[Synthesis]
GENERAL STEPS: To a stirred mixed solution of ethyl (S)-2-amino-3-(4-nitrophenyl)propanoate hydrochloride (23.0 g, CAS No. 58816-66-3) in dichloromethane (230 mL) and water (230 mL) was slowly added 46-48% sodium hydroxide solution (7.7 g, 1.1 eq.). The organic and aqueous layers were separated and the aqueous phase was extracted with dichloromethane (100 mL). The combined dichloromethane layers were washed sequentially with water (100 mL) and saturated brine (100 mL). The organic phase was dried over anhydrous magnesium sulfate and concentrated under reduced pressure to give ethyl (S)-2-amino-3-(4-nitrophenyl)propionate in quantitative yield. The resulting free nitro ester was dissolved in fresh dichloromethane (120 mL), spiro[3,5]nonane-1,3-dione (12.9 g, CAS No. 455264-97-8) [Ref: Wasserman, H.H. et al., J. Org. Chem., 38, 1451-1455 (1973)] was added, and the reaction was stirred. After 16 h, the completion of the reaction was confirmed by HPLC monitoring. The reaction mixture was diluted with dichloromethane (120 mL), washed sequentially with 11% sodium bicarbonate solution (100 mL), saturated brine (100 mL) and then dried over anhydrous magnesium sulfate. After removing the solvent under reduced pressure, ethyl (S)-2-amino-3-(4-nitrophenyl)propionate was obtained in quantitative yield (32.4 g as a slowly crystallizing viscous oil; melting point 120 °C). NMR hydrogen spectrum (DMSO-d6) δ: 8.39 (1H, d), 8.17 (2H, d), 7.56 (2H, d), 4.33 (1H, s), 4.31 (1H, m), 4.14 (2H, q), 3.29 (1H, dd), 3.15 (1H, dd), 1.43-1.70 (8H, m), 1.30 (1H , m), 1.15 (3H, t + 1H, m). Mass spectrum (ESI+) m/z: 373.3 [M+H]+. |