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The reaction of 5-(3,4-dimethoxyphenyl)-1,3,4-oxadiazole-2(3<i>H</i>)-thione <b>3</b> with formaldehyde solution and primary aromatic amines or 1-substituted piperazines, in ethanol at room temperature yielded the corresponding <i>N</i>-Mannich bases 3-arylaminomethyl-5-(3,4-dimethoxyphenyl)-1,3,4-oxadiazole-2(3<i>H</i>)-thiones <b>4a</b>–<b>l</b> or 3-[(4-substituted piperazin-1-yl)methyl]-5-(3,4-dimethoxyphenyl)-1,3,4-oxadiazole-2(3<i>H</i>)-thiones <b>5a</b>–<b>d</b>, respectively. The in vitro inhibitory activity of compounds <b>4a</b>–<b>l</b> and <b>5a</b>–<b>d</b> was assessed against pathogenic Gram-positive, Gram-negative bacteria, and the yeast-like pathogenic fungus <i>Candida albicans</i>. The piperazinomethyl derivatives <b>5c</b> and <b>5d</b> displayed broad-spectrum antibacterial activities the minimal inhibitory concentration (MIC) 0.5–8 μg/mL) and compounds <b>4j</b>, <b>4l</b>, <b>5a,</b> and <b>5b</b> showed potent activity against the tested Gram-positive bacteria. In addition, the anti-proliferative activity of the compounds was evaluated against prostate cancer (PC3), human colorectal cancer (HCT-116), human hepatocellular carcinoma (HePG-2), human epithelioid carcinoma (HeLa), and human breast cancer (MCF7) cell lines. The optimum anti-proliferative activity was attained by compounds <b>4l</b>, <b>5a</b>, <b>5c,</b> and <b>5d</b>.