Heterocycles with an azido-azomethine structural entity are interesting due to their intrinsic dynamic azide-tetrazole tautomeric equilibrium in the solution phase [1] alongside rich azide functional group chemistry [2]. Herein, a method for the synthesis of 5-substituted tetrazolo[1,5-a]pyrido[2,3-e] yrimidines from 2,4-diazidopyrido[3,2-d]pyrimidine in SNAr reactions with N-, O-, and S- nucleophiles is presented [3]. The tetrazolo[1,5-a]pyrimidine derivatives can be regarded as 2-azidopyrimidines due to present azide-tetrazole valance tautomerism and functionalized in copper(I)-catalyzed azide-alkyne dipolar cycloaddition (CuAAC) and Staudinger reactions. Equilibrium constants and thermodynamic values were determined using variable temperature 1H NMR and were found to be ΔG298 = −3.33 to −7.52 (kJ/mol), ΔH = −19.92 to −48.02 (kJ/mol) and ΔS = −43.74 to −143.27 (J/mol·K). The negative Gibbs free energy values assert tetrazole as the major tautomeric form in solutions. Furthermore, the key starting material 2,4-diazidopyrido[3,2-d]pyrimidine shows a high degree of tautomerization in different solvents presenting up to 7 tautomeric forms.