Enantioselective, NHC-Catalyzed Cyclopentene-Forming Annulations

Significance

A highly enantioselective cis-cyclopentene-forming annulation mediated by a chiral N-heterocyclic carbene (NHC) catalyst, which is generated from the corresponding triazolium salt 1, is presented. By reacting aldehydes 2 and 4-oxoenones 3 in the presence of catalyst 1, cis-1,3,4-trisubstituted cyclopentenes 4 were obtained in moderate to high yields and with excellent enantioselectivities, while the formation of the trans diastereomers occurs with much less enantioinduction. Mechanistic and stereochemical investigations support a cascade sequence involving a catalytic asymmetric intermolecular aldehyde-ketone crossed benzoin reaction, followed by a NHC-promoted oxy-Cope rearrangement.