The general procedure for the synthesis of N-Boc-N'-Cbz-L-lysine from N-alpha-Boc-L-lysine and phenylmethoxycarbonylsuccinimide was as follows: to a mixed solution of (2S)-6-amino-2-(tert-butoxycarbonylamino)hexanoic acid (100.00 g, 406.01 mmol, 1.00 equiv) and NaHCO3 (102.33 g 1.22 mol, 3.00 eq.) in a mixed solution of THF (1000 mL) and water (1000 mL) was slowly added CbzOSu (101.19 g, 406.01 mmol, 1.00 eq.) at 0 °C. Subsequently, the reaction mixture was stirred at 30 °C for 16 hours. Upon completion of the reaction, the pH of the reaction mixture was adjusted to 5-6 with 1N HCl and then extracted with ethyl acetate (600 mL x 3). The organic phases were combined, dried with anhydrous Na2SO4 and concentrated under reduced pressure to afford the target product (2S)-6-(benzyloxycarbonylamino)-2-(tert-butoxycarbonylamino)hexanoic acid (142.00 g, 373.26 mmol, 91.9% yield) as a yellow oil.
