Synthesis and reactions of 3-methylthiazolo[3,2-a]benzimidazole-2-carboxylic acid hydrazide: Synthesis of some new pyrazole, 1,3-thiazoline, 1,2,4-triazole and 1,2,4-triazolo[3,4-b]-1,3,4-thiadiazine derivatives pendant to thiazolo[3,2-a]benzimidazole moiety

The reaction of 3-methylthiazolo[3,2-a]benzimidazole-2-carboxylic acid ethyl ester (1) with hydrazine hydrate gives the hydrazide 2 which reacts with CS2/KOH to afford the potassium salt 3. Treatment of 3 with 1-aryl-2-bromoethanones 4a,b afforded the 1,3-thiazoline derivatives 6a,b, respectively, while the reaction of 3 with hydrazine hydrate afforded 1,2,4-triazole-3-thione derivative 9. The reaction of 9 with 1-aryl-2-bromoethanones 4a,b and with hydrazonyl chlorides 11a,b gave the 1,2,4-triazolo[3,4-b]-1,3,4-thiadiazine derivatives 10a,b and 12a,b, respectively. Treatment of hydrazide 2 with phenyl isothiocyanate in refluxing benzene gave the thiosemicarbazide derivative 16. The latter reaction gave 1,3,4-oxadiazole derivative 17 when benzene was replaced by DMF. Cyclization of the thiosemicarbazide derivative 16 with NaOH resulted in the formation of the 1,2,4-triazole-3-thione derivative 18.