The general procedure for the synthesis of tert-butyl 4-(2-ethoxy-2-oxo-substituted)piperidine-1-carboxylate from triethyl phosphonoacetate and N-tert-butoxycarbonyl-4-piperidone was as follows: to a solution of tetrahydrofuran (200 mL) of ethyl diethylphosphonoacetate (28.3 g) was added 60% sodium hydride (4.82 g). The mixture was stirred under cooling in an ice bath for 30 min, followed by the slow dropwise addition of a tetrahydrofuran (200 mL) solution of N-tert-butoxycarbonyl-4-piperidone (20 g). The reaction mixture was stirred continuously for 22 h at room temperature. Upon completion of the reaction, the reaction was terminated by the addition of water (200 mL) and extracted with ethyl acetate. The organic phases were combined and washed with saturated aqueous sodium chloride solution and subsequently dried over anhydrous magnesium sulfate. After concentration under reduced pressure, the crude product was purified by silica gel column chromatography using hexane/ethyl acetate (6:1) as eluent to finally obtain tert-butyl 4-(2-ethoxy-2-oxo-substituted)piperidine-1-carboxylate (27.3 g, 100% yield) as a colorless powder. The product was characterized by 1H NMR (CDCl3): δ 1.28 (3H, t, J = 7.4 Hz), 1.47 (9H, s), 2.24-2.33 (2H, m), 2.90-2.98 (2H, m), 3.43-3.55 (4H, m), 4.16 (2H, q, J = 7.4 Hz), 5.70-5.73 (1H, m ).
