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

5-Ethoxymethylene-2-thioxo-4-thiazolidinone (1) reacts with hydrazine hydrate at room temperature to afford 5-(hydrazinylmethylene)-2-thioxo-4-thiazolidinone (3). Compound 3 condensed with different aromatic aldehydes 6a–d in ethanol in the presence of a few drops of piperidine to give the corresponding Schiff’s bases 7a–d. On the other hand, compound 3 reacts with o-hydroxybenzaldehyde derivatives 8a and 8b in refluxing ethanol catalyzed by a few drops of piperidine to yield 1H-inadzolyl-2-thioxo-4-thiazolidinones 9a and 9b. Reaction of compound 3 with α-ketoesters 10a and 10b or α-diketones 10c–e in refluxing glacial acetic acid furnished the pyrazolyl-2-thioxo-4-thiazolidinone derivatives 11a–e. Also, compound 3 reacts with some different enaminones 12a–f in refluxing glacial acetic acid to afford the new pyrazolyl-2-thioxo-4-thiazolidinone derivatives 13a–f. Pyrazoles 15a–d was obtained via reaction of compound 3 with chalcones 14a–d 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.