General procedure for the synthesis of 4-oxo-1,4-dihydroquinoline-3-carboxylic acid from ethyl 4-hydroxyquinoline-3-carboxylate: ethyl 4-hydroxyquinoline-3-carboxylate (15 g, 69 mmol) was suspended in 2N sodium hydroxide solution (150 mL), and the reaction was stirred under refluxing conditions for 2 hours. After completion of the reaction, the mixture was cooled to room temperature and filtered to remove insoluble material. The filtrate was acidified to pH 4 with 2N hydrochloric acid and a white precipitate was precipitated. The precipitate was collected by filtration, washed several times with deionized water and finally dried under vacuum to afford 4-oxo-1,4-dihydroquinoline-3-carboxylic acid as a light white solid (10.5 g, 92% yield). The product was characterized by 1H NMR (DMSO-d6): δ 15.34 (s, 1H), 13.42 (s, 1H), 8.89 (s, 1H), 8.28 (d, J=8.0 Hz, 1H), 7.88 (m, 1H), 7.81 (d, J=8.4 Hz, 1H), 7.60 (m, 1H).
