[C:1]([OH:4])(=[O:3])[CH3:2].CN(C=O)C.C([O-])(=O)C.[Na+].Br[CH2:16][C:17]([C:19]1[N:20]=[CH:21][C:22]([NH:25][C:26](=[O:31])[C:27]([CH3:30])([CH3:29])[CH3:28])=[N:23][CH:24]=1)=[O:18]>C(OCC)(=O)C.C(=O)(O)[O-].[Na+]>[CH3:28][C:27]([CH3:30])([CH3:29])[C:26]([NH:25][C:22]1[N:23]=[CH:24][C:19]([C:17](=[O:18])[CH2:16][O:3][C:1](=[O:4])[CH3:2])=[N:20][CH:21]=1)=[O:31]
A 500 mL round bottomed flask equipped with a magnetic stirrer, addition funnel, thermocouple probe and nitrogen inlet/outlet was charged with 4.40 mL (76.86 mmol) of acetic acid, 140 mL of DMF and 7.000 g (85.33 mmol) of sodium acetate. Then, 23.30 g (77.62 mmol) of the compound obtained in step 5 was added portionwise over 45 min. After stirring at room temperature for an additional 1 h, HPLC analysis indicated complete reaction. The reaction mixture was diluted with 350 mL of ethyl acetate and 100 mL of saturated sodium bicarbonate was added with stirring. The organic layer was separated, washed with 3×100 mL of water and concentrated at 30° C./60 mmHg to a volume of ca. 60 mL. The resulting slurry was diluted with 200 mL of heptane, concentrated at 30° C./60 mmHg to a volume of ca. 150 mL, and stirred at 50° C. for 30 min. After cooling to room temperature, the solid was collected by filtration, washed with 40 mL of 10% ethyl acetate in heptane and dried by suction, then under reduced pressure (house vacuum) for 24 h, to give 19.52 g (90.0% yield) of the title compound as an off-white solid; 98.81% pure as determined by HPLC analysis (same conditions as in step 4, retention time 6.78 min).
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