[CH2:1]([O:3][C:4]([C:6]1([C:17](=O)[NH2:18])[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].N1C=CC=CC=1.FC(F)(F)C(OC(=O)C(F)(F)F)=O>C1COCC1>[CH2:1]([O:3][C:4]([C:6]1([C:17]#[N:18])[CH2:9][N:8]([C:10]([O:12][C:13]([CH3:15])([CH3:14])[CH3:16])=[O:11])[CH2:7]1)=[O:5])[CH3:2]3-Carbamoyl-azetidine-1,3-dicarboxylic acid 1-tert-butyl ester 3-ethyl ester (25.4 g; 93.5 mmol) is dissolved in THF (260 ml) and pyridine (19.2 g; 243 mmol) and cooled to 0° C. under stirring. Trifluoroacetic acid anhydride (26.4 ml; 187 mmol) is added dropwise over 10 minutes, the mixture 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; heptane/TBME 6:4) to yield the title compound as light brown oil (22.4 g; 94%).
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