[CH2:1]([N:8]1[CH2:12][CH:11]([C:13]2([C:18]([O:20][CH2:21][CH3:22])=[O:19])[CH2:16][CH:15](O)[CH2:14]2)[CH2:10][C:9]1=[O:23])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.C1(C)C=CC=CC=1.C(N(S(F)(F)[F:37])CC)C.C(=O)(O)[O-].[Na+]>C(Cl)(Cl)Cl.ClCCl>[CH2:1]([N:8]1[CH2:12][CH:11]([C:13]2([C:18]([O:20][CH2:21][CH3:22])=[O:19])[CH2:16][CH:15]([F:37])[CH2:14]2)[CH2:10][C:9]1=[O:23])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1
A 495 mg (1.56 mmol) portion of 1-benzyl-4-[3-(ethoxycarbonyl)-1-hydroxycyclobutan-3-yl]-2-pyrrolidone (Isomer A) was dissolved in a mixed solvent consisting of 12 ml of toluene and 6 ml of dichloromethane to which, while cooling in an ice bath with stirring, was then added 839 μl (6.24 mmol) of diethylaminosulfur trifluoride, subsequently carrying out 20 hours of stirring at 40° C. While cooling in an ice bath with stirring, the reaction solution was alkalified by slowly adding saturated sodium bicarbonate aqueous solution and then mixed with chloroform to carry out separation of layers, and the resulting organic layer was washed with saturated brine. The aqueous layer was again extracted with chloroform, and the organic layers were combined and dried over anhydrous sodium sulfate. After filtration, the solvent was evaporated under a reduced pressure and the resulting residue was purified by a silica gel column chromatography (n-hexane:ethyl acetate=2:1) to give 400 mg (80.0%) of the title compound.
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