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16.8 g (0.1 mole) of tert.-butyl glycine hydrochloride were suspended in 100 ml of dry acetonitrile, 10.1 g (0.1 mole) of triethylamine were added and the mixture was stirred for a short time at room temperature. A solution of 31.5 g (0.1 mole) of N-benzoyl-α-(benzoylthio)-glycine in 400 ml of dry acetonitrile and 20.6 g (0.1 mole) of dicyclohexylcarbodiimide were added to the mixture, and stirring was continued for 3 hours at room temperature. The precipitated dicyclohexylurea was filtered off, the filtrate was evaporated down under reduced pressure and the residue was taken up with ethyl acetate. The solution was filtered once again, the filtrate was evaporated down and the residue was chromatographed with 1:1 cyclohexane/ethyl acetate over a column containing deactivated silica gel. Crystallization from ethyl acetate gave 27.8 g of tert.-butyl N-[N-benzoyl-α-(benzoylthio)-glycyl]-glycine of melting point 157°-160° C. (Rf=0.50 in 1:1 cyclohexane/ethyl acetate). The product was dissolved in 100 ml of dry dioxane, 100 ml of 6N HCl/dioxane were added and the mixture was left to stand for 3 hours at 20° C. 500 ml of ethyl acetate were then added to the mixture, the organic phase was extracted by shaking with three times 500 ml of water, dried with anhydrous sodium sulfate and evaporated down under reduced pressure, and the residue was crystallized from diethyl ether. 22.8 g (61%) of N-[N-benzoyl-α-(benzoylthio)-glycyl]-glycine, melting point 194°-197° C. (Rf=0.31 in system I) were obtained.

1. stirringstirring
2. filtrationThe precipitated dicyclohexylurea was filtered off
3. customthe filtrate was evaporated down under reduced pressure
4. filtrationThe solution was filtered once again
5. customthe filtrate was evaporated down
6. customthe residue was chromatographed with 1:1 cyclohexane/ethyl acetate over a column
7. additioncontaining deactivated silica gel
8. customCrystallization from ethyl acetate