A 4-neck flask (500 mL) equipped with a reflux condenser was charged with 2-acetoxymethyl-1-(tetrahydropyran-4-yl)benzimidazole-5-carboxylic acid (5.98 g, 18.79 mmol), anhydrous tetrahydrofuran (158 mL), and oxalyl chloride (2.86 g, 22.50 mmol), and anhydrous N,N-dimethylformamide (1 mL) was then added dropwise at room temperature, after which the reaction mixture was stirred at 50° C. for 5 hours. After being allowed to cool to room temperature, the solvent was distilled off under reduced pressure to give a pale yellow residue, which was taken up in anhydrous tetrahydrofuran (100 mL), and then 2-aminobenzenethiol (2.35 g, 18.79 mmol) was added, and this was stirred for 19 hours at 50° C. This was allowed to cool to room temperature, and after the precipitated crystals were filtered off, they were successively washed with distilled water and dried under reduced pressure with heating to yield the title compound (9.00 g, quant.) as a white solid.
