The general procedure for the synthesis of 7-chloro-1H-indole-3-aldehyde from 7-chloro-1H-indole was as follows: phosphoryl chloride (0.66 mL, 7 mmol) was slowly added dropwise to anhydrous DMF (5 mL) at 0 °C and under argon protection. Subsequently, a solution of 7-chloro-1H-indole (1 g, 6.6 mmol) dissolved in anhydrous DMF (15 mL) was added dropwise to the above mixture at room temperature. The reaction mixture was stirred continuously for 2 hours at room temperature. After completion of the reaction, the mixture was slowly poured into a mixture of ice water and saturated NaHCO3 solution and extracted with ethyl acetate. The organic phases were combined, washed with saturated NaCl solution (10 mL x 3), dried over anhydrous MgSO4, filtered and concentrated to give 990 mg of the target product 7-chloro-1H-indole-3-aldehyde as a yellow-orange solid in 83% yield.
