[1255] To a solution of ethyl 3-oxobutyrate (20.0 g, 153.7 mmol) in tetrahydrofuran (THF, 150 mL) was added N,N-dimethylformamide dimethyl acetal (DMF-DMA, 19.2 g, 161.4 mmol). The reaction mixture was stirred at 70 °C for 2 hours. After completion of the reaction, the solvent was removed by concentration under reduced pressure to give the crude product. The crude product was dissolved in ethanol (EtOH, 150 mL) and hydrazine hydrate (NH2NH2-H2O, 9.2 g, 184.4 mmol) was added slowly and dropwise. The mixture was stirred at 80 °C for 16 hours. At the end of the reaction, the ethanol was removed by concentration under reduced pressure. The residue was diluted with brine (80 mL) and extracted with ethyl acetate (200 mL × 2). The organic layers were combined, dried with anhydrous magnesium sulfate (MgSO4), filtered and concentrated under reduced pressure to afford the target compound ethyl 3-methylpyrazole-4-carboxylate (257a, 21.0 g, 87.8% yield) as a pale green solid.1H NMR (CDCl3, 400 MHz): δ 7.96 (s, 1H), 4.29 (q, J = 7.2 Hz, 2H) 2.55 (s, 3H), 1.34 (t, J = 7.2 Hz, 3H).MS (ESI) m/z (M + H) + 154.8.
