(5S)-5-benzyl-1-(tert-butyloxycarbonyl)-4-hydroxy-1,5-dihydro-pyrrol-2-one

(2'S)-5-[(1-hydroxy-3-phenyl-2-t-butoxycarbonylamino)-propylidene]-2,2-dimethyl-1,3-dioxane-4,6-dione

methyl (3S,4S)-3-O-formyl-4-(tert-butoxycarbonyl)amino-5-phenylpentanoate

(2S,3S)-butyric acid 2-benzyl-5-oxopyrrolidin-3-yl ester