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 (2 H -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, (2 H -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.