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5‐Amino‐3‐methyl‐4‐phenylazo‐1H‐pyrazole and ethyl cyanoacetate reacted in solvent‐free media at 150°C to produce 7‐amino‐3‐phenylazo‐2‐methyl‐4H‐pyrazolo[1,5‐a]pyrimidine‐5‐one (3). A series of aromatic amines was coupled using this compound (3) and nitrous acid to produce new pyrazolo[1,5‐a] pyrimidine derivatives with two arylazo groups 4(a‐m). The structures of these dyes were determined via UV–vis, Fourier transform infrared, proton nuclear magnetic resonance, high‐resolution mass spectral data, and elemental analysis. After synthesis, the solvent and acid–base effects of the dyes were investigated within the UV–vis region. The antimicrobial properties of the dyes were also studied. All dyes exhibited activity against Gram‐positive and Gram‐negative bacteria, and even against fungi. The results were compared to conventional reference results from the antibiotics ciprofloxacin and ketoconazole. Antioxidant potentials were analyzed using in vitro antioxidant models on the basis of DPPH (1,1‐diphenyl‐2‐picrylhydrazyl) radical scavenging activities. Most of the compounds exhibited excellent antioxidant activities. In particular, compound 4b had a higher activity than Vitamin C.

Through a cyclization reaction of 2‐phenylbutyric acid with N‐phenylthiosemicarbazide and POCl3, novel 1,3,4‐thiadiazole derivatives were synthesized. Their structures were confirmed using IR, 1H NMR, and 13C NMR spectroscopies and elemental analysis. The antibacterial activities of the obtained 1,3,4‐thiadiazole derivatives were tested against Gram‐negative bacteria (Salmonella enteritidis, Salmonella typhimurium, Enterobacter aerogenes, Salmonella infantis, Salmonella kentucky, and Escherichia coli) and Gram‐positive bacteria (Staphylococcus aureus, Bacillus subtilis, and Enterococcus durans) using a disk diffusion method. Moreover, an antifungal activity experiment was performed against Candida albicans using the disk diffusion method. It was observed that the synthesized 1,3,4‐thiadiazole derivatives exhibited effective antimicrobial activity against S. aureus, E. coli, and C. albicans. Based on these results, the 1,3,4‐thiadiazole derivatives can be considered as a source of bioactive agents for pharmacological and medicinal applications.

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