To 12 mL of a chloroform solution containing 609 mg of 1-(2-(4-aminopiperidin-1-yl)ethyl)-7-methoxyquinoxalin-2(1H)-one and 414 mg of 7-chloro-3-oxo-3,4-dihydro-2H-pyrido[3,2-b][1,4]thiazine-6-carbaldehyde, 121 mg of acetic acid was added, and stirred at room temperature for 3 hours. To the reaction mixture, 618 mg of sodium triacetoxyborohydride was added, and stirred for 10.5 hours. Aqueous saturated sodium hydrogen carbonate solution was added, and the organic layer was separated. The organic layer was washed with aqueous saturated sodium chloride solution, dried over anhydrous magnesium sulfate, and the solvent was removed under reduced pressure. The residue thus obtained was purified by silica gel column chromatography [Silica Gel; Kanto Chemical Co., Inc., Silica Gel 60N, eluent; chloroform: methanol=10:1], to give 0.73 g of 1-(2-(4-((7-chloro-3-oxo-3,4-dihydro-2H-pyrido[3,2-b][1,4]thiazin-6-yl)methylamino)piperidin-1-yl)ethyl)-7-methoxyquinoxalin-2(1H)-one as a pale yellow foam. To 3 mL of an ethyl acetate solution containing 0.30 g of 1-(2-(4-((7-chloro-3-oxo-3,4-dihydro-2H-pyrido[3,2-b][1,4]thiazin-6-yl)methylamino)piperidin-1-yl)ethyl)-7-methoxyquinoxalin-2(1H)-one, 2 mL of 4 mol/L hydrogen chloride/ethyl acetate was added, and stirred at room temperature. The resulting solid was filtered to give 300 mg of 1-(2-(4-((7-chloro-3-oxo-3,4-dihydro-2H-pyrido[3,2-b][1,4]thiazin-6-yl)methylamino)piperidin-1-yl)ethyl)-7-methoxyquinoxalin-2(1H)-one hydrochloride as a pale yellow solid.
