Enantioselective Diels-Alder reactions of enals and alkylidene diketones catalyzed by N-heterocyclic carbenes.

An electron-withdrawing group was introduced to the α-position of chalcones, and the resulting alkylidene diketones showed new reactivities with enals under the catalysis of N-heterocyclic carbenes (NHCs). Selective activation of enals affords enolate equivalents that undergo highly enantioselective intermolecular Diels-Alder reactions with the alkylidene diketones. No products that might have resulted from typical homoenolate pathways were observed.