Stereoselectivity of epoxidation of substituted cyclohexa-1,4-dienes: influence of an allylic methoxycarbonyl group

Abstract

Epoxidations of methyl 1-methylcyclohexa-2,5-diene-1-carboxylate (1), methyl cyclohexa-2, 5-diene-1-carboxylate (2), 3-hydroxymethyl-3-methylcyclohexa-1,4-diene (3), and the corresponding tetrahydropyranyl ether (4), have been studied. The observed stereoselectivities are explained in terms of steric hindrance to the approaching peracid, and do not require any cis-directing effect of the methoxycarbonyl group to be invoked. Epoxidation of methyl cis-6-acetoxy-1 -methylcyclohex-2-ene-1 -carboxylate (5) was highly stereoselective; the epoxides (15) and (16) were obtained in a 9 : 1 ratio, respectively.