The general procedure for the synthesis of methyl 3-formyl-1H-indole-7-carboxylate from methyl indole-7-carboxylate and N,N-dimethylformamide was as follows: in anhydrous 1,2-dichloroethane (6 mL), phosphorus trichloride (0.43 mL, 4.6 mmol) was slowly added to anhydrous N,N-dimethylformamide (0.35 mL, 4.6 mmol) to prepare a phosphoryl chloride -dimethylformamide mixture and cooled the mixture to below 5°C. A solution of anhydrous 1,2-dichloroethane (6 mL) of 7-methoxycarbonylindole (0.69 g, 4 mmol) was added dropwise to the above mixture below 5 °C. The reaction mixture was stirred at room temperature for 2 hours and subsequently heated at 50 °C for 30 minutes. Upon completion of the reaction, the mixture was cooled, the precipitate was filtered and washed with 1,2-dichloroethane. The precipitate was suspended in 10% aqueous sodium carbonate solution (30 mL) and stirred for 20 minutes at room temperature. Dichloromethane was added and stirring was continued for 10 minutes. The organic phase was separated and the aqueous phase was extracted with dichloromethane. The organic extracts were combined, washed with brine, dried over anhydrous magnesium sulfate, and the solvent was evaporated to give 0.75 g of the title product (93% yield). Melting point: 153-154°C.
