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Fused heterocyclic systems with 1,2,4‐triazole scaffold arouse great interest from researchers in the branch of heterocyclic and medical chemistry because of their wide biological activities as they are considered as fungicidal, antimicrobial, analgesic, bronchodilators, antioxidant, anti‐inflammatory agents, and they are G‐quadruple stabilizers; the molecular docking data for coumarin‐containing 1,2,4‐triazoles indicate their ability to act as a urease inhibitor. Therefore, the present review aims to investigate new trends in the chemistry of heterocycles incorporating thiazolo[3,2‐b][1,2,4]triazoles, thiazolo[2,3‐c][1,2,4]triazoles, thiazino[5,1‐b][1,2,4]triazoles, thiazino[3,4‐b][1,2,4]triazoles, triazolothiazepines, and their biological characteristics. The main sections discuss: (a) the synthetic routes to the production of substituted fused heterocyclic systems, which include condensation reactions, multiple bond annulation, and the reactions of electrophilic heterocyclization. (b) Description of chemical and biological characteristics of these fused heterocycles.

A facile access to novel condensed system of [1,3]thiazolo[2′,3′:3,4][1,2,4]triazolo[1,5-a]pyrimidine is presented. This protocol consists of bromine-assisted direct electrophilic heterocyclization of 3-Nallylamine-5,6-dihydro[1,3]thiazolo[2,3-c] [1,2,4]triazole. The bromination took place in acetic acid and gave a good yield of the target product, which was dehydrobrominated by sodium acetate action.

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A few different theoretical methods for assigning the partial atomic charges were benchmarked for calculation of the hydrophilic/lipophilic index (HLI). The coefficients were selected to produce the best correlation of the HLI values with the experimental octanol-water partition. Different parameters were checked in calculations of partial charges to get the best performance of the HLI values obtained. Thus, four partitioning schemes (Coulson, Mulliken, Merz-Kollman, Ford-Wang) were benchmarked for calculations of atomic charges with six semiempirical methods (AM1, PM3, RM1, PM6, PM6-D3H4, PM7). Moreover, five distinct types of partial atomic charges (Mulliken, Hirshfeld, Löwdin, CHELPG, NPA), obtained at the Hartree-Fock and DFT levels of theory with three basis sets, were tested for their ability to produce the HLI values with the best correlation to experimental logP coefficients of 50 mono-charged organic anions. In the case of the semiempirical methods, the best correlation between the HLI and logP values (the correlation coefficient r = 0.9216) was obtained with the AM1 Ford-Wang parametric electrostatic potential charges. The Mulliken and Coulson charges calculated with the PM7 method can be used as an alternative to AM1, with the r values of 0.9107 and 0.8984, respectively. In the case of the DFT, the PBE/def2-TZVP natural population analysis charges produce the best correlation (r = 0.9220). Nevertheless, in spite of a marginally lower performance (r = 0.9159), the NPA charges computed at the PBE/def2-SVP level are more robust and can be regarded as the optimum choice for calculating the HLI values. Graphical abstract The hydrophilic/lipophilic index (HLI).