Azido-tetrazolo tautomers of methylated azolopyridazines

Azido-tetrazolo tautomeric conversions have been investigated by NMR, X-ray and semiempirical calculations in a series of new 7(8)-methylated azolopyridazines. The transannular effects of the triazole- and tetrazole-ring fusion with pyridazine have been compared, and the thermodynamic equilibrium of the 7- to 8-methyl conversions of the 6-azido-7(8)-methyltetrazolopyridazines monitored in the function of temperature by 1H NMR shifts of the pyridazine hydrogens. General rules governing the azido-tetrazolo conversions in azolopyridazines have been formulated and based on the molecular structure and strains induced in the pyridazine ring by its substituents.