t-Butyloxycarbonyl-p-nitrophenylalanine (4.42g, 14.26 mmol, aminoacetone hydrochloride (1.56 g, 14.26 mmol), triethylamine (1.44 g, 14.26 mmol), 1-hydroxybenzotriazole (1.69 g, 12.55 mmol), were dissolved in ethyl acetate (400 ml). 1,3-Dicyclohexylcarbodiimide (3.23 g, 15.68 mmol) in ethyl acetate (25 ml) was added and the solution allowed to stir for 18 hours. Glacial acetic acid (0.2 ml) was added and the solution was filtered. The filtrate was extracted with saturated salt solution (100 ml), lN HCl solution (3×100 ml), and saturated salt solution (100 ml), 5% bicarbonate solution (3×100 ml), and saturated salt solution (100 ml). The organic phase was dried over MgSO4 , filtered and concentrated to 50 ml. Petroleum ether (50 ml) was added and the solution was cooled to 0° C. for 12 hours. The precipitated product was collected and dried under vacuum.
