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 a-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.