Interactions of 5,6-dimethyl- (1), 3,5,6-trimethylthieno[2,3-d] pyrimidin-4(3H)-ones (2) and 2,3-dimethyl-(5-7),2-methyl-3-methoxycarbonylthieno[2,3-d]dihydropyrrolo-, -tetrahydropyrido-, etrahydroazepino[1,2-a]pyrimidin-4-ones (14-16) with nitrating mixture were investigated. For the first time it is shown, that in dependence on the presence of substituent in position 2 and 3 of pyrimidine and thiophene rings reaction goes in various directions; by electrophilic ipso-substitution of methyl groups at C-5 by nitro group, or its oxidation up to carboxyl groups with formation corresponding 5-carboxy derivatives. It is revealed, that at absence of the substituent in position 3 (compound 1) the electrophilic ipso-substitution of methyl group by nitro group with formation of 5-nitro derivative took