[CH3:1][O:2][C:3]1[CH:4]=[C:5]2[C:10](=[CH:11][C:12]=1[CH:13]=[O:14])[N:9]([CH3:15])[C:8](=[O:16])[CH2:7][CH2:6]2.P([O-])(O)(O)=[O:18].[Na+].CC(=CC)C.Cl([O-])=O.[Na+]>C(O)(C)(C)C.O>[CH3:1][O:2][C:3]1[CH:4]=[C:5]2[C:10](=[CH:11][C:12]=1[C:13]([OH:18])=[O:14])[N:9]([CH3:15])[C:8](=[O:16])[CH2:7][CH2:6]2To a mixture of 6-methoxy-1-methyl-2-oxo-1,2,3,4-tetrahydroquinoline-7-carbaldehyde (2.0 g) synthesized by a known method (WO2001/077100), sodium dihydrogen phosphate (1.63 g) and 2-methyl-2-butene (4.3 ml) in tert-butanol (52 mL) and water (8 mL) was slowly added sodium chlorite (2.89 g) at 0° C., and the mixture was stirred at room temperature for 2 hr. After cooling to 0° C., the reaction was quenched with 6N hydrochloric acid, and the mixture was extracted twice with ethyl acetate. The organic layer was washed with brine and dried, and the solvent was evaporated under reduced pressure. The obtained crystals were collected by filtration, and washed with hexane to give 6-methoxy-1-methyl-2-oxo-1,2,3,4-tetrahydroquinoline-7-carboxylic acid (0.72 g, 34%) as white crystals.
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