Organocatalyzed cyclopropanation of alpha-substituted alpha,beta-unsaturated aldehydes: enantioselective synthesis of cyclopropanes bearing a chiral quaternary center.

An enantioselective cyclopropanation of alpha-substituted alpha,beta-unsaturated aldehydes with bromomalonate has been successfully developed through a domino Michael/alpha-alkylation strategy. The method allows the efficient formation of cyclopropanes bearing a quaternary carbon stereocenter at the alpha-position of the aldehydes by using iminium/enamine catalysis and gives a nice extension on the organocatalytic cyclopropanation of bromomalonate and alpha,beta-unsaturated aldehydes previously reported by other groups. Very good yields (up to 81%) and enantioselectivities (up to 97% ee) have been obtained. The optically active cyclopropane derivatives are of high synthetic interest as useful targets for further elaboration into more complex structures.