The general procedure for the synthesis of 5,6-dimethoxyindoline from 2-(2-bromo-4,5-dimethoxyphenyl)ethanamine hydrochloride was as follows: to a sealed round-bottom flask under nitrogen protection were added cuprous iodide (CuI, 0.11 g, 0.55 mmol), L-proline (0.13 g, 1.1 mmol) and potassium carbonate (K2CO3, 1.62 g, 11.1 mmol). 11.1 mmol). The system was evacuated and backfilled with nitrogen and the process was repeated twice. Subsequently, a dimethyl sulfoxide (DMSO, 5.5 mL) solution of 2-(2-bromo-4,5-dimethoxyphenyl)ethylamine (1.45 g, 5.5 mmol) was added and the reaction was stirred at 70 °C for 45 h under nitrogen atmosphere. After completion of the reaction, water (100 mL) was added to the mixture and extracted with chloroform (CHCl3, 3 x 100 mL). The organic layers were combined, dried over anhydrous sodium sulfate (Na2SO4), concentrated and purified by short-range chromatographic column with chloroform/methanol (60:1) as eluent to afford the target product 5,6-dimethoxyindoline (0.36 g, 36% yield) as a light brown solid. The product was characterized by 1H NMR (CDCl3): δ 6.76 (s, 1H), 6.35 (s, 1H), 3.82 (s, 3H), 3.81 (s, 3H), 3.54 (t, J = 8.4 Hz, 2H), 2.98 (t, J = 8.4 Hz, 2H). The data obtained are in agreement with the spectral data of the compounds prepared by other methods.
