ClC[C:3](=O)[C:4]([OH:6])=[O:5].[CH3:8][O:9][C:10]1[CH:11]=[C:12]2[C:16](=[CH:17][CH:18]=1)[NH:15][C:14](=O)[C:13]2=[O:20].[OH-].[K+].S([O-])(O)=O.[Na+].Cl>O>[OH:20][C:13]1[CH:14]=[N:15][C:16]2[C:12]([C:3]=1[C:4]([OH:6])=[O:5])=[CH:11][C:10]([O:9][CH3:8])=[CH:18][CH:17]=2
3-Chloro-2-oxo-propionic acid (11.46 g, 91.10 mmol, 1.61 eq) is added portionwise at room temperature to a stirred solution of 5-methoxy-1H-indole-2,3-dione (10.0 g, 56.45 mmol, 1.0 eq) and potassium hydroxide (30.5 g, 543.6 mmol, 9.6 eq) in water (60 mL). After 6 days stirring at room temperature, a solution of sodium hydrogen sulfite (2.3 g, 22.10 mmol, 0.4 eq) in water (4 mL) is added and the reaction mixture is acidified by the addition of concentrated hydrochloric acid (12N, 30 mL). The resulting yellow precipitate is collected by filtration, washed with a saturated sulfur dioxide aqueous solution and water, then purified by column chromatography (silica gel, eluent: ethyl acetate:acetonitrile:methanol:water, 70:5:2.5:2.5, v/v/v/v) to afford 3-hydroxy-6-methoxyquinoline-4-carboxylic acid as a light brown solid (2.66 g, 21% yield).
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