To a mixed liquid of 1.40 g of methyl 3-chloro-2-hydrazino-7-methylquinoxaline-6-carboxylate and 28.0 mL of acetic acid was added 820 mg of tetrahydro-2H-thiopyran-4-ylacetaldehyde, followed by stirring at room temperature for 2 hours. The solvent was evaporated under reduced pressure, and to the residue was added 28.0 mL of N,N-dimethylformamide, followed by replacement with argon. Then, a mixed liquid of 706 mg of copper(II) chloride and 28.0 mL of N,N-dimethylformamide was added thereto, followed by stirring at 50° C. for 20 minutes and at 100° C. for 1 hour. To the reaction liquid were added water and chloroform, followed by extraction with chloroform, the organic layer was washed with 1 M hydrochloric acid, water, a saturated aqueous sodium hydrogen carbonate solution, and saturated brine and, dried over anhydrous magnesium sulfate, and then the solvent was evaporated under reduced pressure. The residue was purified by silica gel column chromatography (chloroform/methanol) to obtain 600 mg of methyl 4-chloro-8-methyl-1-(tetrahydro-2H-thiopyran-4-ylmethyl)[1,2,4]triazolo[4,3-a]quinoxaline-7-carboxylate as a brown solid.
