NHC-Catalyzed Asymmetric α-Regioselective [4 + 2] Annulation to Construct α-Alkylidene-δ-lactones.

The unprecedented NHC-catalyzed [4 + 2] annulation of α-bromoenals with dioxopyrrolidines is described. This protocol features broad substrate scope and allows rapid assembly of α-alkylidene-δ-lactones in good to high yields with excellent enantioselectivities. Notably, this process includes α-regioselective activation of azolium dienolate intermediates, which has not yet been reported.