[CH2:1]([O:3][C:4]([C:6]1([C:17]([OH:19])=O)[CH2:9][N:8]([C:10]([O:12][C:13]([CH3:16])([CH3:15])[CH3:14])=[O:11])[CH2:7]1)=[O:5])[CH3:2].CC[N:22](CC)CC.ClC(OCC(C)C)=O>C1COCC1.N>[CH2:1]([O:3][C:4]([C:6]1([C:17](=[O:19])[NH2:22])[CH2:9][N:8]([C:10]([O:12][C:13]([CH3:16])([CH3:15])[CH3:14])=[O:11])[CH2:7]1)=[O:5])[CH3:2]
Azetidine-1,3,3-tricarboxylic acid 1-tert-butyl ester 3-ethyl ester (Example 11b; 21.5 g; 78.9 mmol) and NEt3(17.6 ml; 126 mmol) are dissolved in THF (200 ml) and cooled to 0° C. Isobutyl chloroformate (15.5 ml; 118 mmol) is added dropwise. The thick suspension is stirred for another 10 minutes at 0° C., diluted with THF (200 ml) and NH3-gas introduced for 5 minutes, stirred for 15 minutes at 0° C. followed by a second NH3-gas treatment for 5 minutes. The reaction mixture is warmed to room temperature and stirred for 30 minutes, poured on water and extracted with TBME three times. The combined organic phases are dried over Na2SO4, filtered, evaporated and purified via chromatography (SiO2; TBME ) to yield the title compound as colorless crystals (17.3 g; 80%).
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