Quantum chemical studies of pyrimidin-4-ones 4.* Electronic structure and reactivity of substituted 2-oxo(thioxo)thieno[2,3-d]pyrimidin-4-ones. Mechanism of ipso-substitution of methyl group by nitro group

The quantum chemical calculations of the energies, electronic structures, and geometries for the products and intermediates of the ipso-nitration of 5,6-dimethyl-substituted 2-oxo- and 2-thioxothieno[2,3-d]pyrimidin-4-ones, as well as 6-methyl-5-nitrothieno[2,3-d]pyrimidin-4-one and 3,6-dimethyl-4-oxo-3,4-dihydrothieno[2,3-d]pyrimidine-5-carboxylic acid formed under the action of a nitrating mixture on 5,6-dimethylthieno[2,3-d]pyrimidin-4-one and 3,5,6-trimethylthieno[2,3-d]pyrimidin-4-one, respectively, were performed in terms of the HF and DFT (B3LYP) approximations in the 3-21G basis set. Taking into account the substituent effects in the pyrimidine cycle, solvents, and prototropic tautomerism, possible reasons for the predominant ipso-substitution of the methyl group at the C(5) atom by the nitro group of the thienopyrimidine system and differences in behavior of 3-unsubstituted and 3-substituted thieno[2,3-d]pyrimidin-4-ones were considered.