After adding thionyl chloride (10 ml) and a catalytic amount of dimethylformamide to phenyl 7-benzyloxy-4-oxo-1,4-dihydro-6-quinolinecarboxylate (2.32 g, 6.25 mmol), the mixture was heated to reflux for 2 hours while stirring. The reaction solution was concentrated under reduced pressure and azeotropically distilled twice with toluene, the residue was dissolved in a dimethylformamide (10 ml) and triethylamine (5 ml) mixed solvent, 2M ethylamine (tetrahydrofuran solution) (6.25 ml, 12.5 mmol) was gradually added thereto while cooling in an ice water bath, and the mixture was stirred at room temperature for 3 hours. The reaction solution was distributed between ethyl acetate and water, and then the organic layer was washed with ammonia water, water and saturated saline and dried over anhydrous magnesium sulfate. The solvent was distilled off, after which ethyl acetate and then diethyl ether were added for crystallization and the crystals were filtered out and blow-dried to obtain the title compound (723 mg, 2.12 mmol, 34%) as yellow crystals.
