1-[(S)-1-(tert-butyldimethyl-silanyloxymethyl)-2-methylpropyl]-6-(3-chloro-2-fluorobenzyl)-7-methoxy-4-oxo-1,4-dihydroquinoline-3-carboxylic acid ethyl ester (900 mg) was added to isopropyl alcohol (3.6 mL), and 1N sodium hydroxide (3.6 mL) was added. After stirring at 70° C. for 3 hr, the completion of the reaction was confirmed by HPLC. The reaction mixture was cooled, and allowed to cool to room temperature. n-Heptane (5 mL) was added and, after stirring, the mixture was partitioned. The aqueous layer was washed with n-heptane (5 mL). 35% Hydrochloric acid (400 mg) was added to the aqueous layer, methylisopropylketone (10 mL) was added and, after stirring, the mixture was partitioned. The organic layer was washed successively with 8.5% aqueous sodium hydrogencarbonate solution (5 mL) (3 times), 0.5N hydrochloric acid (5 mL) containing sodium chloride (250 mg), and 10% brine (5 mL). After the washing, methylisopropylketone was evaporated under reduced pressure to give 6-(3-chloro-2-fluorobenzyl)-1-[(S)-1-hydroxymethyl-2-methylpropyl]-7-methoxy-4-oxo-1,4-dihydroquinoline-3-carboxylic acid (680 mg, yield 99.3%) as a yellow-white solid.
