To 20 mL of a chloroform solution containing 205 mg of 1-(2-(4-aminopiperidin-1-yl)ethyl)-7-methoxyquinoxalin-2(1H)-one and 120 mg of 2,3-dihydro-1,4-benzodithiin-6-carbaldehyde, 41 mg of acetic acid was added, and stirred at room temperature for 4.5 hours. To the reaction mixture, 216 mg of sodium triacetoxyborohydride was added, and stirred for 1 hour. Aqueous saturated sodium hydrogen carbonate solution was added, and the organic layer was separated. The organic layer was 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 195 mg of 1-(2-(4-((2,3-dihydro-1,4-benzodithiin-6-ylmethyl)amino)piperidin-1-yl)ethyl)-7-methoxyquinoxalin-2(1H)-one as a brown oil.
