Ring-Opening Reactions of cis- and trans-2,3-Bis(4-methoxybenzyl)oxirane: Competition between Assistance by and Migration of an Aryl Group

Article abstract of DOI:10.1021/jo00364a026

The study of the aryl participation in the oxirane ring-opening reactions in acidic conditions was extended to (p-methoxybenzyl)oxiranes cis-2b and trans-3b analogous to the ones unsubstituted on the phenyl ring, 2a and 3a, previously studied.The introduction of the p-methoxy group causes in the trans epoxide 3 the appearance of the syn adduct in the methanolysis reactions and a large increase in the same adduct in the hydrolysis, acetolysis, and trichloroacetolysis reactions.For the cis epoxide 2, the presence of the para substituent causes the loss of complete anti stereoselectivity: in all the reactions but acetolysis, significant amounts of syn adduct are formed.Furthermore some reactions (see methanolysis) of the p-methoxy-substituted compounds 2b and 3b reveal the presence of significant amounts of rearranged 1,3-addition products of the type 31 and 29, respectively, contrary to the corresponding reactions of the epoxides 2a and 3a.Except for the trichloroacetolysis in CH2Cl2, all the reactions of the trans epoxide 3b show a higher syn stereoselectivity than do the corresponding reactions of the cis epoxide 2b.The results obtained for the reactions of p-methoxy-substituted epoxides 2b and 3b further confirm the mechanism previously proposed in order to rationalize the reactions of the unsubstituted epoxides 2a and 3a, which involves the neighboring participation of the phenyl.However the presence of rearranged 1,3-addition compounds among the reaction products led us to insert into the mechanistic scheme the necessary modifications which imply competition between assistance by and migration of the aryl group.