After dissolving 7-methoxy-4-oxo-1,4-dihydroquinoline-6-carboxylic acid (2.58 g) in N,N-dimethylformamide (50 ml), there were added benzyl alcohol (3.29 ml), diphenylphosphoryl azide 2.51 ml) and triethylamine (1.63 ml), and the mixture was heated and stirred at 95° C. for 5 hours. The reaction solution was poured into saturated brine and extracted with ethyl acetate, and the organic layer was dried over magnesium sulfate and concentrated. The obtained residue was subjected to NH silica gel column chromatography and eluted with a solvent (ethyl acetate:methanol=5:1) to obtain the title compound (2.03 g) as a solid.
