2-(1,3-Dithiol-2-ylidene)-3-methoxy-3-oxopropionic acid (3) (109 g) was suspended in dichloromethane (1 l), followed by the addition of 61 g of triethylamine under ice-cooled stirring. To the resulting mixture, ethyl chlorocarbonate (60 g) was added dropwise under ice-cooled stirring over 30 minutes. To a solution of 88.5 g of γ-methyl L-glutamate and 61 g of triethylamine in dichloromethane (1 l), the thus-obtained solution was added dropwise at 10° C. over 1.5 hours. After the resulting mixture was stirred at room temperature for 10 hours, the solvent was removed under reduced pressure. Ethyl acetate (2 l) was added to the residue, and the resultant precipitate was filtered off. After the filtrate was washed with 1 N hydrochloric acid and then with water, the solvent was removed under reduced pressure. The residue was dissolved in methanol (800 ml). Activated carbon (15 g) was added, followed by stirring at 40° C. for 1 hour. The activated carbon was filtered off, and the methanol was removed under reduced pressure. The residue was dissolved in toluene (500 ml) and the toluene was then removed under reduced pressure, whereby 2-((2-(1,3-dithiol-2-ylidene)-3-methoxy-3-oxopropanoyl)amino)-5-methoxy-5-oxopentanoic acid (4) (164 g, yield: 91%) was obtained as a brown oil.
