[I-].[CH3:2][S+](C)(C)=O.[H-].[Na+].[O:9]=[C:10]([N:25]1[CH2:30][CH2:29][C:28](=[O:31])[CH2:27][CH2:26]1)[CH2:11][N:12]1[CH2:17][CH2:16][N:15]([C:18]([O:20][C:21]([CH3:24])([CH3:23])[CH3:22])=[O:19])[CH2:14][CH2:13]1.O>CS(C)=O>[O:31]1[C:28]2([CH2:27][CH2:26][N:25]([C:10](=[O:9])[CH2:11][N:12]3[CH2:13][CH2:14][N:15]([C:18]([O:20][C:21]([CH3:23])([CH3:24])[CH3:22])=[O:19])[CH2:16][CH2:17]3)[CH2:30][CH2:29]2)[CH2:2]1Trimethylsulfoxonium iodide (2.02 g, 9.20 mmol) was gradually added to a suspension of sodium hydride (385 mg, 9.64 mmol) in dimethylsulfoxide (50 ml), and the resultant mixture was stirred at room temperature for 1 hour. To which tert-butyl 4-[2-oxo-2-(4-oxopiperidin-1-yl)ethyl]piperazine-1-carboxylate (2.85 g, 8.76 mmol) in DMSO (10 ml) was added, and the mixture was stirred at room temperature for 2 hours, and stirred at 55° C. for 1 hour. The reaction mixture was poured into iced water, and extracted with diethyl ether. The organic phase was washed with a saturated saline solution, dried over magnesium sulfate, and filtered. The resultant filtrate was concentrated under a reduced pressure to afford tert-butyl 4-[2-(1-oxa-6-azaspiro[2,5]octan-6-yl)-2-oxoethyl]piperazine-1-carboxylate (2.97 g, yield 99%) as a white solid.
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