To a mixture of 6-chloro-5-[4-(oxetan-3-ylmethoxy)phenyl]-1H-indole-3-carbaldehyde (70 mg, 0.2 mmol) in acetonitrile/t-butanol=1/1(5 mL) was added 2-methyl-2-butene (0.5 mL). The reaction was cooled to 0° C. and an aqueous solution of sodium chlorite (180 mg, 2.00 mmol) and sodium dihydrogen phosphate (270 mg, 6.00 mmol) in water (0.5 mL) were added dropwise via additional funnel. Then ice bath was removed, the reaction was stirred at room temperature for 12 h. The reaction was quenched with sodium sulfite. The mixture was partitioned between EtOAc (20 mL×3) and water. The combined organics was concentrated to give a residue, which was purified by reverse phase HPLC to give the title compound (25 mg, 35% yield) as an off-white solid. MS (AP+) 358.1 (M+H)+. 1H NMR (400 MHz, DMSO-d6) δ 8.04 (s, 1H), 7.95 (s, 1H), 7.61 (s, 1H), 7.35 (d, 2H), 7.04 (d, 2H), 4.74 (t, 2H), 4.46 (t, 2H), 4.26 (t, 2H), 3.44-3.42 (m, 1H).
