A solution of ((1S,2R)-3-amino-1-benzyl-2-hydroxy-propyl)-carbamic acid tert-butyl ester (D102) (25.6 g, 91.4 mmol, 1 equiv) in DMF (250 ml) at 0° C. was treated with NEt3 (15 ml, 108 mmol, 1.2 equiv) and then with benzyl chloroformate (14 ml, 98 mmol, 1.1 equiv) in DMF (50 ml) dropwise. The resulting solution was stirred at 0° C. for 1 h and at room temperature for 16 h and then concentrated in vacuo. The residue was partitioned between AcOEt and saturated aqueous NaHCO3 solution. The resulting precipitate was diluted with H2O and filtered to give ((2R,3S)-3-tert-butoxycarbonylamino-2-hydroxy-4-phenyl-butyl)-carbamic acid benzyl ester (D103) (31.5 g, 83%) as a white solid.
