A solution of N-(tert-butyloxycarbonyl)-O-benzyl-L-tyrosine (Bachem, 2.00 g, 5.38 mmol) in 20 mL of DMF was cooled to 0° C. and treated with iPr NEt (1.5 mL) followed by O-benzotriazol-1-yl-N,N,N′,N′-bis(tetramethylene)-uronium hexafluoro phosphate (2.04 g, 5.38 mmol, Novabiochem, La Jolla, Calif.). The resulting suspension was stirred for 30 minutes at 0° C. and then treated with tert-butylamine (0.48 g, 6.56 mmol). The reaction mixture was stirred for 1 hour at 0° C. and warmed to room temperature. The reaction mixture was poured into Et O and washed sequentially with saturated aqueous NaHCO3 solution, and saturated aqueous NaCl solution. The organic phase was dried (MgSO4), filtered, and concentrated. The crude residue was purified by chromatography (silica gel, 3:1 heptane/ethyl acetate) to give (S)-[2-[(1,1-dimethylethyl)amino]-2-oxo-1-(phenylmethyl)ethyl]-carbamic acid 1,1-dimethylethyl ester (2.65 g).
