[C:1]([O:5][C:6](=[O:13])[NH:7][C:8]1[CH:12]=[CH:11][S:10][CH:9]=1)([CH3:4])([CH3:3])[CH3:2].C([Li])CCC.CN(C)[CH:21]=[O:22].O>O1CCCC1>[C:1]([O:5][C:6](=[O:13])[NH:7][C:8]1[CH:12]=[CH:11][S:10][C:9]=1[CH:21]=[O:22])([CH3:4])([CH3:2])[CH3:3]Thiophen-3-yl-carbamic acid tert-butyl ester described in Preparation Example U-1 (1.00 g, 5.02 mmol) was dissolved in tetrahydrofuran (20 mL), to which n-butyl lithium (2.47M hexane solution, 4.47 mL, 11.0 mmol) was added at −78° C., and the mixture was stirred at −78° C. for 1 hour. N,N-dimethylformamide (466 μl, 6.02 mmol) was added to the reaction mixture at −78° C., and the solution was stirred for 1 hour at room temperature. Water was added to the reaction solution, which was then extracted with ethyl acetate, the organic layer was washed with brine and dried over anhydrous magnesium sulfate. The solvent was evaporated in vacuo, the residue was purified by silica gel column chromatography (hexane:ethyl acetate=10:1), and the title compound (1.14 g, quantitatively) was obtained as a colorless oil.
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