Concentrated sulfuric acid (27.54 mL, 517 mmol) was slowly added to a solution of tetrahydro-3-furancarboxylic acid (20.00 g, 172 mmol) in methanol (350 mL). The reaction mixture was heated to reflux and kept for 18 hours. Upon completion of the reaction, the mixture was cooled to room temperature and concentrated in a rotary evaporator to remove most of the methanol. The concentrated residue was dissolved in water (500 mL) and dichloromethane (200 mL) for partitioning. The organic and aqueous phases were separated and the aqueous phase was extracted once more with dichloromethane (200 mL). All organic phases were combined and washed sequentially with saturated aqueous sodium bicarbonate (200 mL) and brine (200 mL). The organic phases were dried with anhydrous sodium sulfate, filtered and concentrated to afford methyl 3-tetrahydrofurancarboxylate (15.0 g, 67% yield) as a light yellow oil. The product was confirmed by 1H NMR (400 MHz, DMSO-d6): δ 3.85 (t, J = 8.4 Hz, 1H), 3.77-3.66 (m, 3H), 3.62 (s, 3H), 3.07-3.22 (m, 1H), 1.97-2.12 (m, 2H).
