To a solution of 1-benzyl-2-isopropyl-6-methoxy-1H-indole-3-carbaldehyde (Compound 5, 608 mg, 1.98 mmol) in t-BuOH (15 ml), CH3CN (15 ml), and 2-methyl-2-butene (10 ml) was added a solution of KH2PO4 (5.4 g, 39.6 mmol) and NaClO2 (80%, 4.5 g, 39.6 mmol) in H2O (50 ml). The mixture was stirred at room temperature and additional 2-methyl-2-butene, KH2PO4, and NaClO2 were added at the above ratio every 16-24 h until the starting material was consumed. The reaction mixture was extracted with EtOAc (×3) and the combined organic layer was washed with brine, dried over Na2SO4, and concentrated in vacuo. The residue was purified by chromatography on silica gel (0→25% EtOAc-hexanes) to yield the title compound as a yellow solid.
