[CH3:1][O:2][C:3]([C:5]1([CH2:16][NH2:17])[CH2:8][N:7]([C:9]([O:11][C:12]([CH3:15])([CH3:14])[CH3:13])=[O:10])[CH2:6]1)=[O:4].C(N(CC)CC)C.[CH2:25]([O:27][C:28](=[O:33])[CH2:29][C:30](Cl)=[O:31])[CH3:26].C([O-])(O)=O.[Na+]>C(Cl)Cl>[CH3:1][O:2][C:3]([C:5]1([CH2:16][NH:17][C:30](=[O:31])[CH2:29][C:28]([O:27][CH2:25][CH3:26])=[O:33])[CH2:8][N:7]([C:9]([O:11][C:12]([CH3:14])([CH3:13])[CH3:15])=[O:10])[CH2:6]1)=[O:4]3-Aminomethyl-azetidine-1,3-dicarboxylic acid 1-tert-butyl ester 3-methyl ester (3.11 g; 12.7 mmol) and triethylamine (1.95 ml; 14.01 mmol) in CH2Cl2 (55 ml) are cooled to 0° C. under stirring. Chlorocarbonyl-acetic acid ethyl ester (1.9 g; 12.74 mmol) in CH2Cl2 (15 ml) is added dropwise within 15 minutes. The reaction mixture is warmed to room temperature and stirred for 30 minutes, poured on 10% aqueous NaHCO3 and extracted with CH2Cl2 three times. The combined organic phases are dried over Na2SO4, filtered, evaporated and purified via chromatography (SiO2; TBME/MeOH 97:3) to yield the title compound as colorless resin (3.8 g; 77%), which crystallizes slowly.
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