[CH2:1]([N:8]1[CH2:13][CH2:12][C:11](=[O:14])[CH2:10][CH2:9]1)[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.[N+](=[CH:17][C:18]([O:20][CH2:21][CH3:22])=[O:19])=[N-].C(=O)(O)[O-].[Na+]>O1CCCC1>[CH2:1]([N:8]1[CH2:9][CH2:10][C:11](=[O:14])[CH:17]([C:18]([O:20][CH2:21][CH3:22])=[O:19])[CH2:12][CH2:13]1)[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1Commercially available 1-benzylpiperidin-4-one (78.5 g, 0.415 mol) was dissolved in tetrahydrofuran (300 mL) and the solution was cooled to −25° C. Ethyl diazoacetate (56.8 g) and boron trifluoride diethyl ether complex (128 mL) were simultaneously added dropwise to the solution over one hour, followed by stirring for one hour while elevating the temperature from −25° C. to 0° C. The reaction mixture was mixed with a saturated aqueous sodium bicarbonate solution and was extracted with ethyl acetate. The organic layer was washed with brine and was dried over anhydrous sodium sulfate. After distilling off the solvent, the residue was purified by silica gel column chromatography (n-hexane:ethyl acetate=4:1) and thereby yielded ethyl 1-benzyl-5-oxoperhydroazepine-4-carboxylate (30.6 g, in a yield of 27%).
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