A solution of 2-(2-oxo-1,2-dihydro-3-quinolinyl)-1H-indole-5-carbaldehyde (5-8, 500 mg, 1.29 mmol, 1 equiv) in a 4:1 mixture of TBF and t-BuOH was treated with 2-methyl butene (8 mL), an aqueous solution of sodium phosphate monobasic (0.14 M 355.2 mg, 2.57 mmol, 2.00 equiv), and sodium chlorite (232.8 mg, 2.57 mmol, 2.00 equiv). Additional solid sodium phosphate monobasic (380 mg, 2.76 mmol, 2.14 equiv) and sodium chlorite (300 mg, 3.32 mmol, 2.57 equiv) was added in 2 equal portions over 2.5 hours. The reaction mixture was concentrated, and the residue dissolved in EtOAc (60 mL), then washed twice with a 25:1 mixture of aqueous 10% sodium bisulfite solution and 10% potassium hydrogen sulfate solution (2×50 mL). The organic layer was dried over sodium sulfate, concentrated, and combined with a precipitate in the aqueous layer which was filtered and dried to 2-(2-oxo-1,2-dihydro-3-quinolinyl)-1H-indole-5-carboxylic acid (6-1) as an off-white solid. 1H NMR (500 MHz, DMSO) δ 12.13 (s, 1H), 8.27 (s, 1H), 8.14 (m, 3H), 7.95 (d, 1H, J=7.8 Hz), 7.76 (d, 1H, J=7.8 Hz), 7.54 (t, 1H, J=7.8), 7.36 (d, 1H, J=7.8), 7.24 (t, 1H, J=7.8) 1.36 (s, 9H).
