[C:1]([O:5][C:6]([N:8]1[CH2:13][CH2:12][C:11](=[CH:14][CH2:15][OH:16])[CH2:10][CH2:9]1)=[O:7])([CH3:4])([CH3:3])[CH3:2].C(Cl)(Cl)Cl>CCCCCC.[O-2].[O-2].[Mn+4]>[C:1]([O:5][C:6]([N:8]1[CH2:13][CH2:12][C:11](=[CH:14][CH:15]=[O:16])[CH2:10][CH2:9]1)=[O:7])([CH3:4])([CH3:3])[CH3:2]To a solution of 4-(2-hydroxyethylidene)piperidine-1-carboxylic acid t-butyl ester (10.4 g, 46 mmol) in a mixture of hexane (300 mL)-chloroform (100 mL) was added manganese dioxide (100 g), and stirred for 10 hours. After the reaction was completed, insoluble materials were filtered off through Celite and the filtrate was concentrated under a reduced pressure to obtain 9.2 g (yield 89%) of 4-(2-oxoethylidene)piperidine-1-carboxylic acid t-butyl ester (VII-18, 20, 21, 24, 30, 41, 42, 43, 44, 45).
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