Example 1 [0041] 7.2 g of L-methionine was dissolved in a solvent mixture of 50 mL of water and 50 mL of acetonitrile. To this solution was added 2 g of sodium hydroxide (0.05 mol). The reaction mixture was cooled to 0 °C, followed by the slow addition of 10.9 g of di-tert-butyl dicarbonate (0.05 mol). After addition, the reaction system was gradually warmed to room temperature (24-25 °C) and the reaction was continuously stirred for 12 hours. After completion of the reaction, the acetonitrile was removed by rotary evaporation. To the residual aqueous phase, potassium carbonate was added and the pH was adjusted to 12. The aqueous phase was extracted twice with 50 mL of dichloromethane and the organic phase was discarded. To the aqueous phase, 1 N hydrochloric acid was added dropwise and the pH was adjusted to 6. The aqueous phase was again extracted twice with 50 mL of dichloromethane, the organic phases were combined, washed with 50 mL of saturated sodium chloride solution, dried with anhydrous sodium sulfate, and concentrated under reduced pressure to give viscous Boc-L-methionine (11.4 g) in 95% yield. The product was characterized by 1H NMR (500 MHz, CDCl3): δ 11.62 (br, 1H), 6.91 (br, 1H), 4.40 (m, 1H), 2.52 (t, J = 4.8 Hz, 2H), 2.05 (s, 3H), 1.92-2.15 (m, 2H), 1.42 (s, 9H). Mass spectrometry (MS) showed a molecular ion peak (M + 1) of 250.
