Synthesis
N,N-dimethylformamide (65 mL) was cooled at 0 °C and trichlorophosphorus (39 mL) was slowly added dropwise over a period of 30 minutes and stirring was continued at 0 °C for 1 hour. Subsequently, 2-(propane-2-ylidene)hydrazinecarboxamide (13 g, 114 mmol) was added in batches at 0 °C, and then the reaction mixture was warmed up to 70 °C and kept for 4 hours. Upon completion of the reaction, the mixture was poured into crushed ice (500 g) and neutralized to neutral with 10% sodium hydroxide solution, followed by extraction with ethyl acetate (3 x 150 mL). The organic phases were combined, washed sequentially with water (2 x 100 mL) and saturated aqueous sodium chloride solution (100 mL), dried over anhydrous sodium sulfate and concentrated to give the crude product. The crude product was purified by column chromatography (60-120 mesh silica gel), using chloroform solution containing 2-4% methanol as eluent, and finally 3-methyl-1H-pyrazole-4-carboxaldehyde (4 g, 32% yield) was obtained as solid. The product was characterized by 1H NMR (CDCl3): δ 9.96 (s, 1H), 8.0 (s, 1H), 2.6 (s, 3H).
References
[1] Patent: WO2009/144554, 2009, A1. Location in patent: Page/Page column 37
[2] Synthesis, 1997, # 10, p. 1140 - 1142
