Structure of the cationic forms of pyrimido [4,5-b][1,4]thiazines — A new type of folic acid antagonist

The protonation of 4-methoxy-, 4-amino-, and 4-dimethylamino-6-aminopyrimido4,5-b]1,4]thiazines and N-(4-methoxy-5-pyrimidinyl)acetamidine was studied by ¹H and ¹³C NMR spectroscopy. It is shown that the addition of a proton in the first three compounds takes place at the N₅ atom of the thiazine ring, whereas in the case of N-(4-methoxy-5-pyrimidinyl) acetamidine primary protonation is observed at the nitrogen atom of the amidine group, and the second proton adds to the N₁ atom of the pyrimidine ring.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 49–54, January, 1979.