Divergent Synthesis of Pyrazolo[1,5-a]pyridines and Imidazo[1,5-a]pyridines via Reagent-Controlled Cleavage of the C–N or C–C Azirine Bond in 2-Pyridylazirines

Abstract

The three-membered ring in 2-(2-pyridyl)azirine-2carboxylic esters and thioesters can undergo selective cleavage of either the N−C2 bond under copper(II) catalysis or the C−C bond under the action of HCl to provide isomeric azirine ring expansion products of pyrazolo[1,5-a]pyridine or imidazo[1,5a]pyridine series, respectively. Mild catalytic reaction conditions for the formation of pyrazolopyridines make it possible to obtain them directly from 4-bromoisoxazoles by a one-pot, three-stage procedure without isolating the intermediate 2-bromoazirines and 2-(2-pyridyl)azirines.

2H‐Azirines: Recent Progress in Synthesis and Applications

“…Pyrazolo[1,5-a]pyridines have gained the great attention of researchers because of their potential in medicinal chemistry as well as in ubiquitous natural compounds. Very recently, Novikov and coworkers [68] reported the synthesis of pyrazolo Combination of a C2À N3 fragment with a N1À C4À C5 unit has been explored for the synthesis of imidazoles. In recent years, the synthesis of imidazoles employing azirines or their equivalents as N1À C4À C5 unit has been actively studied.…”