The optically active compound of 1-(2,4-dimethoxybenzyl)-6-oxopiperidine-3-carboxylic acid (1.36 g) was added to anisole (758 μL) and trifluoroacetic acid (10 mL) as a starting material, and the reaction mixture was stirred for 6 hours at 80 °C. Upon completion of the reaction, the reaction solution was cooled to room temperature and subsequently concentrated under reduced pressure. Diisopropyl ether was added to the concentrated residue and the mixture was stirred at room temperature. The insoluble solid was collected by filtration and dried under reduced pressure to afford the target product 6-oxopiperidine-3-carboxylic acid (628 mg). The structure of the product was confirmed by 1H-NMR (DMSO-D6): δ 1.75-1.88 (m, 1H), 1.91-2.01 (m, 1H), 2.11-2.24 (m, 2H), 2.66-2.73 (m, 1H), 3.21-3.32 (m, 2H), 7.45 (s, 1H), 12.51 (br s, 1H).
