STEREOCHEMISTRY OF EPOXIDATION OF ALLYLIC AND HOMOALLYLIC CYCLOHEXENE ALCOHOLS

The reactivity of cyclohexene-type allylic alcohols towards epoxidation reagents (peroxy acids and Bu(t)O(2)H 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 metal catalysed oxidation with Bu(t)O(2)H 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 alpha,beta-unsaturated ketones.