Copper-Catalyzed Annulation of Electrophilic Benzannulated Heterocycles with 2-Aminopyridine and 2-Aminoquinoline: Direct Access toward Polyring-Fused Imidazo[1,2-a]pyridines

We have developed a direct method for the synthesis ofpolyring-fusedimidazo[1,2-a]pyridines via a copper-catalyzed annulationof electrophilic benzannulated heterocycles with 2-aminopyridine and2-aminoquinoline. From 3-nitroindoles and 2-aminopyridine, we couldsynthesize tetracenes, viz., indole-fused imidazo[1,2-a]pyridines, and by starting from 2-aminoquinoline, we could generatepentacenes, viz., indolo-imidazo[1,2-a]quinolines.In addition, we could also extend the methodology toward the synthesisof benzothieno-imidazo[1,2-a]pyridines starting from3-nitrobenzothiophene. Furthermore, the basic photophysical propertiesof these synthesized heteroacenes were evaluated.