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0 °C

(2R)-2-Amino-3-phenylpropionic acid methyl ester (3.2 g; 15.0 mmol) and (2-oxoethyl)carbamic acid tert-butyl ester (3.2 g; 20.0 mmol) (prepared as in Dueholm et al, Org. Prep. Proced. Int. 25(4), 457-61 (1993)) were dissolved in a mixture of methanol (100 ml) and acetic acid (5 ml). Molecular sieves (3 Å; 100 g) were added and the reaction mixture was cooled to 0° C. Sodium cyanoborohydride (1.38 g; 22.0 mmol) was added portionwise and the reaction mixture was left without stirring for 12 h. The reaction mixture was filtered and water (100 ml), aqueous sodium hydrogencarbonate/sodium carbonate (10%; 100 ml; pH 9) and methylene chloride (50 ml) were added to the filtrate. The aqueous phase was extracted with methylene chloride (2×50 ml) and the combined organic phases were dried over magnesium sulfate. The organic phase was evaporated in vacuo and the residue was chromatographed on silica (4×40 cm) using ethyl acetate/heptane (3:2) as eluent to afford 2.07 g of (2R)-2-(2-(tert-butoxycarbonylamino)ethylamino)-3-phenylpropionic acid methyl ester.

1. additionMolecular sieves (3 Å; 100 g) were added
2. waitthe reaction mixture was left
3. filtrationThe reaction mixture was filtered
4. additionwater (100 ml), aqueous sodium hydrogencarbonate/sodium carbonate (10%; 100 ml; pH 9) and methylene chloride (50 ml) were added to the filtrate
5. extractionThe aqueous phase was extracted with methylene chloride (2×50 ml)
6. dry with materialthe combined organic phases were dried over magnesium sulfate
7. customThe organic phase was evaporated in vacuo
8. customthe residue was chromatographed on silica (4×40 cm)