[CH2:1]([C@H:8]1[CH2:12][O:11][C:10](=[O:13])[NH:9]1)[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.C([Li])CCC.CCCCCC.[C:25](Cl)(=[O:29])[CH2:26][CH2:27][CH3:28].[Cl-].[NH4+]>O1CCCC1>[CH2:1]([C@H:8]1[CH2:12][O:11][C:10](=[O:13])[N:9]1[C:25](=[O:29])[CH2:26][CH2:27][CH3:28])[C:2]1[CH:3]=[CH:4][CH:5]=[CH:6][CH:7]=1(S)-4-benzyl-2-oxazolidinone (2.26 g, 15.0 mmol) and 30 ml of dehydrated tetrahydrofuran were mixed, which was cooled to −78° C. under an atmosphere of argon. Under stirring, 1.6 mol/l solution of n-butyl lithium in hexane (10.3 ml, 16.5 mmol) was added dropwise for over 10 minutes and the mixture was stirred for 30 minutes as it was. Next, butyryl chloride (1.56 ml, 15.0 mmol) dissolved into 30 ml of dehydrated tetrahydrofuran was added dropwise for over 10 minutes and the mixture was stirred for 30 minutes, followed by stirring for 3 hours at room temperature. Saturated aqueous solution of ammonium chloride was added to the reaction mixture and concentrated. Water was added to the residue, which was extracted with ethyl acetate. The extract was washed with water and with brine, then dried over anhydrous sodium sulfate and concentrated. The residue was purified by silica gel chromatography (eluate n-hexane:ethyl acetate=1:1 v/v) to obtain 3.64 g (98%) of the title compound as a colorless oil.
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