[O:1]=[C:2]1[NH:6][CH:5]([CH2:7][CH:8]([OH:12])[C:9]([OH:11])=[O:10])[CH2:4][CH2:3]1.C(N(CC)CC)C.ClC(O[CH2:24][C:25]1[CH:30]=[CH:29][CH:28]=[CH:27][CH:26]=1)=O>O1CCCC1>[O:1]=[C:2]1[NH:6][CH:5]([CH2:7][CH:8]([OH:12])[C:9]([O:11][CH2:24][C:25]2[CH:30]=[CH:29][CH:28]=[CH:27][CH:26]=2)=[O:10])[CH2:4][CH2:3]15-oxo-alpha-hydroxy-2-pyrrolidinepropanoic acid (1.1 g), triethylamine (1.2 g), N-N-dimethylaminopyridine (1.5 g), and benzyl chloroformate (17 g) in 40 ml tetrahydrofuran is heated at reflux for 16 hours. After cooling and filtering off the triethylamine-hydrochloride, the solution was concentrated and chromatographed on SiO2 (elution with methanol:chloroform; 3:97) to yield 5-oxo-alpha-hydroxy-2-pyrrolidinepropanoic acid, benzyl ester mp 95°-96° C. NMR (CDCl3) and 7.37 (s, 5H), 7.93 (d, 1H), 5.40 (s, 1H), 1.70 (m, 2H). IR (KBr) 3330, 3200, 1733, 1691, 1497, 1456, 1389, 1351, 1287, 1216.
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