)] or slightly hindered allylic alcohols (4, 8 and 12) the reaction is dominated by the syn-stereodirecting effect of the hydroxy group which results in the exclusive or predominant formation of cis-epoxy alcohols. In contrast, this well established type of stereocontrol fails with sterically congested substrates (23, 26 and 27), which give trans-epoxy alcohols on m-chloroperoxybenzoic acid treatment while the transition metalcatalysed oxidation with Bu^tO₂H affords conjugated ketones as the sole products. The latter reaction can serve as a mild procedure for the selective oxidation of hindered allylic alcohols to α,β-unsaturated ketones.