1: 4-(N,N-dimethlyamino)pyridine; bis-(2-hydroxyethyl)-sulfide; (S)-5-tert-butoxy-4-tert-butoxycarbonylamino-5-oxo-pentanoic acid; dicyclohexyl-carbodiimide in ethyl acetate T=0 - 20°C; Esterification; 16 h; 2: trifluoroacetic acid; tert-butyl 2-(2-hydroxyethylthio)ethyl (2S)-2-[(tert-butoxy)carbonylamino]pentane-1,5-dioate in trifluoroacetic acid T=20°C; saponification; 5 h;