General procedure for the synthesis of 6-chloro-2-quinoline carboxylic acid from 6-chloro-2-quinoline carboxaldehyde: A solution of sodium chlorite (2.4 g) and sodium dihydrogen phosphate (2.4 g) in water (24 ml) was added slowly and dropwise over a period of 5 min to a solution of 6-chloroquinoline-2-carboxaldehyde (536 mg; 2.8 mmol) dissolved in a tert-butanol (56 ml) and 2-methyl-2-butene (14 ml) mixed solution. The reaction mixture was stirred at room temperature for 4 hours. After completion of the reaction, the organic solvent was removed by distillation under reduced pressure. Water (30 ml) was added to the residue and a white precipitate was precipitated. The precipitate was collected by filtration, washed with water and dried under vacuum in the presence of phosphorus pentoxide to afford 6-chloroquinoline-2-carboxylic acid in white powder form (505 mg; yield = 87%). Mass spectrometry/liquid chromatography analysis: m/z=208.01 (M+H), retention time=8.55 min (condition 1). NMR hydrogen spectrum (DMSO-d6,400MHz) δ: 8.41-8.39 (m, 1H, aromatic H); 8.20-8.11 (m, 3H, aromatic H); 7.82-7.79 (m, 1H, aromatic H).
