C[O:2][C:3](=[O:21])/[C:4](/[C:8]1[N:9]=[C:10]([NH:13][C:14]([O:16][C:17]([CH3:20])([CH3:19])[CH3:18])=[O:15])[S:11][CH:12]=1)=[CH:5]\[CH2:6][CH3:7].[OH-].[Na+].Cl>C(O)(C)C.O>[C:17]([O:16][C:14]([NH:13][C:10]1[S:11][CH:12]=[C:8](/[C:4](=[CH:5]/[CH2:6][CH3:7])/[C:3]([OH:21])=[O:2])[N:9]=1)=[O:15])([CH3:20])([CH3:19])[CH3:18]To a solution of (Z)-2-(2-t-butoxycarbonylaminothiazol-4-yl)-2-pentenoic acid methyl ester (425 mg: 1.4 mMol.) in isopropanol (1.3 ml) and water (3.7 ml) is added sodium hydroxide (163 mg: 4.1 mMol.), and the mixture is stirred at 65° C. for 90 minutes. The reaction mixture is adjusted to pH 4.7 with 35% hydrochloric acid and kept at 20° C. for 1 hour. The separating crystals are collected by filtration, washed with isopropanol, and dried to give (Z)-2-(2-t-butoxycarbonylaminothiazol-4-yl)-2-pentenoic acid (325 mg). Yield: 86%. mp. 187° C. (decomposition). NMR δ(CDCl3) ppm: 1.09(t, J=7.6 Hz, 3H), 1.55(s, 9H) 2.64(qd, J=7.6 Hz, 2H), 6.69(t, J=7.6 Hz, 1H), 6.96(s, 1H). IR ν(CHCl3) cm-1 : 3160, 2548br, 1721, 1685, 1556, 1252, 1156.
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