2.3 g (0.023 mol) of N-methylpiperidine are added to 5.0 g of t-butoxycarbonyl-L-valine (0.023 mol), dissolved in 50 ml of CH2Cl2, at -20° C. 3.2 g (0.023 mol) of isobutyl chloroformate are then rapidly added dropwise at -20° C., the mixture is subsequently stirred at the same temperature for 10 minutes and cooled to -60° C. and 3.5 g (0.023 mol) of 4-methoxy-1-phenylethylamine are allowed to run in, the temperature being kept below -15° C. After 2 hours at -15° C., the mixture is subsequently stirred at room temperature for 15 hours, the solid is filtered off and rinsed with CH2Cl2, the filtrate is concentrated, the residue is introduced into water, the mixture is extracted twice with ethyl acetate and the combined ethyl acetate phases are washed with NaHCO3 solution and water, dried and concentrated. 17.5 g (85% of theory) of N-(t-butoxycarbonyl)-L-valine 4-methoxy-1-phenylethylamide of melting point 126°-127° C. are obtained.
