Enantioselective, Cyclopentene-Forming Annulations via NHC-Catalyzed Benzoin−Oxy-Cope Reactions

Chiral N-heterocyclic carbene catalysts generated from triazolium salts promote the cyclopentene-forming annulation of α,β-unsaturated aldehydes and 4-oxoenoates with excellent levels of enantioinduction and preference for the cis-1,3,4-trisubstituted cyclopentene diastereomer. Although the observed products could arise by conjugate additions of catalytically generated homoenolates, our mechanistic and stereochemical investigations strongly support a novel reaction manifold featuring an intermolecular crossed-benzoin reaction and an NHC-catalyzed oxy-Cope rearrangement.