To a pressure-resistant reaction flask was added 5-methoxy-2-nitrobenzoic acid (3.98 g, 20.0 mmol) and anhydrous methanol (10 mL). Concentrated sulfuric acid (150 μL) was added slowly and dropwise. The reaction vial was sealed and the reaction was stirred at 85 °C for 4 days. Upon completion of the reaction, the mixture was cooled to room temperature and concentrated under reduced pressure to half the original volume. Ethyl acetate (25 mL) was added to dilute, and the organic phase was washed sequentially with water (25 mL), saturated aqueous sodium bicarbonate solution (2 × 25 mL), saturated saline (2 × 25 mL) and water (2 × 25 mL). The organic layer was dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure to afford methyl 5-methoxy-2-nitrobenzoate as a yellow oil (3.57 g, 87% yield). Its nuclear magnetic resonance hydrogen spectrum (1H NMR, 300 MHz, CDCl3) data were as follows: δ 8.04 (d, J = 8.8 Hz, 1H, H-6), 7.04 (d, J = 2.3 Hz, 1H, H-3), 7.00 (dd, J = 8.8, 2.8 Hz, 1H, H-4), 3.93 (s, 3H, OCH3), 3.91 (s. 3H, OCH3). The nuclear magnetic resonance carbon spectrum (13C NMR, 75 MHz, CDCl3) data were as follows: δ 166.7, 163.5, 140.2, 131.4, 126.8, 115.9, 114.2, 56.4, 53.5. The obtained data are in agreement with the literature reports.
