Stereoselective Synthesis of Pyrroloquinolines by a BrOnsted Acid Catalyzed [4+2]-Heterocyclization of Indole-7-Carbaldehydes, Anilines and Electron-Rich Alkenes

The pyrrolo[3,2,1-ij]quinoline heterocyclic core is found in the structure of a variety of compounds with interesting applications and then, new efficient and flexible strategies to construct this skeleton are required. Here, a new diastereoselective tetrafluoroboric acid (HBF4)-catalyzed three-component coupling reaction of 1H-indole-7-carbaldehyde derivatives, anilines and electron-rich alkenes to give pyrrolo[3,2,1-ij]quinolines is described. The reaction involves an unusual [4+2]-heterocyclization between an in situ formed imine and an alkene. The new catalytic method, where water is the only by-product, is efficient, robust and flexible, and allows for multigram-scale synthesis.