Example 1: Synthesis of 4-hydroxy-1H-indole-3-carbaldehyde
1. 7.35 mL of phosphorus oxychloride was slowly added dropwise to 15 mL of dry N,N-dimethylformamide under cooling conditions in an ice-methanol bath with stirring for 15 minutes.
2. 5.0 g of 4-hydroxyindole dissolved in 10 mL of anhydrous N,N-dimethylformamide was added slowly dropwise to the above mixture under cooling in an ice bath.
3. The reaction mixture was stirred at room temperature for 2 hours.
4. Under cooling in an ice bath, water was added to the reaction mixture and adjusted to alkaline with 30% aqueous sodium hydroxide solution and stirring was continued for 15 minutes.
5. The mixture was acidified to pH 4 with 5N hydrochloric acid and the precipitate was collected by filtration.
6. The precipitate was washed with water and dried to give 4.99 g of 4-hydroxy-1H-indole-3-carbaldehyde crude product in 82% yield.
7. The crude product was recrystallized with methanol to give yellow crystals.
Product characterization.
1H-NMR (DMSO-d6) δ: 6.54 (1H, dd, J = 8.1, 0.9 Hz), 6.95 (1H, dd, J = 8.1, 0.9 Hz), 7.13 (1H, t, J = 8.1 Hz), 8.37 (1H, s), 9.64 (1H, s), 10.54 (1H, br s), 12.35 (1H, br s). br s).
Melting point: 190-193 °C.
