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General procedure for the synthesis of 1H-benzimidazole-2-carboxylic acid from 1H-benzimidazole-2-carboxylic acid: 3.0 g (18.6 mmol) of 1H-benzimidazole-2-carboxylic acid was dissolved in 100 ml of solvent at room temperature, 30 ml of thionyl chloride was added, and the reaction was carried out by refluxing the reaction for 4 hours at 79°C. After completion of the reaction, the solvent was removed by distillation under reduced pressure. To the residue was added 30 ml of ammonia and the reaction was carried out at 70 °C for 5 hours. The reaction solution was cooled and filtered to give a yellow solid product, which was dried to give the target compound. The yield was 55.0%.

[1] Patent: CN106905241, 2017, A. Location in patent: Paragraph 0133-0135
[2] Patent: CN107118249, 2017, A. Location in patent: Paragraph 0132; 0133; 0134
[3] Biochemische Zeitschrift, 1956, vol. 327, p. 422,447