*3.3. General Procedure of SeO2 Mediated Allylic Oxidation* (**7a–c, 7e**)

To a stirred solution of the corresponding 3-methyleneoxindole **5** (0.256 mmol) in *p*-xylene (3.0 mL) was added SeO2 (170.4 mg, 2.0 mmol, 2.0 equiv.). The reaction mixture was refluxe for 24 h, then, the mixture was diluted with EtOAc (50 mL) and brine (30 mL). The organic layer was separated, dried over Na2SO4, filtered, and concentrated under reduced pressure. The crude residue was purified by column chromatography to afford aldehye **7**.
