Triazolo- and Tetrazolo-Pyridines

2-Azido-azines are in equilibrium with fused tetrazoles, the position of the equilibrium being very sensitive to substituent influence, for example for tetrazolo[l,5-fl]pyridine, the equilibrium lies predominantly towards the closed form, whereas the analogous 5-chloro compound is predominantly open.  

Direct lithiation of l,2,3-triazolo[l,5-a]pyridines occurs with ease, at C-7, subsequent reaction with electrophiles being unexceptional, for example conversion into the 7-bromo derivative then allows nucleophiles to be introduced via displacement of bromide, thus providing, overall, a route to 2,6-disubstituted pyridines.  

4-triazolo[4,3-fl]pyridine nuclens can be accessed by cyclocondensation of 2-hydrazino-pyridines the synthesis of the antidepressant trazodone, shown below, is an example. Condensation of 2-hydrazinopyridines with carboxylic acids, promoted by polymer-supported triphenylphosphine with tri-chloroacetonitiile and heated with microwave irradiation, is also efficient. ° 

Oxidative closure of pyridin-2-yl hydrazones also produces l,2,4-triazolo[4,3-fl]pyridines.  

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