N,N-dimethylformamide (15 ml), 2,4-dichlorobenzyl bromide (4.45 g) and sodium bicarbonate (1.23 g) are added to 2-benzyl-5-ethoxycarbonyl-benzimidazole (2.37 g) (example 11) and the solution is heated for one hour at 60° C. After adding ethyl acetate (70 ml) and water (70 ml) and separating the solution, the organic layer is washed with water three times and extraction is performed on the aqueous layer using ethyl acetate three times. By concentrating the obtained organic layer under reduced pressure, a mixture of 2-benzyl-1-(2,4-dichlorobenzyl)-6-ethoxycarbonylbenzimidazole (0.49 g) and 2-benzyl-1-(2,4-dichlorobenzyl)-5-ethoxycarbonylbenzimidazole (0.52 g) is obtained. Separation and purification is performed using medium pressure silica gel column chromatography (eluate: hexane/ethyl acetate=1/4˜0/100).
