Asymmetric synthesis of pyrrolo[1,2-a]indoles via bifunctional tertiary amine catalyzed [3 + 2] annulation of 2-nitrovinylindoles with azlactones†

Abstract

The chiral pyrrolo[1,2-a]indole skeleton represents a privileged structural motif in many natural products and pharmaceutical agents. Herein we developed an efficient [3 + 2] annulation of 2-nitrovinylindoles with azlactones via the cascade Michael addition and intramolecular acylation under the catalysis of a bifunctional tertiary amine in combination with DABCO, delivering a wide range of pyrrolo[1,2-a]indoles in good yields with high diastereo- and enantioselectivities. Alternatively, 7-nitrovinylindoles served as 4C synthons to perform enantioselective [4 + 2] annulation under identical conditions, smoothly affording pyrrolo[3,2,1-ij]quinoline skeletons with good to high stereoselectivities. Furthermore, a novel enantio-enriched indole-based tetracyclic skeleton was facilely derived from the obtained pyrrolo[1,2-a]indoles.