Synthesis of ethyl 3-oxo-2,3-dihydro-1H-pyrazole-4-carboxylate: Hydrazine monohydrate (10.0 mL, 0.20 mol) was slowly added to an ethanolic solution (400 mL) of sodium ethoxide (20.8 g, 0.31 mol) and diethyl ethoxymethylenemalonate (20 mL, 0.10 mol) under the cooling condition of ice-water bath. Subsequently, the reaction mixture was heated to reflux at 80 °C for 3 hours. Upon completion of the reaction, the mixture was diluted with water (200 mL) and the pH was adjusted to 6 with 10 M HCl solution. separation was carried out by multiple extractions with chloroform. The aqueous phase was further acidified to pH 2 and extracted again with chloroform. All organic phases were combined, dried with anhydrous sodium sulfate and concentrated under reduced pressure to give the crude product. The crude product was washed with methanol and ethyl ether and dried to afford the target compound ethyl 3-oxo-2,3-dihydro-1H-pyrazole-4-carboxylate (13.2 g, 80% yield) as a white solid. The product was characterized by 1H NMR (400 MHz, DMSO-d6): δ 7.89 (s, 1H), 4.15 (q, J = 7.12 Hz, 2H), 1.23 (t, J = 7.11 Hz, 3H).
