An Efficient N-Heterocyclic Carbene Precursor for the Asymmetric Intramolecular Stetter Reaction

T. Rovis et al. has developed a chiral bicyclic triazolium salt (1) and has used it as a N-heterocyclic carbene precursor. 1 affords the related N-heterocyclic carbene 2 by a deprotonation under basic conditions. The chemical property of 2 is in showing a lower basicity effected by the electron-deficient 4-(trifluoromethyl)phenyl group of 2. Therefore, in an intramolecular Stetter reaction using 2 as an organocatalyst, the reactions proceed with high enantio- and diastereoselectivities even when substances easy to epimerize are employed. In addition, in a case using substances which easily proceed to intramolecular aldol condensations, the intramolecular Stetter reactions preferentially proceed. Furthermore, aliphatic aldehydes can be used and afford the desired 1,4-dicarbonyl compounds with high stereoselectivities. Thus, 1 is an efficient asymmetric organocatalyst because of its availability for various substances.