1,2,3-Dithiazole chemistry in heterocyclic synthesis
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The chemistry of various 5H-1,2,3-dithiazoles is investigated with emphasis on assisted ring opening and ring closure reactions leading to new heterocycles. Thus on treatment with catalytic tetraalkylammonium iodide N-(2-chloropyrid-3-yl)- and N-(4-chloropyrid-3-yl)-4-chloro-1,2,3-dithiazol-5H- imines 19 and 20 give thiazolo[5,4-b]pyridine-2-carbonitrile 16 and thiazolo-[4,5-c]pyridine-2-carbonitrile 17 respectively. Similar treatment of bisdithiazoles 29 and 30 afford high yielding routes to 1,3,4-thiadiazole-2,5- dicarbonitrile 31 and thiazole-2,4,5-tricarbonitrile 32 respectively. N-(Pyrid-3-yl)-4-chloro-1,2,3-dithiazol-5H-imine 36 reacts with secondary alkylamines to give as main product pyrido[2,3-d]pyrimidines 37 and several minor byproducts including a deep green quinoidal 2,2′-bithiazole 40. Dithiazolylidenacetonitriles 43 react with either anhydrous HBr or tetraalkylammonium chloride to afford a series of 3-halo-4-substituted- isothiazole-5-carbonitriles 45 and 52 respectively. The reactions of dithiazoles 43 with tetraalkylammonium chloride are complicated owing to the formation of isothiazolo-pentathiepin-8-carbonitrile 53, isothiazolodithiin-4,5,7- tricarbonitrile 54, tetracyanothiophene 56 and an unidentified compound 55 whose possible structures are proposed. The mechanistic rationales for the formation of the identified products are proposed.