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.

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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.

The development of methods of obtaining new condensed heterocyclic compounds from 1,2,4-triazolethiones is an urgent problem, since these compounds possess a broad spectrum of physiological activity as analgetic, vasodilating, anti-inflammatory, and bactericidal agents and show tranquilizing action [1-3]; they are used as highly effective additives to photomaterial [4], and also are inhibitors of corrosion of nonferrous metals and alloys based on them [5].

A procedure for the preparation of the title salts via regioselective halocyclization of 3-S-propargylthio-4Н-1,2,4-triazoles is reported. Stereoselectivity of electrophilic heterocyclization depends on the nature of the electrophilic reagent: bromination is better than iodobromination and iodination. The heterocyclization with tellurium tetrahalogenides leads to the formation of a mixture of geometric isomers of the salts. Their structure was confirmed by 1H NMR, 13C NMR, НМВС and single crystal X-ray diffraction analysis.

Condensed heterocycles are well-known for their excellent biological effects and they are undeniably important compounds in organic chemistry. Electrophilic cyclization reactions are widely used for the synthesis of mono-heterocyclic compounds. This review highlights the utility of electrophilic cyclization reactions as an effective generic tool for the synthesis of various condensed heterocycles containing functional groups that are able to undergo further chemical transformations, such as nucleophilic substitution, elimination, re-cyclization, cleavage, etc. This review describes the reactions of unsaturated derivatives of different heterocycles with various electrophilic agents (halogens, arylsulfanyl chlorides, mineral acids) resulting in annulation of an additional partially saturated heterocycle. The electrophilic reaction conditions, plausible mechanisms and the use of such transformations in organic synthesis are also discussed. The review mainly focuses on research published since 2002 in order to establish the current state of the art in this area. 1 Introduction2 Electrophilic Cyclization Pathways Involving a Nitrogen Nucleo­philic Center3 Electrophilic Cyclization Pathways Involving a Chalcogen Nucleophilic Center3.1 Sulfur Centers3.2 Oxygen Centers3.3 Selenium Centers4 Strategies and Mechanisms5 Conclusion

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