General procedure for the synthesis of tert-butyl 3-benzyl-4-oxopiperidine-1,3-dicarboxylate from 1-tert-butyl 3-ethyl 3-benzyl-4-oxopiperidine-1-carboxylate: the compound obtained in Method 1 (6.0 g) was dissolved in a mixture of methanol (100 mL) and 6 N hydrochloric acid (250 mL), and the reaction was stirred for 24 hours at 110 °C. Subsequently, concentrated hydrochloric acid (100 mL) was added to the reaction system and stirring was continued at 110 °C for 24 hours. Upon completion of the reaction, the solvent was removed by concentration under reduced pressure. To the residue, 4 N aqueous sodium hydroxide solution was added to ice-cooled conditions, the pH was adjusted to alkaline, and subsequently extracted with dichloromethane. The organic phases were combined, washed with saturated brine, dried over anhydrous sodium sulfate and concentrated under reduced pressure to remove the solvent. The resulting residue was dissolved in tetrahydrofuran (60 mL), cooled to 0 °C, and di-tert-butyl dicarbonate (5.0 mL) was added slowly, and the reaction was stirred for 1 hour at room temperature. After the reaction was completed, the solvent was removed by concentration under reduced pressure, and the residue was separated and purified by silica gel column chromatography (eluent: hexane/ethyl acetate=4:1) to obtain tert-butyl 3-benzyl-4-oxopiperidine-1-carboxylate white crystals (4.20 g, 87% yield). Melting point: 74-75°C. 1H-NMR (CDCl3): δ 1.42 (9H, s), 2.43-2.60 (3H, m), 2.70 (1H, m), 2.97 (1H, dd, J=13.2,9.8 Hz), 3.14-3.41 (2H, m), 3.9-4.2 (2H, m), 7.15-7.35 (5H , m).
