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The general procedure for the synthesis of 1,3-dimethyl-1H-pyrazole-4-carboxylic acid from ethyl 1,3-dimethyl-1H-pyrazole-4-carboxylate is as follows: ethyl 1,3-dimethyl-1H-pyrazole-4-carboxylate (2) was stirred in an ethanol solution of sodium hydroxide for 12 hours at room temperature. After completion of the reaction, the solvent ethanol was removed by distillation under reduced pressure. The remaining solid was dissolved in an appropriate amount of water and hydrochloric acid was added slowly to a pH of about 2, at which time a white solid 1,3-dimethyl-1H-pyrazole-4-carboxylic acid (3) precipitated. The precipitate was collected by filtration and washed with cold water and finally dried in a vacuum drying oven to give the target product 1,3-dimethyl-1H-pyrazole-4-carboxylic acid (3).

[1] Journal of Heterocyclic Chemistry, 2018, vol. 55, # 4, p. 946 - 950
[2] Molecules, 2015, vol. 20, # 3, p. 4383 - 4394
[3] Patent: CN107033082, 2017, A. Location in patent: Paragraph 0015; 0016; 0028
[4] Patent: CN107033084, 2017, A. Location in patent: Paragraph 0029
[5] Patent: CN106632044, 2017, A. Location in patent: Paragraph 0014