[C:1]([O:5][C:6]([NH:8][CH:9]([C:13]([SH:16])([CH3:15])[CH3:14])[C:10]([OH:12])=[O:11])=[O:7])([CH3:4])([CH3:3])[CH3:2].[OH-].[K+].[C:19](OC(=O)C)(=[O:21])[CH3:20]>O>[C:19]([S:16][C:13]([CH3:15])([CH3:14])[CH:9]([NH:8][C:6]([O:5][C:1]([CH3:4])([CH3:2])[CH3:3])=[O:7])[C:10]([OH:12])=[O:11])(=[O:21])[CH3:20]2-(tert-butoxycarbonylamino)-3-mercapto-3-methylbutanoic acid (2.96 g, 11.87 mmol) was suspended in water (33 ml) and cooled with an ice/water bath. KOH (1.67 g, 29.7 mmol) dissolved in water (16 ml) followed by acetic anhydride (1.41 ml, 14.9 mmol) were added to the reaction flask and stirred at 0-5° C. for 4 hours (a white precipitated appears after ca. 30 minutes of stirring). The reaction mixture was partitioned between EtOAc and HCl 1M. The aqueous layer was extracted 3 times with EtOAc. The combined organic layers were washed with brine and dried over Na2SO4, filtered and solvent removed under vacuum. Crude compound was used in the next step without further purification. (3.45 g, quantitative yield).
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