Enantioselective synthesis of aza-beta-lactams via NHC-catalyzed [2 + 2] cycloaddition of ketenes with diazenedicarboxylates.

N-Heterocyclic carbenes were found to be efficient catalysts for the formal [2 + 2] cycloaddition of aryl(alkyl)ketenes and diazenedicarboxylates to give the corresponding aza-beta-lactams in good yields with up to 91% ee. The N-substituent (carboxylate vs benzoyl) of diazenes played an important role to control the reaction modes (formal [2 + 2] vs [4 + 2] cycloaddtions).