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The cooperative catalysis of an achiral Lewis base and a chiral N-heterocyclic carbene (NHC) enables a highly enantioselective [4 + 3] annulation of benzoxazinanones with isatin-derived enals. DMAP serves as a nucleophilic Lewis base to promote decarboxylation of benzoxazinanones, which leads to aza-ortho-xylylene intermediates that undergo [4 + 3] annulations with NHC-bound homoenolates. This method breaks the substrate scope limitation of transition-metal-catalyzed variants, thus a broader range of benzoxazinanones are tolerated, and provides a straightforward entry to enantioenriched spirooxindoles in high structural diversity.