[CH3:1][C:2]([O:5][C:6]([N:8]1[CH2:11][CH:10]([CH2:12][C:13]([OH:15])=[O:14])[CH2:9]1)=[O:7])([CH3:4])[CH3:3].Cl.CN(C)[CH2:19][CH2:20]CN=C=NCC.C(O)C>C(OCC)C.CN(C)C1C=CN=CC=1>[CH2:19]([O:14][C:13](=[O:15])[CH2:12][CH:10]1[CH2:11][N:8]([C:6]([O:5][C:2]([CH3:1])([CH3:3])[CH3:4])=[O:7])[CH2:9]1)[CH3:20]
In an oven-dried 250 mL round bottom flask under nitrogen, a mixture of (1-{[(1,1-dimethylethyl)oxy]carbonyl}-3-azetidinyl)acetic acid (10 g, 46.5 mmol) in diethyl ether (100 mL) was treated with N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (9.80 g, 51.1 mmol), 4-(dimethylamino)pyridine (0.568 g, 4.65 mmol), and ethanol (5.97 mL, 102 mmol) at room temperature and the white mixture was stirred over the weekend. The white reaction mixture became a colorless solution with gummy material at the bottom of the flask after 1 h. The reaction mixture was diluted with ether (300 mL) and washed with 1M aq NaHSO4 solution (150 mL) and saturated aq NaHCO3 solution (150 mL). The organic phase was isolated, dried over MgSO4, and concentrated in vacuo to give the title compound (10.55 g, 93%) as a clear colorless liquid. MS(ES)+ m/e 244.4 [M+H]+.
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