General procedure for the synthesis of 5-azaindole-3-carboxaldehyde from 5-azaindole and 1,1-dichlorodimethyl ether: 5-azaindole (0.400 g; 3.386 mmol) was dissolved in a solvent mixture of 1,2-dichloroethane (10 mL) and nitromethane (10 mL) and cooled to 0 °C. The reaction was followed by the addition of aluminum trichloride (1.544 mL; 16.92 mmol). Under argon protection, 1,1-dichlorodimethyl ether (1.544 mL; 16.92 mmol) was slowly added dropwise, followed by batchwise addition of aluminum trichloride (1.5500 g; 11.25 mmol), which was controlled to be completed within 1 hour. Upon completion of the reaction, the reaction was quenched by the addition of water and saturated sodium bicarbonate solution. The reaction mixture was extracted with a solvent mixture of dichloromethane and ethanol (9:1, 6 x 100 mL, v/v). The organic layers were combined, washed with saturated brine, dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure to afford the crude product 1H-pyrrolo[3,2-c]pyridine-3-carboxaldehyde (0.333 g, 67% yield), which can be used directly in subsequent reactions without further purification. The product was characterized as follows: ESI/APCI(+): m/z 147 [M+H]+; ESI/APCI(-): m/z 145 [M-H]-. 1H NMR (DMSO-d6) δ: 10.00 (1H, s), 9.29 (1H, s), 8.42 (1H, s), 8.35 (1H, d), 7.53 (1H, d). .
