Azido/tetrazole equilibrium in the thiazoloacridinone series

Compounds 2-azido-6H-thiazolo[5,4-a]acridin-11-one 3 and 2-azido-6-butylthiazolo[5,4-a]acridin-11-one 4 have been prepared for the first time. The azido/tetrazole equilibria of the most soluble compound 4 has been studied in six solvents. The resulting tautomeric equilibrium constants have been compared with those of 2-azido-4,5-dimethylthiazole 1 and 2-azidobenzothiazole 2 . The rather insoluble NH derivative 3 when dissolved in DMSO contains 66% of tetrazole 3b while the corresponding anion (obtained by adding sodium hydride) is all in the tetrazole 13b form.