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 2 N hydrochloric acid, then with saturated sodium bicarbonate, dried over magnesium sulfate, filtered and concentrted to give N-[4-pivaloyloxymethoxycarbonyl-(R*,R*)-2,4-dibenzylbutyryl]-L-leucine benzyl ester.
