General procedure for the synthesis of 4-methyl-1H benzimidazole-2-carboxylic acid from (7-methyl-1H-benzo[d]imidazol-2-yl)methanol: To a suspension of (4-methyl-1H-benzoimidazol-2-yl)methanol (0.84 g, 5.18 mmol) in water (10 mL) was added 2M Na2CO3 solution (10 mL). To the mixture was slowly added 0.1 M KMnO4 solution (1.4 g, 8.8 mmol) dropwise. The reaction mixture was heated to 100 °C and kept for 2 h. After completion of the reaction, it was filtered while hot. The filtrate was cooled and acidified with 3N acetic acid. The resulting solid was collected by filtration, washed with water and dried under vacuum to afford the crude 4-methyl-1H-benzimidazole-2-carboxylic acid (0.56 g, 62% yield), which could be used for the next reaction without further purification. The above carboxylic acid (111.6 mg, 0.63 mmol) was suspended in DMF (3 mL), CDI (108.9 mg, 0.67 mmol) was added and the mixture was stirred for 1 hour. Subsequently, methylpiperazine (80 μL) was added and the reaction mixture continued to be stirred at room temperature for 16 hours. Upon completion of the reaction, the mixture was poured into water (50 mL) and extracted with dichloromethane. The organic phases were combined, concentrated under reduced pressure, and the residue was purified by silica gel column chromatography (silica gel 10 g; eluent: 1-8% methanol (containing 2M NH3)/dichloromethane) to give 124.3 mg of white solid product (76% yield). The mass spectrum and 1H NMR data were consistent with the target product.
