[Cl:1][C:2]1[CH:3]=[C:4]2[C:9](=[CH:10][C:11]=1[O:12]C)[NH:8][C:7](=[O:14])[CH2:6][CH2:5]2.Br>>[Cl:1][C:2]1[CH:3]=[C:4]2[C:9](=[CH:10][C:11]=1[OH:12])[NH:8][C:7](=[O:14])[CH2:6][CH2:5]2The thus obtained 30 g of 6-chloro-7-methoxy-3,4-dihydrocarbostyril are dispersed in 300 ml of 47%-hydrobromic acid aqueous solution and heated for 4 hours under refluxing condition. After cooling of the reaction mixture, the insoluble matter is separated by filtration, washed with water and dried. Recrystallization from methanol-chloroform obtains 25 g of 6-chloro-7-hydroxy-3,4-dihydrocarbostyril in the form of colorless needle-like crystals with a melting point of 264°-266° C.
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