General procedure for the synthesis of 5-cyanoindole-3-carboxaldehyde from 5-cyanoindole: Phosphoryl chloride (4.24 mL, 45.48 mmol) was slowly added dropwise to dimethylformamide (3.52 mL, 45.48 mmol), and stirred at room temperature for 30 min. Subsequently, a solution of 1H-indole-5-carbonitrile (5.39 g, 37.9 mmol) in dimethylformamide (10 mL) was added dropwise. To promote stirring, solid precipitated dimethylformamide (10 mL) was added and the reaction mixture continued to be stirred for 3 hours at room temperature. Upon completion of the reaction, the reaction mixture was quenched by the addition of water and stirring was continued for 18 hours. Stirring was stopped and the reaction mixture was allowed to stand for 24 h. A pink solid precipitated from the organic layer. The layers were separated, the organic layer was filtered, washed with water and dried to afford the target product 5-cyanoindole-3-carbaldehyde (5.44 g, 84% yield). The product was characterized by 1H NMR (DMSO): δ 7.60-7.80 (m, 2H), 8.20-8.30 (m, 2H), 10.00 (s, 1H), 12.20-12.35 (s, br, 1H).
