Unusually Reactive Cyclic Anhydride Expands the Scope of the Castagnoli–Cushman Reaction

In the course of synthesizing and testing various “azole-including” cyclic anhydrides in the Castagnoli–Cushman reaction with imines, a remarkably reactive, pyrrole-based anhydride has been identified. It displayed a remarkably efficient reaction with N-alkyl and N-aryl imines, in particular, with “enolizable” α-C-H imines which typically fail to react with a majority of known cyclic anhydrides. The reactivity of this anhydride has been justified by an efficient resonance stabilization of its enol form. This finding expands the existing arsenal of highly reactive cyclic anhydrides and further confirms the importance of anhydride enolization for an efficient Castagnoli–Cushman reaction.