Synthesis Characterization and Antimicrobial Activity of Pyrazolo[3,4-D]thiazole Derivatives of 1,2,4-Triazole

A novel series of five pyrazolo[3,4-d]thiazole derivatives of 1,2,4-triazole [9a–e] were synthesized via a two-step condensation–cyclization sequence starting from 4-thiazolidinone intermediates [5a–e]. The first step involved condensation with furfuraldehyde to form 5-arylidene derivatives [8a–e], followed by cyclization with isoniazid in glacial acetic acid. The target compounds were obtained in moderate yields (53–56 %) as stable crystalline solids and were fully characterized by elemental analysis, FT-IR, ¹H NMR, ¹³C NMR, and LC-MS spectroscopy. All spectral data confirmed the successful formation of the fused pyrazolo[3,4-d]thiazole ring system and the incorporation of the isonicotinoyl and substituted furan moieties. The synthesized derivatives were screened for in vitro antibacterial activity against two Gram-positive (Bacillus megaterium and Staphylococcus aureus) and two Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacterial strains using the agar cup diffusion method at 1000 ppm concentration. All compounds exhibited moderate to good antibacterial activity, with the chloro-substituted derivative 9b emerging as the most potent analogue, producing the largest zones of inhibition (15–17 mm) across all tested strains. The results demonstrate that the pyrazolo[3,4-d]thiazole–1,2,4-triazole hybrid scaffold functionalized with a para-chlorophenylfuran unit represents a promising pharmacophore for the development of new broad-spectrum antimicrobial agents.