Concentrated sulfuric acid (10 mL) was slowly added to a methanolic (500 mL) suspension of 2,3-pyridinedicarboxylic acid (50 g, 300 mmol). The reaction mixture was heated to reflux for 48 hours and subsequently cooled to 40 °C and bromine (38 mL, 750 mmol) was added slowly and dropwise over 2 hours. The reaction mixture was continued to be heated at 55 °C for 48 h, followed by vacuum concentration. The concentrated residue was dissolved in isopropyl acetate (500 mL), and the resulting solution was washed sequentially with saturated aqueous sodium thiosulfate (200 mL x 4) and brine (400 mL), dried over anhydrous sodium sulfate, filtered, and concentrated again under vacuum. The residue was purified by silica gel column chromatography (petroleum ether/ethyl acetate, v/v=5/1) to afford dimethyl 5-bromo-pyridine-2,3-dicarboxylate as a yellow solid (52 g, 63% yield). Mass spectrum (ESI, positive ion mode) m/z: 274.0 [M + H]+; 1H NMR (400 MHz, CDCl3) δ (ppm): 8.75-8.76 (d, J = 2.2 Hz, 1H), 8.23-8.24 (d, J = 2.2 Hz, 1H), 3.93 (s, 3H), 3.89 (s, 3H).
