To a solution of 2,5-furandicarboxylic acid (1.00 g, 6.41 mmol) in anhydrous ethanol (50 mL) was slowly added 98% aqueous sulfuric acid (0.05 mL), and the reaction mixture was heated and refluxed under nitrogen protection for 18 hours. Upon completion of the reaction, the solution was cooled to room temperature and subsequently concentrated under reduced pressure to remove the solvent. The concentrated residue was separated by filtration and washed sequentially with saturated aqueous sodium bicarbonate solution (3 x 10 mL) and deionized water (3 x 10 mL) to afford finally diethyl furan-2,5-dicarboxylate as a white solid (1.33 g, 98% yield). The structure of the product was confirmed by nuclear magnetic resonance hydrogen spectroscopy (1H NMR, 500 MHz, CDCl3) and carbon spectroscopy (13C NMR, 125 MHz, CDCl3) with the following data: 1H NMR δ 1.38 (t, J = 7.1 Hz, 6H), 4.39 (q, J = 7.1 Hz, 4H), 7.19 (s, 2H); 13C NMR δ 14.2, 61.6, 118.2, 146.9, 158.1.
