C([O-])(=O)C.[Na+].Cl.[NH2:7][CH2:8][C:9](=O)[CH2:10][CH3:11].F[B-](F)(F)F.[NH:18]=[C:19](SC)[C:20]([O:22][CH2:23][CH3:24])=[O:21]>C(O)(=O)C>[CH2:10]([C:9]1[NH:18][C:19]([C:20]([O:22][CH2:23][CH3:24])=[O:21])=[N:7][CH:8]=1)[CH3:11]Sodium acetate (9.3 g, 113.4 mmol) was added to a solution of 1-amino-butan-2-one hydrochloride (3.41 g, 27.60 mmol) and ethyl imino(methylthio)acetate tetrafluoroborate (7.45 g, 31.84 mmol) synthesized according to the method described in the document (J. Med. Chem., 38, 1995, 2196-2201) in acetic acid (120 mL), and the mixture was stirred at 100° C. for four hours. The reaction solution was concentrated under reduced pressure and then saturated aqueous sodium bicarbonate solution was added to the reaction solution, followed by extraction with ethyl acetate. Then, the organic layer was washed with brine and dried over anhydrous sodium sulfate. Following concentration under reduced pressure, the residue was purified by silica gel column chromatography (elution solvent: hexane/ethyl acetate=5/1, 2/1, 1/2) to obtain 3.35 g of the title compound as a pale yellow solid (72%).
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