Facile Synthesis of Some New Pyrazole-Based 2-Thioxo-4-thiazolidinone

5-Ethoxymethylene-2-thioxo-4-thiazolidinone (<b>1</b>) reacts with hydrazine hydrate at room temperature to afford 5-(hydrazinylmethylene)-2-thioxo-4-thiazolidinone (<b>3)</b>. Compound <b>3</b> condensed with different aromatic aldehydes <b>6a–d</b> in ethanol in the presence of a few drops of piperidine to give the corresponding Schiff’s bases <b>7a–d</b>. On the other hand, compound <b>3</b> reacts with o-hydroxybenzaldehyde derivatives <b>8a</b> and <b>8b</b> in refluxing ethanol catalyzed by a few drops of piperidine to yield 1H-inadzolyl-2-thioxo-4-thiazolidinones <b>9a</b> and <b>9b</b>. Reaction of compound <b>3</b> with α-ketoesters <b>10a</b> and <b>10b</b> or α-diketones <b>10c–e</b> in refluxing glacial acetic acid furnished the pyrazolyl-2-thioxo-4-thiazolidinone derivatives <b>11a–e</b>. Also, compound <b>3</b> reacts with some different enaminones <b>12a–f</b> in refluxing glacial acetic acid to afford the new pyrazolyl-2-thioxo-4-thiazolidinone derivatives <b>13a–f</b>. Pyrazoles <b>15a–d</b> was obtained via reaction of compound <b>3</b> with chalcones <b>14a–d</b> in dimethylformamide (DMF). The structures of all the newly synthesized products were confirmed on the basis of their elemental and spectral data, and a plausible mechanism has been postulated to account for their formation.