Enantioselective, NHC-Catalyzed Bicyclo-β-Lactam Formation via Direct Annulations of Enals and Unsaturated <i>N</i>-Sulfonyl Ketimines

A wide range of α,β-unsaturated aldehydes, including 3-alkyl derivatives, undergo N-heterocyclic carbene (NHC)-catalyzed annulations with N-sulfonyl ketimines under mild conditions to provide bicyclo[3.2.0]lactams with outstanding diastereo- and enantioselectivity. This concise route to β-lactams established four new chiral centers in a single operation. Although this process could occur via the intermediacy of a catalytically generated homoenolate equivalent, the stereochemical outcome supports a tandem or concerted aza-Benzoin/oxy-Cope reaction as the key bond-forming step.