[CH2:1]([O:3][C:4]([C:6]1(C(O)=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].C([N:22](CC)CC)C.C1(P(N=[N+]=[N-])(C2C=CC=CC=2)=O)C=CC=CC=1.[C:44]([O-:47])([O-])=[O:45].[Na+].[Na+].[C:50]1([CH3:56])[CH:55]=CC=C[CH:51]=1>C(O)(C)(C)C>[CH2:1]([O:3][C:4]([C:6]1([NH:22][C:44]([O:47][C:50]([CH3:56])([CH3:55])[CH3:51])=[O:45])[CH2:7][N:8]([C:10]([O:12][C:13]([CH3:14])([CH3:15])[CH3:16])=[O:11])[CH2:9]1)=[O:5])[CH3:2]Azetidine-1,3,3-tricarboxylic acid 1-tert-butyl ester 3-ethyl ester (1 g; 3.67 mmol), triethyl-amine (0.52 ml; 3.7 mmol) and diphenylphosphoryl azide (0.8 ml; 3.67 mmol) in toluene (6 ml) are heated to 80° C. for 4 hours. tert-Butanol (6 ml) is added and heating continued for 4 hours. The reaction mixture is poured on 2N Na2CO3, extracted with TBME three times, the organic phases are combined, dried over Na2SO4 and evaporated to dryness. Purification via chromatography (SiO2; TBME/hexanes 2:8>1:0) yields the desired compound as colorless crystals. 1H-NMR (400 MHz; DMSO-d6: 8.04 (s, 1H); 4.14 (q, 2H); 4.09 (bd, 2H); 3.81 (bd, 2H); 1.39 (s, 18H); 1.19 (t, 3H). MS (m/z) ES+: 345 (MH+).
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