General procedure for the synthesis of (S)-5-oxo-2-tetrahydrofuran carboxylic acid from L-glutamic acid: L-glutamic acid (10.07 g, 0.068 mol, JK CHEMICA) was dissolved in 20 mL of water in a conical flask. A solution of NaNO2 (7.0 g, 0.102 mol, Shantou Xilong Chemical Factory) in 20 mL of water was slowly added to this solution at -5°C, along with HCl and 40 mL of water. The reaction mixture was stirred continuously at room temperature for 12 hours. Subsequently, the reaction mixture was subjected to vacuum evaporation at less than 50 °C to obtain a yellow oily substance. This oily substance was dissolved in EtOAc, filtered to remove the solid formed and the solid was washed with EtOAc. The filtrate and washings were combined and dried with Na2SO4. Finally, the solvent was removed by vacuum concentration to afford (S)-5-oxo-2-tetrahydrofuran carboxylic acid as a light yellow oil (8.1 g, 91.6% yield). The product was analyzed by mass spectrometry (ESI, cation mode), m/z: 130.9 (M+1); 1H NMR (400 MHz, CDCl3) data were as follows: δ 2.27-2.41 (m, 1H), 2.44-2.65 (m, 3H), 5.09 (m, 1H), 9.12-9.55 (m, 1H).
