General procedure for the synthesis of 4-methoxyindoline-2,3-dione from 4-methoxyindole: 4-methoxyindole (18.25 mg, 0.17 mmol, 1.0 eq.) was dissolved in 1,2-dichloroethane (DCE) (2 mL), which was subsequently added to a solution containing pyridinium chlorochromate (PCC) (273 mg, 1.36 mmol, 8.0 equivalents) in a DCE solution (2 mL) and aluminum chloride (7 mg, 0.05 mmol, 0.3 equivalents) was added. The reaction mixture was heated at 80 °C for 2 h and then concentrated to dryness. The residue was purified by fast column chromatography (FCC) (eluent: ethyl acetate-hexane, 1:1) to afford 4-methoxyindoline-2,3-dione (16 mg, 0.09 mmol, 53% yield) in yellow powder form. The spectral data of the obtained compound were in agreement with literature reports.
