Benzyloxycarbonyl-L-leucyl-L-threonine methyl ester (I, 3.8 g, 10 mmole) was dissolved in a mixture of 95% EtOH (50 ml) and 1 N HCl (10 ml) and hydrogenated in the presence of a 10% Pd on charcoal catalyst (0.8 g). After removal of the catalyst and the solvent the residue was dissolved in DMF (35 ml), treated with DIEA (1.6 ml, 10 mmole), benzyloxycarbonyl-L-valine p-nitrophenyl ester (4.3 g, 12 mmole) and HOBt (1.5 g, 10 mmole). After a negative spot test with ninhydrin indicated the completion of the reaction, the solvent was removed in vacuo and the residue triturated with CHCl3 (40 ml). The resulting solid was washed with CHCl3 (20 ml) and dried: 4.5 g (94%, mp 217°-8° C.,) unchanged after extraction with solvents, [α]D24 -11.7° (c 1.5, DMF); RfC 0.75; Thr 1.08; Val 0.98; Leu 1.0.
