Synthesis
Over 45 minutes, dimethyl sulfate (236 g, 177 mL, 1.87 mol) was slowly added dropwise to a stirred solution of pyrazole-3-carboxylic acid (200 g, 1.78 mol) dissolved in 20% aqueous sodium hydroxide (850 mL) at 40 °C. The reaction mixture was heated at 80 °C for 2 h and subsequently cooled to room temperature. The reaction mixture was filtered and the filtrate was acidified to pH 1 with concentrated hydrochloric acid, the precipitate was collected by filtration, washed with water and dried under vacuum to afford 1-methyl-1H-pyrazole-5-carboxylic acid (85 g, 38% yield). The remaining filtrate was concentrated to 800 mL in vacuum and extracted with chloroform (15 x 400 mL) several times. The organic phases were combined, dried with anhydrous magnesium sulfate and concentrated in vacuum to remove the solvent. The residue was recrystallized by isopropanol to give 1-methyl-1H-pyrazole-3-carboxylic acid (74 g) as a white crystalline solid.
References
[1] J. Gen. Chem. USSR (Engl. Transl.), 1982, vol. 52, # 11, p. 2291 - 2296
[2] Zhurnal Obshchei Khimii, 1982, vol. 52, # 11, p. 2592 - 2598
[3] J. Gen. Chem. USSR (Engl. Transl.), 1982, vol. 52, # 11, p. 2291 - 2296
[4] Zhurnal Obshchei Khimii, 1982, vol. 52, # 11, p. 2592 - 2598
[5] Patent: WO2004/58702, 2004, A2. Location in patent: Page 30-31; 24
