The derivatives of thieno[2,3-d]pyrimidine and quinazoline play an important role in a large number of heterocyclic systems. They are successfully used in agriculture, industry, medicine. These heterocycles with an exocyclic alkenyl (alkynyl) fragment and an additional nucleophilic center are interesting for the studying the regio of stereoselectivity of the electrophilic intramolecular cyclization.In order to increase of the variation of unsaturated thiothers for the study of this heterocyclic reaction, we have carried out the alkylation of the starting 2,3-dihydro-2-thioxo-3-phenyl-quinazoline-4(1H)-one and 5,6-dimethyl-2,3-dihydro-2-thioxo-3-phenylthieno[2,3-d]pyrimidin-4(1H)-one. 4-Bromo-but-2-ene, 4-bromo-2-methylbut-2-ene and 4-bromobut-1-ene was used as alkylating agents. Alkylation of the starting substrates was carried out in an alcoholic-alkaline medium at room temperature. As a result of the reaction the alkenyl thioether 2-thioxo-3-phenylquinazolin-4(1H)-one and 5,6-dimethyl-2,3-dihydro-2-thioxo-3-phenylthieno[2,3-d]pyrimidin-4(1H) – one were observed. The structure of all the compounds was confirmed by NMR spectroscopy recorded on nuclei of 1H and 13C. The confirmation of the formation of thioeters is the presence in their spectra of PMR signals of the alkenyl moiety and the absence of signals of the thioamide proton. An additional evidence of the formation of thioethers is the data of the 13C spectra, which indicate the absence of a carbon atom signal of the group C=S, whereas the presence of a chemical shift characteristic of the C-S bond confirms the direction of alkylation on the exocyclic Sulfur atom.Thus, we have obtained alkenyl thioeters of functionalized quinazolone and thienopyrimidinone, which potentially can exhibit biological activity and can be used in reactions of intramolecular electrophilic heterocyclicization. It should be noted that the position of the multiple bond and the presence of a substituent in the butenyl substituent can significantly affect the regio chemistry of heterocyclicity during halogenation, chologenogens, halogenation of thiothers, and aryltellurohalogenation.