To a 500 mL four-necked flask equipped with a stirrer, thermometer, Dimroth condenser tube, and constant-pressure dropping funnel was added 17.6 g (0.1 mol, optical purity 99.5% ee) of (S)-3-amino-1-benzylpyrrolidine. Subsequently, 158.7 g of water and 0.2 g of cationic DS (manufactured by Sanyo Chemical Industries, Ltd.) were added, and the pH of the mixture was adjusted to 11.0 ± 0.5 using a 48% aqueous sodium hydroxide solution.The mixture was stirred at 50 to 60 °C while 26.2 g (0.12 mol) of di-tert-butyl dicarbonate (abbreviated as DiBoc), the titration process lasted about 2 hours. During this time, the pH of the reaction solution was maintained at 11.0 ± 0.5 with 48% aqueous sodium hydroxide.After the dropwise addition, stirring was continued for 1 h. The reaction solution was then cooled to room temperature and the crystals were separated by filtration. The resulting crystals were dried under vacuum at 50 °C to afford 26.0 g of (S)-1-benzyl-3-(tert-butoxycarbonylamino)pyrrolidine in 94.1% yield, 99.1% chemical purity, and 99.5% ee optical purity.
Using the same apparatus as above, 26.0 g (optical purity 99.5% ee) of (S)-1-benzyl-3-(tert-butoxycarbonylamino)pyrrolidine, 120 g of water, and 2.6 g of 5% Pd/C (PE-type, 55.27% water content, produced by NE Chemcat Corp.) were added and stirred at a reaction temperature of 40 °C for 10 h under hydrogen atmosphere. The reaction solution was analyzed by GC and it was confirmed that the feedstock peak disappeared and only 3-(tert-butoxycarbonylamino)pyrrolidine and toluene were detected. Upon completion of the reaction, the Pd/C was removed by filtration and the filtrate was concentrated using a rotary evaporator to 30 g. Subsequently, the concentrate was mixed with toluene and concentrated again to 20 g. Water was removed by azeotropic distillation. Under stirring, 25 g of hexane was slowly added to the concentrate to precipitate crystals and stirring was continued for 2 hours in an ice bath. The precipitated crystals were separated by filtration and dried under vacuum to give a final 15.4 g of (S)-3-(tert-butoxycarbonylamino)pyrrolidine in 87.4% yield, 99.5% chemical purity, 99.5% optical purity ee, and 0.4% moisture content.
