Triazolo pyridines, formation

There is a single example of formation of the l,8a-bond. The pyridinium salts 66 can be cyclized by chloranil or by DDQ to give 3-substituted triazolo-pyridines.94... 

The formation of thioethers as by-products occurs because the triazolo-pyridines are in equilibrium with a diazo form, and the position of the equilibrium depends on the substitution pattern on triazolopyridine (050BC3905). The presence of an electron-withdrawing sulphoxide in position 6, as is the case in 98 or 99, shifts the equUibrium to the diazo form. The formation of thioethers 104 and 105 from the diazo form, can be explained on the assumption that a carbene intermediate formed by extrusion of nitrogen (07ARK297) is trapped by an oxygen transfer from the corresponding sulphoxide (67TL2363), which is reduced to a thioether (Scheme 21). 

The oxidation of A -phenylhydrazones in the presence of pyridine leads to the formation of 5-triazolo[4,3a]pyridinium salts hy attack of pyridine as a nucleophile on the nitrilimine intermediate (Scheme 54) [77]. 

The [l,2,4]triazolo[l,5- ]pyridine system is usually constructed by the closure of the triazole ring either by oxidative formation of the NN bond or by condensation of the N-aminopyridine derivatives . Various compounds 78 were obtained by the oxidation of the amidine 77 with Mn02 (Scheme 47) <2003IJB2901, 2005ARK(xiii)21>. A dehydrative cyclization by treatment of formamidoximes 79 with trifluoroacetic anhydride (TFAA) has been used in the synthesis of 80 (Scheme 48), which are key building blocks in the preparation of DPP (IV) inhibitors <2005EJ03761, 2006JME3614>. 

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