Synthesis of diethyl 1H-indole-2,6-dicarboxylate (3): ethyl 4-(3-ethoxy-2,3-dioxopropyl)-3-nitrobenzoate (2) (8.5 g, 27.5 mmol) was suspended in acetic acid (75 mL) and heated to 75°C. After the solid was completely dissolved, water (56 mL) was slowly added, followed by activated zinc powder (17.9 g, 27.5 mmol). 27.5 mmol). The reaction temperature was maintained at 80°C with continuous stirring for 2 hours. Upon completion of the reaction, the mixture was cooled to room temperature, diluted with ethyl acetate (50 mL) and filtered through a diatomaceous earth pad. The filtrate was washed sequentially with water (2 x 100 mL), saturated sodium bicarbonate solution (2 x 100 mL) and brine (100 mL). The organic layer was dried with anhydrous sodium sulfate and concentrated under reduced pressure. The residue was purified by silica gel column chromatography to afford the yellow solid product 3 (6.0 g, 83% yield).1H NMR (500 MHz, DMSO-d6) δ 1.33 (m, 6H), 4.34 (m, 4H), 7.21 (s, 1H), 7.65 (d, J = 8.5 Hz, 1H), 7.75 (d, J = 8.5 Hz, 1H), 7.12 (s, 1H), 7.12 (s, 1H), 8.12 (s, 1H). 8.12 (s, 1H), 12.25 (s, 1H). Mass spectrum (MS) m/z (M-H): 259.9.
