4-(Pivaloyloxymethoxycarbonyl)-2,4-(R*,R*)-dibenzylbutyryl chloride (1.2 g) in 10 ml of methylene chloride is added slowly to a solution of 0.60 g of (L)-leucine benzyl ester and 0.30 ml of triethylamine in 20 ml of methylene chloride. The reaction is stirred overnight at room temperature. The solution is washed with 2N hydrochloric acid, then with saturated sodium bicarbonate, dried over magnesium sulfate, filtered and concentrated to give N-[4-pivaloyloxymethoxycarbonyl-(R*,R*)-2,4-dibenzylbutyryl]-L-leucine benzyl ester.
