Preparation of 5‐substituted‐ and 3,5‐disubstituted‐ s ‐triazolo[4,3‐ a ]pyridines

Cyclization of N ‐acyl‐ N ′‐(6‐chloropyrid‐2‐yl)hydrazines ( 2a‐2e ) with phosphorus oxychloride has produced several 5‐chloro‐ s ‐triazolo[4,3‐ a ]pyridines ( 3a‐3e ). Nucleophilic displacement of the chlorosubstituent of 5‐chloro‐ s ‐triazolo[4,3‐ a ]pyridine ( 3a ) availed the 5‐ethoxy ( 4a ) and 5‐thioethoxy ( 4b ) derivatives and di( s ‐triazolo[4,3‐ a ]pyrid‐5‐yl)sulfide ( 8 ) while reaction of 5‐ethylsulfonyl‐ s ‐triazolo[4,3‐ a ]pyridine ( 4d ) with potassium hydroxide yielded the 5‐hydroxy/5‐one system ( 4c or 6 ). Further reaction of 3a with bromine to give 3‐bromo‐5‐chloro‐ s ‐triazolo‐[4,3‐ a ]pyridine ( 3g ) has provided the corresponding 3‐cyano‐ and 3‐carboxamido‐5‐chloro‐ s ‐triazolo[4,3‐ a ]pyridine derivatives ( 3h and 3i ). Treatment of 6‐chloro‐2‐hydrazinopyridine ( 1 ) with cyanogen bromide has provided 3‐amino‐5‐chloro‐ s ‐triazolo[4,3‐ a ]pyridine ( 3f ) which, with bromoacetaldehyde dimethyl acetal, transformed into 7‐chloroimidazo[1,2‐ b ]‐ s ‐triazolo[4,3‐ a ]‐pyridine ( 7 ). Finally, attempts at cyclizing N ‐oxalyl‐ N ′‐(6‐chloropyrid‐2‐yl)hydrazine derivatives ( 2g‐2i ) with intentions of preparing various 3‐acyl‐5‐chloro‐ s ‐triazolo[4,3‐ a ]pyridines for entry into other 3,5‐disubstituted systems were unsuccessful.