Aim. Theoretical substantiation of directions of the cyclization reaction in different solvents by means of quantum-chemical calculations of thermodynamic parameters of three tautomers of the initial N-ethyl-N'-[4-(6,7,8,9-tetrahydro-5H-[1,2,4]triazol[4,3-a]azepin-3-yl) phenyl]thiourea. Methods. Quantum-chemical calculations of relative energies, interconversion barriers, structural and thermodynamic parameters of tautomers of thiourea in dioxane, ethanol, dimethylformamide (DMFA) and

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... methane were performed on the basis of the density functional theory applying the GAUSSIAN 09W software. The influence of the solvent was taken into account within the framework of the continuum polarized model. Results. In all the solvents under study, the cyclization reaction must proceed in one direction to form the N-ethyl-4-phenyl-N'-[4-(6,7,8,9-tetrahydro-5H-[1,2,4]triazol[4,3a]azepin-3-yl) phenyl]-1,3-thiazole-2(3H)-imine. The smallest barrier of initial thiourea tautomers interconversion was observed in the presence of dioxane as a solvent; this fact indicated the advantage of synthesis conducting in this solvent precisely in comparison to ethanol, water, tetrachlormethane and DMFA. Conclusions. Dioxane is the most suitable solvent for cyclization. K e y w o r d s: 5Н-[1,2,4]triazol[4,3-а]azepines, cyclization, quantum-chemical calculations, activating energy, relative energy, saddle point (transitional state).