BOC-Cys(Acm)-OSu (77.5 g, 198 mM) was dissolved in THF (400 ml). H-Asp(OBzl)-OH (44.64 g, 200 mM) was added thereto, and the mixture was adjusted to pH 7 by adding NMM (18 ml) at 0° C.; DMF (300 ml) was added and the mixture was stirred overnight. The DMF was removed in vacuo, and 5% sodium bicarbonate (150 ml) was added to the residue, which was then washed twice with ether. The aqueous layer was adjusted to pH 3 by adding 1 N HCl (200 ml) to precipitate an oily material, which was extracted three times with ethyl acetate (300 ml), and the ethyl acetate layer was washed three times with aqueous sodium chloride and water. After drying with anhydrous sodium sulfate, the solution was concentrated in vacuo. Ether and hexane were added to the residue to obtain a precipitate. Recrystallization was effected with ethyl acetate-ether to yield the substance [42] (61.98 g, yield: 62.9%).
