Design, synthesis and antimicrobial activities of novel 1,3,5-thiadiazine-2-thione derivatives containing a 1,3,4-thiadiazole group

A series of novel 1,3,5-thiadiazine-2-thione derivatives containing a 1,3,4-thiadiazole group was designed and synthesized. The structures of all the compounds were well characterized using (1)H NMR, (13)C NMR and high-resolution mass spectrometer, and further confirmed by the X-ray diffraction analysis of 8d. The antimicrobial activities of all the target compounds against Xanthomonas oryzae pv. oryzicola, X. oryzae pv. oryzae, Rhizoctonia solani and Fusarium graminearum were evaluated. The in vitro antimicrobial bioassays indicated that some title compounds exhibited noteworthy antimicrobial effects against the above strains. Notably, the compound N-(5-(ethylthio)-1,3,4-thiadiazol-2-yl)-2-(5-methyl-6-thioxo-1,3,5-thiadiazinan-3-yl)acetamide (8a) displayed obvious antibacterial effects against X. oryzae pv. oryzicola and X. oryzae pv. oryzae at 100 μg/mL with the inhibition rates of 30% and 56%, respectively, which was better than the commercial bactericide thiodiazole-copper. In addition, the anti-R. solani EC(50) value of 8a was 33.70 μg/mL, which was more effective than that of the commercial fungicide hymexazol (67.10 μg/mL). It was found that the substitutes in the 1,3,5-thiadiazine-2-thione and the 1,3,4-thiadiazole rings played a vital role in the antimicrobial activities of the title compounds. More active title compounds against phytopathogenic microorganisms might be obtained via further structural modification.