N-Heterocyclic Carbene-Catalyzed δ-Carbon LUMO Activation of Unsaturated Aldehydes.

An N-heterocyclic carbene (NHC) catalyzed domino reaction triggered by a delta-LUMO activation of alpha,beta-gamma,delta-diunsaturated enal has been developed for the formal [4 + 2] construction of multisubstituted arenes and 3-ylidenephthalide. These two products, formed in a highly chemo- and regioselective manner, were obtained via different catalytic pathways due to a simple change of the substrate. The activation of the remote delta-carbon of unsaturated aldehydes expands the synthetic potentials of NHC organocatalysis.