Reaction of cyclohexanone benzylimines with ethylidenemalonate diesters.: Diphenyl 2-ethylidenemalonate:: A highly electrophilic synthetic equivalent of crotonic esters

The diastereoselective Michael alkylation of alpha -substituted and alpha,alpha'-disubstituted cyclohexanone benzylimines with ethylidenemalonate diesters was carried out for mechanistic and synthetic purposes. In the first case, an inverse regioselectivity occurred in comparison with what is generally observed since the Michael adducts resulted from alkylation of the non substituted enamine tautomer. With a,a'-disubstituted imines, in all cases, the stereochemistry of the major diastereomer was the one anticipated from a mechanism including a chairlike complex approach with a preferred exo position for the B-methyl group of the ethylidenemalonic acid diesters. Furthermore, diphenyl 2-ethylidenemalonate 4 was found to be a highly electrophilic synthetic equivalent of crotonic esters.