Design, Synthesis, and Antimicrobial Evaluation of Novel Pyrazoles and Pyrazolyl 1,3,4-Thiadiazine Derivatives

A novel series of pyrazolyl 1,3,4-thiadiazines 5a–c, 8a–c, 12, 15a–c, 17a–c, and 20 was prepared from the reaction of pyrazole-1-carbothiohydrazide 1a,b with 2-oxo-N′-arylpropanehydrazonoyl chloride, 2-chloro-2-(2-arylhydrazono)acetate, and 3-bromoacetylcoumarin. Moreover, the regioselective reaction of 5-pyrazolone-1-carbothiohydrazide 1a with 4-substituted diazonium salts and 4-(dimethylamino)benzaldehyde gave the corresponding hydrazones 21a–c and 22. The newly prepared compounds were characterized by spectroscopy and elemental analysis. Many new synthesized compounds showed considerable antimicrobial activity against tested microorganisms. Hydrazones 21a–c and 22 showed remarkable antibacterial and antifungal activities. 4-(2-(p-tolyl)hydrazineylidene)-pyrazole-1-carbothiohydrazide 21a displayed the highest antibacterial and antifungal activities with minimum inhibitory concentration (MIC) values lower than standard drugs chloramphenicol and clotrimazole, in the range of 62.5–125 and 2.9–7.8 µg/mL, respectively.