Highly Stereoselective Formal [3 + 3] Cycloaddition of Enals and Azomethine Imines Catalyzed by <i>N</i>-Heterocyclic Carbenes

N-Heterocyclic carbenes derived from N-mesityl benzimidazolium salts are effective catalysts for generating homoenolate species from α,β-unsaturated aldehydes. The nucleophilic intermediate adds to azomethine imines, and the resulting activated carbonyl unit undergoes an intramolecular acylation event. This formal [3 + 3] cycloaddition between α,β-unsaturated aldehydes and azomethine imines catalyzed by NHC has been developed to give substituted pyridazinones in good to excellent yields with high diastereoselectivity (>20:1). The NHC-catalyzed reaction accommodates aromatic and alkyl α,β-unsaturated aldehydes and various aromatic azomethine imines. The pyridazinones can undergo selective six-membered ring opening upon addition of alcohols or amines to give esters and amides.