N-Heterocyclic Carbene-Catalyzed Umpolung Formal [3+3] Cycloaddition of Enals with Isatogens: Access to Fused Indolin-3-ones with a Tetrasubstituted Carbon Stereocenter

A novel synthetic method to access fused indolin-3-ones with a tetrasubstituted carbon stereocenter has been developed via NHC-catalyzed umpolung formal [3+3] cycloaddtion of enals with isatogens. This methodology could be also applied for the quick construction of the 6-5-5 tricyclic pyrrolo[1,2-a]indole skeleton which is frequently found as a core structure of many indole alkaloids.