Synthesis of Imidazo[1,2‑a]pyridines via Near UV Light-Induced Cyclization of Azirinylpyridinium Salts

An efficient one-pot synthesis of imidazo­[1,2-a]­pyridines from 2-bromoazirines and pyridines has been developed. The construction of the bicyclic framework of imidazo­[1,2-a]­pyridines occurs in two steps through the formation of (2H-azirin-2-yl)­pyridinium bromides followed by dehydrobrominative UV light-induced cyclization. The method can also be applied for the synthesis of imidazo­[2,1-a]­isoquinolines. Unstable in solution, (2H-azirin-2-yl)­pyridinium/isoquinolinium bromides were quantitatively converted to stable tetrafluoroborates, which can be cyclized to imidazo­[1,2-a]­pyridines under UV irradiation in the presence of bromide ions.