Cyclic sulfonyl imines derived from ketones were identified as stable and readily prepared compounds that serve as superior electrophiles for N-heterocyclic carbene (NHC)-catalyzed annulations with a,p-unsaturated aldehydes to afford highly substituted y-lactams. Their superior reactivity and properties are highlighted by the first example of NHC-catalyzed reactions of enals with low catalyst loadings (0.5 mol %) and broad substrate scope encompassing alkyl, aryl, and heteroaromatic substituents. These findings and supporting studies suggest an alternative, ene-like mechanism rather than the catalytic generation of a catalyst-bound homoenolate equivalent.