The general procedure for the synthesis of (S)-2-amino-4-((trifluoromethyl)thio)butyric acid from L-homocysteine and trifluoroiodomethane was as follows: first, 10.0 g (37.3 mmol) of L-homocysteine was added to a 500 mL three-necked flask equipped with a liquid ammonia cooling unit, glass stirring bar, thermometer, and diaphragm rubber cap. The flask was then flushed thoroughly with nitrogen to remove air. After cooling the flask to -78°C, ammonia dried through a KOH tube was liquefied in a liquid ammonia cooling unit cooled in a dry ice-acetone bath, about 100 mL of liquid ammonia was added to the L-homocysteine, and stirring was started. Next, 3.58 g (156 mmol) of sodium metal was slowly added while controlling the reaction temperature to not exceed -35°C. During this process, the color of the solution (mixture) changed to a dark blue. Then, 18.2 g (93.3 mmol) of trifluoromethyl iodine weighed with a balloon was added and stirred in a bath at -78°C for 20 min. Afterwards, the ammonia was gradually evaporated, the residue was dissolved in ultrapure water, and the solution was purified by passing it through an ion exchange resin DOWEX-50X8 pre-treated with hydrochloric acid. After rinsing with a sufficient amount of ultrapure water, the solution was eluted using 2% ammonia solution, and 14.1 g of the target product, L-trifluoromethionine, was finally obtained in 93% yield. The structure of the product was confirmed by 1H-NMR (D2O, 200 MHz) δ: 2.07-2.37 (m, 2H), 3.01 (t, 2H, J = 7.6 Hz), 4.03 (t, 1H, J = 6.6 Hz) and 19F-NMR (D2O, 188 MHz) δ: -41.3 (s).
