N-Heterocyclic Carbene-Catalyzed Aza-Benzoin–Michael Addition–Lactamization Cascade Incorporating a Formal [4 + 1] Annulation to Access Tricyclic Indeno[2,1- b ]pyrrole Derivatives

In this letter, we disclose a N-heterocyclic carbene (NHC)-catalyzed one-pot cascade annulation that brings together ortho-formyl cinnamates and benzothiazole imines through an umpolung-driven strategy. This reaction proceeds through the NHC-catalyzed aza-benzoin reaction, followed by intramolecular C-Michael addition and γ-lactamization. The presented cascade quickly builds complex multicyclic indanone-fused heterocycles by forming two carbon-carbon bonds and one amide bond, with two five-membered ring-forming events that feature two stereogenic centers in one go. The densely functionalized products containing benzothiazole were generated in good yields under mild reaction conditions with a broad range of substrates.