To a stirred solution of S-benzyl-N-(3-benzylthio-2,2-dimethylpropionyl)-L-cysteine (4.01 g), Nε -t-butoxycarbonyl-L-lysine methyl ester (2.08 g) and NMM (1.1 ml) in DMF (20 ml), HOBt (2.16 g) and then a solution of DCC (1.82 g) in DMF (10 ml) were added slowly under ice cooling. The mixture was stirred for 1 hr. under ice cooling and for 18 hr. at room temperature. After an addition of a mixture of ethyl acetate and benzene (2:1, 500 ml), the mixture was washed with 10% citric acid solution, water, saturated sodium bicarbonate solution, water and then saturated sodium chloride solution. The organic layer was dried over anhydrous sodium sulfate and concentrated in vacuo. The oily residue was purified by a silica gel column chromatography to give 3.22 g (61%) of the titled compound.
