[Synthesis]
Step 4: Preparation of [1-(tert-butoxycarbonyl)-4-methylpiperidin-4-yl]acetic acid: tert-butyl 4-(cyanomethyl)-4-methylpiperidine-1-carboxylate (4.5 g, 18.9 mmol) was dissolved in concentrated hydrochloric acid (100 mL) and the reaction was reacted at reflux for 48 hr. Upon completion of the reaction, the mixture was cooled to room temperature, diluted with water (200 mL), and the pH was subsequently adjusted to 12 with 2 M NaOH solution. di-tert-butyl dicarbonate (4.12 g, 18.9 mmol) was added to the reaction system and the reaction was stirred for 16 h at room temperature. At the end of the reaction, the solvent was removed by evaporation and the pH was adjusted to 5 with 2 M HCl solution.The aqueous phase was extracted with dichloromethane (200 mL), the organic phases were combined, dried over anhydrous sodium sulfate, and concentrated under reduced pressure to afford the brown oily product [1-(tert-butoxycarbonyl)-4-methylpiperidin-4-yl]acetic acid (3.54 g, 72% yield). NMR hydrogen spectral (CDCl3) data: δ 1.2 (s, 3H), 1.5-1.7 (m, 13H), 2.4 (s, 2H), 3.4 (m, 2H), 3.6 (m, 2H). |