To access, purchase, authenticate, or subscribe to the full-text of this article, please visit this link: http://dx.doi.org/10.1007/s11172-015-0897-3 Byline: M. Kh. Mamarakhmonov (1), L. I. BelenA[acute accent]kii (2), N. D. Chuvylkin (2), M. A. Ashirmatov (1), B. Zh. Elmuradov (1), I. S. Ortikov (1), Kh. M. Shakhidoyatov (1) Keywords: thieno[2,3-d]pyrimidin-4-ones; nitric acid oxidation; ipso-substitution; quantum chemical calculations; Hartree--Fock method (HF); DFT/B3LYP-3-21G method Abstract: Quantum chemical calculations of energies, electronic structures, and molecular geometries of 3,5,6-trimethylthieno[2,3-d]pyrimidin-4-one (1), tricyclic 2,3-tri-, 2,3-tetra-, and 2,3-pentamethylene-substituted 5,6-dimethylthieno[2,3-d]pyrimidin-4-ones (2--4), carboxylic acids 5--8 resulting from oxidation of compounds 1--4 at the methyl group in position 5 of thienopyrimidine, methyl esters of these acids (9--12), and products of ipso-nitration of esters (13--16) were performed in terms of the HF and DFT (B3LYP) approximations with the 3-21G basis set. Also, calculations for mixed anhydrides of nitric acid and carboxylic acids 17--20 were carried out. The possible causes of differences in the behaviors of 3-unsubstituted and 3-substituted thieno[2,3-d]pyrimidin-4-ones were discussed. Author Affiliation: (1) S. Yu. Yunusov Institute of Plant Products, Uzbekistan Pepublic Academy of Sciences, 77 ul M. Ulugbek, 100170, Tashkent, Republic of Uzbekistan (2) N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prospect, 119991, Moscow, Russian Federation Article History: Registration Date: 17/11/2015 Received Date: 15/12/2014 Online Date: 19/11/2015 Article note: For Part 4, see Ref. 1. Dedicated to Academician of the Russian Academy of Sciences V. I. Minkin on the occasion of his 80th birthday. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 0534--0539, March, 2015.