C[N:2]1[CH2:7][CH2:6][O:5]C[CH2:3]1.[C:8]([O:12][C:13]([NH:15][C@@H:16]([CH:20]1[CH2:23][CH2:22][CH2:21]1)[C:17]([OH:19])=O)=[O:14])([CH3:11])([CH3:10])[CH3:9].CN(C(ON1N=NC2C=CC=CC1=2)=[N+](C)C)C.[B-](F)(F)(F)F.Cl.N1CC(O)C1>C(Cl)Cl.CN(C=O)C.C([O-])(O)=O.[Na+]>[CH:20]1([C@H:16]([NH:15][C:13](=[O:14])[O:12][C:8]([CH3:9])([CH3:10])[CH3:11])[C:17]([N:2]2[CH2:7][CH:6]([OH:5])[CH2:3]2)=[O:19])[CH2:23][CH2:22][CH2:21]1NMM (4.26 mL, 38.8 mmol) was added to a mixture of (S)-2-(tert-butoxycarbonylamino)-2-cyclobutylacetic acid (4.04 g, 17.6 mmol), which is commercially available, in DCM (45 mL) and DMF (14 mL). The mixture was cooled to 0° C. and TBTU (5.66 g, 17.6 mmol) was added. After 10 min was azetidin-3-ol hydrochloride (2.40 g, 21.9 mmol) added. The resultant mixture was stirred at rt over night. The mixture was washed with NaHCO3 (8% aq., 30 mL). A second wash with NaHCO3 (8% aq., 30 mL) resulted in an emulsion. The emulsion was diluted with DCM (50 mL) and NaHCO3 (8% aq., 30 mL), mixed well and left to separate. The organic layer was dried (Na2SO4) and concentrated under reduced pressure. The residue was co-evaporated with toluene to give the title compound (4.10 g, 82%) as a solid. The obtained crude product was used without further purification. 1H NMR (400 MHz, CD3OD) δ 1.44 (9H), 1.76-2.09 (6H), 2.51-2.65 (1H), 3.67-3.82 (1H), 3.97-4.27 (3H), 4.45-4.69 (2H). Expected Number of protons: 24. OH- and NH-protons are exchanged with CD3OD.
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