A suspension of (S)-2-tert-butoxycarbonylamino-3-(phenyl-pyrimidin-2-ylamino)-propanoic acid (11.9 g, 33.2 mmol, >99.9% ee) in acetonitrile (36 ml) is admixed at −15° C. with a solution of 1,1′-carbonyldiimidazole (8.36 g, 51.6 mmol) in acetonitrile (125 ml), and the reaction mixture is stirred for 2 h at this temperature. Ammonia (4.0 g, 222 mmol) is then introduced, and the mixture is stirred for further 1 h and then adjusted to pH 4.5 using 2N sulfuric acid. Acetonitrile is distilled off, and the precipitated solid is filtered off with suction and dried to give (S)-2-tert-butoxycarbonylamino-3-(phenyl-pyrimidin-2-ylamino)-propanoic acid amide (11.3 g, 95%, >99.9% ee) as a colorless solid.
