General procedure for the synthesis of 1-aminoindole from indole and hydroxylamine sulfonic acid: refer to Example 4. 24.55 g (440 mmol) of potassium hydroxide, 3.201 g (27.3 mmol) of 1H-indole, and 6.15 g (54.4 mmol) of hydroxylamine-O-sulfonic acid were added slowly with stirring to 50 mL of N,N-dimethylformamide. After stirring the reaction mixture for 1 hour at room temperature, 50 mL of water was added and extracted three times with 100 mL of benzene. The combined organic phases were washed with water, dried and concentrated under reduced pressure. The residue was purified by silica gel column chromatography using 50% dichloromethane/hexane as eluent to give 1.16 g of the target product 1-aminoindole in 32% yield. Melting point: 41°C~41.5°C. 1H NMR (CDCl3, δ): 4.73 (br, 2H, NH2), 6.37 (d, 1H), 7.01~7.65 (m, 5H, phenyl ring).
