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 alpha,beta-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.