To a solution of 5-nitroindole-3-carboxylic acid (2.45 g, 12 mmol) in methanol (50 mL) was slowly added concentrated sulfuric acid (1 mL), and the reaction mixture was stirred under reflux conditions for 6 hours. Upon completion of the reaction, the reaction solution was allowed to cool to room temperature and subsequently poured slowly into ice water. It was neutralized with saturated sodium bicarbonate solution to pH neutral and the precipitated yellow solid product was collected by filtration. The crude product was purified by recrystallization from methanol to afford methyl 5-nitro-1H-indole-3-carboxylate as an off-white solid in a yield of 1.76 g in 67%. The melting point of the product was 282-284°C. Nuclear magnetic resonance hydrogen spectroscopy (1H NMR) data: δ 3.85 (3H, s, OCH3), 7.65 (1H, d, J = 9 Hz, H-7), 8.08 (1H, dd, J1 = 9 Hz, J2 = 2.3 Hz, H-6), 8.35 (1H, s, H-2), 8.83 (1H, d, J = 2.3 Hz, H-4), 8.5 (1H, s, NH). Ultraviolet spectral (UV) data: λmax 320 nm (ε = 15000), 251 nm (ε = 32450), λmin 284 nm, 213 nm; λmax 366 nm (ε = 13100), 271 nm (ε = 31500), 213 nm (ε = 48100) at pH 12. Mass spectrometry (MS) data: m/z 243.1 ([M + Na]+). The calculated value ([M + Na]+ C10H8N2O4Na) from the high-resolution mass spectrometry (HRMS) was 243.0396 with a measured deviation of -5.73 ppm.
