Heterocyclic synthesis via catalysis of N-heterocyclic carbenes: very classical and very modern chemical species.

After a long history of medical and biochemical investigations of vitamin B1, N-heterocyclic carbenes (NHCs) have recently been used as organocatalysts for a variety of synthetic reactions. This review article highlights the application of NHC-catalyzed reactions including enantioselective reactions to the heterocyclic synthesis. NHC-catalyzed benzoin condensation and Stetter reaction of ether- and amine-linked aldehydes give four- to six-membered oxygen and nitrogen heterocycles. NHC-catalyzed conjugate additions of enals to ketones and imines afford γ-lactones and γ-lactams. When the NHC-catalyzed reaction intermediates of enals are protonated, the reaction of the resulting enols with enones and ene-imines furnish formal hetero-Diels-Alder products. Acyl fluoride and esters can be activated by NHCs, and subsequent aldol and Michael reactions give β-lactones. Oxidation of intermediates derived from enals or the intermediates from ynals provide unsaturated acyl azolium which are transformed into cyclic products via subsequent nucleophilic reactions. NHCs also catalyze [2+2] cycloadditions of ketenes and other heterocumulenes and ring expansion of cyclic aldehydes. [ABSTRACT FROM AUTHOR]