Synthesis and study of properties of 5-r-2-propargylsulfanylbenzimidazoles

Synthesis of new medicinal substances and biologically active compounds, in particular, heterocyclic ones, is an integral part of modern pharmacology development. This is one of the reasons for steady synthetic and biological interest in the chemistry of imidazoles and benzimi-dazoles. Synthesis of 2-propargylsulfanylbenzimidazole (2a) (yielding 85 %) and 5-methyl-2-propargylsulfanylbenzimidazole (2b) (yielding 56 %) has been carried out for the first time by alkylation of benzimidazole-2-thiol (1a) and 5-methylbenzimidazole-2-thiol (1b), respectively, with propargyl bromide in the KOH-DMF system. The structure of the synthesized compounds 2a,b has been studied by the ¹H and ¹³C NMR spectroscopy, as well as GC-MS. It has been found that propargyl sulfides 2a,b undergo acetylene-allene rearrangement and subsequent chemical transformations during chromatography under the GC-MS analysis conditions due to the high temperature in the injector (> 200 °C). The literature data on the synthesis of halogen- and selenium derivatives of [1,3]thiazinobenzimidazole based on 2-propargylsulfanylbenzimidazoles exist. We synthesized tricyclic fused [1,3]thiazolo[3,2- a ]benzimidazolium systems by heterocyclization of compounds 2a,b under conditions of acetylene-allene rearrangement (heating in the KOH-DMSO or MeONa-MeOH system). According to the 1H NMR data, the reactions of propargyl sulfides 2a,b with a twofold excess of iodine in chloroform proceeded to give 3-iodomethyleno-2,3-dihydro-9 H -[1,3]thiazolo[2,3- b ]benzimidazolium iodide (11) and 3-iodomethyleno-6-methyl-2,3-dihydro-9 H -[1,3]thiazolo[2,3- b ]benzimidazolium polyiodide (10b), respectively. When the solvent was changed to a more polar one and the iodination of compound 2a in glacial acetic acid at the same ratio of starting reagents (1:2) was carried out, isomerization of the heterocyclization product having an exocyclic double bond into a heterocyclization product having an endocyclic double bond in the thiazolium ring was observed. As a result, an individual 3-iodomethyl-9 H -[1,3]thiazolo[2,3-b]benzimidazolium polyiodide (12) was isolated, and its structure was studied by 1H NMR spectroscopy.