Ac-L-proline (3.05 g, 19.41 mmol) was dissolved in dry tetrahydrofuran (100 ml) under an argon atmosphere in a flask fitted with an overhead stirrer and an internal thermometer. The solution was cooled to -15° C. and N-methylmorpholine (2.13 ml, 19.41 mmol) was added followed by isobutylchloroformate (2.51 ml, 19.41 mmol) at such a rate as to maintain the internal reaction temperature at -10° to -15° C. Five minutes after the addition was completed, a solution of L-alanyl-L-proline benzyl ester hydrochloride (6.08 g, 19.41 mmol) in chloroform (70 ml) was added followed by a second portion of N-methylmorpholine (2.13 ml, 19.41 mmol) at such a rate as to maintain the internal reaction temperature at -10° to -15° C. The reaction mixture was then allowed to slowly warm to room temperature and stirred at room temperature for 2.5 hours. The reaction mixture was concentrated to a small residue on a rotary evaporator, diluted with chloroform (500 ml) and poured into a separatory funnel where it was washed with 0.2N hydrochloric acid (3×200 ml) and 5% sodium bicarbonate (200 ml). The organic layer was dried over magnesium sulfate, filtered, concentrated on a rotary evaporator and chromatographed using methylene chloride-methanol as an eluent to give Ac-L-prolyl-L-alanyl-L-proline benzyl ester; Rf: 0.35 (CH3OH/CH2Cl2 - 7:93).
