General procedure for the synthesis of 7-methylindole-3-carboxaldehyde from 7-methylindole and ouvertropine: To a 50 mL round-bottomed flask fitted with a magnetic stirrer were added sequentially 7-methylindole (1.0 mmol, 1.0 eq.), hexamethylenetetetramine (HMTA, 2.0 mmol, 0.2803 g, 2.0 eq.), activated carbon (0.1 g), and N,N- dimethylformamide (DMF, 2 mL). Iodine (I2, 0.2 mmol, 0.0507 g, 20 mol%) was then added and the flask was assembled with a reflux condenser tube. The reaction mixture was stirred at 120 °C and the progress of the reaction was monitored by thin layer chromatography (TLC). After completion of the reaction, the mixture was cooled to room temperature. The resulting mixture was filtered through a diatomaceous earth pad and the filter cake was washed well with ethyl acetate (EtOAc, 4 x 6 mL). The combined filtrates were washed sequentially with 0.5 M aqueous hydrochloric acid (10 mL), saturated sodium bicarbonate (NaHCO3) solution (10 mL), and saturated sodium chloride (NaCl) solution (10 mL), dried over anhydrous sodium sulfate (Na2SO4) and concentrated under reduced pressure. The crude product was purified by silica gel fast column chromatography, and the target product 7-methylindole-3-carbaldehyde was obtained by using a mixed solvent of hexane and ethyl acetate as eluent.
