[C:1]([O:5][C:6](=[O:22])[C:7]([S:10][C:11]1[S:12][CH:13]=[C:14]([CH2:16][C:17]([O:19]CC)=[O:18])[N:15]=1)([CH3:9])[CH3:8])([CH3:4])([CH3:3])[CH3:2].[OH-].[Na+]>CO>[C:1]([O:5][C:6](=[O:22])[C:7]([S:10][C:11]1[S:12][CH:13]=[C:14]([CH2:16][C:17]([OH:19])=[O:18])[N:15]=1)([CH3:9])[CH3:8])([CH3:2])([CH3:3])[CH3:4]2-{[4-(2-Ethoxy-2-oxoethyl)-1,3-thiazol-2-yl]thio}-2-methylpropionic acid tert-butyl ester (150 g) synthesized in Example 2 was dissolved in methanol (700 mL), a 1N aqueous sodium hydroxide solution (448 mL) was added at room temperature, and the mixture was stirred for 3 hr. The reaction solution was concentrated under reduced pressure, an aqueous 10% citric acid solution (1 L) was added, and the mixture was extracted with ethyl acetate (1 L). The organic layer was washed three times with water (500 mL) and dried over anhydrous magnesium sulfate. The solvent was evaporated under reduced pressure to give the title compound (123 g) as a slightly yellow solid.
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