[CH2:1]([N:6]1[C:10]2[CH:11]=[CH:12][C:13]([C:15]([O:17]C)=[O:16])=[CH:14][C:9]=2[N:8]([CH2:19][CH2:20][CH:21]([CH3:23])[CH3:22])[C:7]1=[O:24])[CH2:2][CH:3]([CH3:5])[CH3:4].[OH-].[Na+].O.Cl>CO.C(Cl)(Cl)Cl>[CH2:1]([N:6]1[C:10]2[CH:11]=[CH:12][C:13]([C:15]([OH:17])=[O:16])=[CH:14][C:9]=2[N:8]([CH2:19][CH2:20][CH:21]([CH3:23])[CH3:22])[C:7]1=[O:24])[CH2:2][CH:3]([CH3:5])[CH3:4]In 7 ml of methanol is dissolved 0.34 g of methyl 1,3-diisopentyl-2-oxo-2,3-dihydro-1H-benzimidazole-5-carboxylate. After adding 1.5 ml of 1 mol/L solution of sodium hydroxide, the mixture is stirred at 50° C. for one hour. Water and chloroform are successively added to the reaction mixture, pH is adjusted to 2 with 6 mol/L hydrochloric acid, and the organic layer is separated. The organic layer thus obtained is washed with water and saturated aqueous solution of sodium chloride successively and dried over anhydrous magnesium sulfate, and then the solvent is distilled off under reduced pressure. Thus, 0.29 g of 1,3-diisopentyl-2-oxo-2,3-dihydro-1H-benzimidazole-5-carboxylic acid is obtained as a white solid product.
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