A solution of 2.65 g. of Nα -benzyloxycarbonyl-D-leucine [This material is prepared by the same procedure as used by Grassman and Wunsch, Ber. 91, 462 (1968) for the DL-leucine and for the L-leucine see Losse and Demuth, Ber. 94, 1762 (1961). The material is an oil as described for the Nα -benzyloxycarbonyl-L-leucine enantiomer. See also Farthing, J. Chem. Soc. 1950, 3213 and Bergmann, J. Biol. Chem., 115, 593 (1936).] in 50 ml. of dimethylformamide is treated with 1.98 g. of L-leucine methyl ester hydrochloride and cooled in an ice bath. The solution is treated with 1.4 ml. of triethylamine, then with 1.5 g. of 1-hydroxybenztriazole, and finally with 2.26 g. of dicyclohexylcarbodiimide. The reaction is stirred overnight with initial ice bath cooling and gradual warming to room temperature, and then an additional 24 hours at room temperature. The mixture is filtered and the filtrate evaporated. The residue is dissolved in ethyl acetate and the solution washed with 1N hydrochloric acid, saturated sodium chloride solution, 5% sodium bicarbonate solution and again with saturated sodium chloride solution. The ethyl acetate solution is separated and dried over magnesium sulfate, filtered and evaporated. The residue crystallizes and is recrystallized from isopropyl ether; [α]D23 -5.3° (c 2.06, methanol), m.p. 80°-82° C.
