[C:1]([NH:11][C@H:12]([C:18]([OH:20])=[O:19])[CH2:13][CH2:14][CH2:15][CH2:16][NH2:17])([O:3][CH2:4][C:5]1[CH:10]=[CH:9][CH:8]=[CH:7][CH:6]=1)=[O:2].C(O[C:25]([CH3:28])([CH3:27])[CH3:26])(=O)C.Cl(O)(=O)(=O)=O.[OH-].[Na+]>O.C(OCC)(=O)C>[C:1]([NH:11][C@H:12]([C:18]([O:20][C:25]([CH3:28])([CH3:27])[CH3:26])=[O:19])[CH2:13][CH2:14][CH2:15][CH2:16][NH2:17])([O:3][CH2:4][C:5]1[CH:10]=[CH:9][CH:8]=[CH:7][CH:6]=1)=[O:2]
A mixture of 25 g of Nα-CBZ-L-lysine from Sigma, where CBZ is benzyloxycarbonyl, 250 mL of t-butyl acetate and 14 mL of 70% perchloric acid was vigorously mixed until all solids were dissolved. After stirring overnight, 375 mL of ethyl acetate was added followed by 250 mL of water. The pH of the aqueous layer was raised to 5.5 with 20% NaOH whereupon the organic phase was separated. The aqueous phase was extracted with the ethyl acetate solution and the pH again raised to 10.5 whereupon it was again extracted with 200 mL of fresh ethyl acetate. The organic extracts were combined, washed with saturated brine, dried over Na2SO4 and concentrated. The residue was dried overnight under high vacuum to yield 23.2 g of a colorless, viscous material.
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