6-chloro-5-[2-(dimethylamino)pyrimidin-5-yl]-1H-indole-3-carbaldehyde (100 mg, 0.333 mmol) was dissolved in acetonitrile (6 mL) and warm tert-butanol (6 mL) and treated with 2-methyl-2-butene (4 mL). The reaction mixture was cooled to 0° C. and treated with a solution of sodium chlorite (450 mg, 6.65 mmol) and sodium phosphate monobasic dihydrate (1.04 g, 6.65 mmol) in water (3 mL) dropwise via addition funnel. The reaction mixture was warmed to room temperature and stirred for 20 hours. The reaction mixture was concentrated to remove the organics and extracted with EtOAc (3×20 mL). The combined organic layers were washed with brine (20 mL), dried and concentrated in vacuo. The crude product was purified by prep-HPLC (Column: Agella venusil ASB C18 150×21.2 mm×5 μm; Mobile phase: from 20% MeCN in water (0.1% HCl) to 45% MeCN in water (0.1% HCl) Wavelength: 220 nm) to give 6-chloro-5-[2-(dimethylamino)pyrimidin-5-yl]-1H-indole-3-carboxylic acid (43 mg, 41% yield) as a yellow solid. MS (ES+) 316.9 (M+H)+. 1H NMR (400 MHz, DMSO-d6) δ 11.98 (s, 1 H), 8.42 (s, 2 H), 8.08 (d, 1 H), 7.93 (s, 1 H), 7.65 (s, 1 H), 3.17 (s, 6 H).
