Pyrimidine is essential building block of nucleic acids. Pyrimidine based drugs show a wide spectrum of biological activities, such as antiviral, antimicrobial and anticancer [1]. Therefore modifications of pyrimidine moiety and new synthesis methods toward modified pyrimidines are continuously developing. Our group has previously discussed azido group mediated aromatic substitution rearrangements in purines and quinazolines and the importance of azido-tetrazolo equilibrium in diazidopurine [2]. Herein we extend our research to pyrido[2,3 d]pyrimidines. SNAr regioselectivity patterns of the latter via different substitution routes (Scheme 1) have been explored. Also dynamic azido-tetrazolo equilibrium in various organic solvents is studied in detail and solid state tautomer forms are determined by single crystal x-ray spectroscopy.