Synthesis of (pyridinyl)‐1,2,4‐triazolo[4,3‐a]pyridines

Abstract Methods for the synthesis of (pyridinyl)‐1,2,4‐triazolo[4,3‐ a ]pyridines were developed. The principal route to the required intermediate 2‐chloropyridines was based on rearrangements of mono N ‐oxides of 2,2′‐bipyridine, 2,3′‐bipyridine, 3,3′‐bipyridine, 2,4′‐bipyridine and 4,4′‐bipyridine with phosphorus oxychloride. Reaction of 3,3′‐bipyridine 1‐oxide or 2,2′‐bipyridine 1‐oxide with phosphorus oxychloride gave mixtures of chloro isomers. Reaction with acetic anhydride, 3,3′‐bipyridine 1‐oxide and 2,2′‐bipyridine 1‐oxide gave exclusively [3,3′‐bipyridine]‐2(1 H )‐one and [2,2′‐bipyridine]‐6(1 H )‐one, respectively. 1,2,4‐Triazolo[4,3‐ a ]pyridines with pyridinyl groups at the 5,6,7 and 8 positions were synthesized.