The chemistry of [1,2,3]triazolo[1,5-a ]pyridines

The reactivity of [1,2,3]triazolo[1,5- a ]pyridines 1 is described. Triazolopyridines react with electrophiles in two contrasting ways, giving 3-substituted triazolopyridines 2 , or products 3 , resulting from triazolo ring opening with loss of molecular nitrogen. The triazolopyridines can be lithiated at -40degreesC by lithium diisopropylamide in ether giving regiospecifically the 7-lithio derivative. Bromotriazolopyridines have activation towards nucleophilic substitution at position 5 and 7, and benzenoid inertness at position 6. The parent compound 1a is easily hydrogenated giving tetrahydrotriazolopyridine 11a in high yield; when the triazolopyridines have substituents, the hydrogenation reaction strongly depends on the position of the substituent. Triazolopyridinium ylides of type 18 and 26 react with acetylenic esters; these reactions are influenced by the nature of solvent and the acetylenic ester used, giving different types of adducts: stable disubstituted triazolo-pyridinium ylides of type 19 and 20 , indolizines 21 , or pyrroleninylpyrazolo[5,1- a ] pyridines 22 . Photochemistry, and photochemical reactions with MP and DMAD of these ylides are also described. A new way to 2,2'-bipyridines, in two steps from triazolopyridines is reported.