tert-Butyl 8-amino-1-benzyl-1,2,3,4-tetrahydro-2-oxoquinolin-3-ylcarbamate (1.0 g) was dissolved in methanol (10 mL), and acetaldehyde (599 mg), acetic acid (10 mg), and sodium cyanoborohydride (171 mg) were sequentially added to the solution under cooling on ice. The mixture was stirred at room temperature for 30 minutes, and the reaction mixture was concentrated under reduced pressure. The thus-recovered residue was dissolved in ethyl acetate, and the solution was sequentially washed with water, saturated aqueous sodium bicarbonate solution, and saturated aqueous sodium chloride solution. The organic layer was dried over sodium sulfate, and solvent was evaporated under reduced pressure. The recovered residue was purified through silica gel column chromatography (ethyl acetate: n-hexane=1:3), whereby the title compound (600 mg) was yielded.
