Chiral N-Heterocyclic Carbene Catalyzed Staudinger Reaction of Ketenes with Imines:  Highly Enantioselective Synthesis of N-Boc -Lactams.

N-Heterocyclic carbenes (NHCs) were demonstrated to be efficient catalysts for the Staudinger reaction of ketenes with N-tosyl, N-benzyloxycarbonyl, or N-tert-butoxycarbonyl imines. Chiral NHC 8b, conveniently prepared from l-pyroglutamic acid, catalyzed the reactions of arylalkylketenes with a variety of N-tert-butoxycarbonyl arylimines to give the corresponding cis--lactams in good yields with good diastereoselectivities and excellent enantioselectivities (up to 99% ee). Two possible catalytic pathways, initiated by the addition of NHC to ketenes or imines, were discussed. [ABSTRACT FROM AUTHOR]