General procedure for the synthesis of indole-7-carboxaldehyde from (1H-indol-7-yl)methanol: (1H-indol-7-yl)methanol (8.0 g, 54.3 mmol) was dissolved in 400 mL of dichloromethane, and activated manganese(IV) oxide (85%, 41.0 g, 0.40 mol) was added. The reaction mixture was stirred at room temperature for 72 hours. Upon completion of the reaction, 200 mL of dichloromethane and 400 mL of methanol were added to the mixture, followed by filtration through a silica gel pad to remove solid impurities. The filtrate was concentrated to give the crude product, which was further purified by silica gel column chromatography to afford the target product 1H-indole-7-carboxaldehyde (I-1) (6.55 g, 83% yield). The structure of the product was confirmed by 1H-NMR (CDCl3).
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