Ac-L-prolyl-L-alanyl-L-proline (0.65 g, 2.00 mmol) wassuspended in dry acetonitrile (20 ml) under an argon atmosphere in a flask fitted with an overhead stirrer and internal thermometer. The suspension was cooled to -15° C. and N-methylmorpholine (0.22 ml, 2.00 mmol) was added followed by isobutylchloroformate (0.26 ml, 2.00 mmol) at such a rate as to maintain the internal reaction temperature at -10° to -15° C. After 10 minutes, a solution of methyl (3S)-3-amino-2-hydroxybutanoate (0.53 g, 4.00 mmol) in chloroform (2.5 ml) was added at such a rate as to maintain the internal reaction temperature at -10° to -15° C. The reaction mixture was slowly warmed to room temperature and then stirred for 3 hours. The reaction mixture was evaporated on a rotary evaporator, the residue dissolved in water (20 ml) and treated with a mixed bed resin (J. T. Baker--ANGMI-615, 11.0 g). After 15 minutes the mixture was filtered and the filtrate evaporated on a rotary evaporator. Chromatography using methylene chloride-methanol as the eluent gave the above-named product (0.32 g) in 36% yield.
