To a solution of N-Cbz-L-valine (1.0 g, 3.98 mmol) dissolved in CH2Cl2 (80 mL) under nitrogen was added hydroxybenzotriazole (0.538 g, 3.98 mmol) and PS-Carbodiimide (2.25 g, 5.17 mmol). After stirring for 10 min, triethylamine (0.665 mL, 4.78 mmol) and ((R)-3-amino-2-hydroxy-propyl)-cyclohexylmethyl-phosphinic acid benzyl ester hydrochloride (1.44 g, 3.98 mmol) were added. The reaction was stirred for 2 h, then filtered and concentrated in vacuo. The residue was purified by flash chromatography with methanol/dichloromethane on a 35 g RediSep disposable column to give [(R)-3-((S)-2-benzyloxycarbonylamino-3-methyl-butyrylamino)-2-hydroxy-propyl]-cyclohexylmethyl-phosphinic acid benzyl ester (0.60 g, 1.07 mmol) as a white foam. NMR and MS spectra were substantially similar to the product obtained via Method A set forth above.
