[CH:1]([CH:3]1[CH2:8][CH2:7][N:6]([C:9]([O:11][C:12]([CH3:15])([CH3:14])[CH3:13])=[O:10])[CH2:5][CH2:4]1)=O.[OH-].[K+].[CH3:18][C:19](=[O:22])[CH:20]=[CH2:21]>CCO>[O:22]=[C:19]1[CH2:20][CH2:21][C:3]2([CH2:8][CH2:7][N:6]([C:9]([O:11][C:12]([CH3:15])([CH3:14])[CH3:13])=[O:10])[CH2:5][CH2:4]2)[CH:1]=[CH:18]1To a solution of tert-butyl 4-formylpiperidine-1-carboxylate (10.0 g, 46.9 mmol) and KOH (1.3 g, 23.5 mmol) in EtOH (200 mL) was added but-3-en-2-one (3.9 g, 56.3 mmol), the mixture was stirred at 70° C. for 16 h and concentrated in vacuo. The residue was purified by silica gel column chromatography (EtOAc/PE (v/v)=1/4) to give the product as brown oil (5.2 g, 41.8%).
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