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According to the method of O. Keller and J. Rudinger (Helv. Chim. Acta., 58, 531 (1975)), 6.2 g (40 mmols) of N-methoxycarbonylmaleimide was added at 0° C. to a solution of 8.44 g (40 mmols)of S-benzyl-L-cysteine in a mixture of 200 ml of saturated aqueous solution of sodium hydrogen carbonate and 100 ml of THF. The mixture was stirred for 10 minutes and then diluted with 300 ml of water and 150 ml of THF. The reaction mixture was stirred at room temperature for 30 minutes and subsequently at 40° C. for 30 minutes, neutralized with concentrated sulfuric acid and thereafter concentrated to about 200 ml in a vacuum. The concentrate was subjected to extraction with ethyl acetate, and the extract was dried over anhydrous sodium sulfate (Na2SO4) and concentrated to give a white solid. The product was subjected to silica gel chromatography (90:10:5 chloroform-ethyl acetate-acetic acid to 40:60:5 chloroform-ethyl acetate-acetic acid), giving 2.29 g of (2S)-3-benzylthio-2-maleimidopropionic acid. M.p.: 86°-87° C.

1. stirringThe reaction mixture was stirred at room temperature for 30 minutes and subsequently at 40° C. for 30 minutes
2. concentrationconcentrated to about 200 ml in a vacuum
3. extractionThe concentrate was subjected to extraction with ethyl acetate
4. dry with materialthe extract was dried over anhydrous sodium sulfate (Na2SO4)
5. concentrationconcentrated
6. customto give a white solid