[OH-:1].[Na+].BrC(Br)[C:5]([C:7]1[CH:8]=[C:9]2[C:14](=[CH:15][CH:16]=1)[NH:13][C:12](=[O:17])[CH2:11][CH2:10]2)=[O:6]>O>[C:5]([C:7]1[CH:8]=[C:9]2[C:14](=[CH:15][CH:16]=1)[NH:13][C:12](=[O:17])[CH2:11][CH2:10]2)([OH:6])=[O:1]Into 250 ml of water, 26 g sodium hydroxide was dissolved, then at a temperature of 90°-100° C. and under stirring condition, 35 g of 6-dibromoacetyl-3,4-dihydrocarbostyril was added thereinto and reacted for 3 hours. After the reaction was completed, the reaction mixture was cooled and insoluble matters formed in the mixture were removed by filtration. The mother liquor was acidified with concentrated hydrochloric acid and the crystals precipitated were collected by filtration and washed with water. The crystals thus obtained were recrystallized from ethanol twice to obtain 10.5 g of 6-carboxy-3,4-dihydrocarbostyril in the form of light yellow amorphous crystals. Melting point: 324.5°-327° C. (decomposed).
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