To a solution of BOC-proline (Anderson G W and McGregor A C: J. Amer. Chem. Soc.: 79, 6810, 1994) (10 mmol) in dichloromethane (50 ml), cooled to 0° C., was added triethylamine (1.39 ml, 10 mmol) and ethyl chloroformate (0.96 ml, 10 mmol). The resultant mixture was stirred at 0° C. for 30 minutes. A solution of dibenzyl-L-glutamate (10 mmol) was then added and the mixture stirred at 0° C. for 2 hours then warmed to room temperature and stirred overnight. The reaction mixture was washed with aqueous sodium bicarbonate and citric acid (2 mol 1-1) then dried (MgSO4) and concentrated at reduced pressure to give BOC-L-proline-L-glutamic acid dibenzyl ester (5.0 g, 95%).
