N-Heterocyclic Carbene-Catalyzed Enantioselective Synthesis of N-Fused Polycyclic Dihydropyrido[1,2-α]-indolones.

An NHC-catalyzed enantioselective annulation reaction of enals with an activated ketone is disclosed. The approach proceeds from a formal [3 + 2] annulation of homoenolate with activated ketone and a subsequent ring expansion of γ-lactone by the nitrogen atom of indole. This strategy features a broad substrate scope, affording the corresponding DHPIs in moderate to good yields and with excellent levels of enantioselectivities. Controlled experiments have been conducted to elucidate a plausible mechanism.