By using an analogous procedure to that described for Reference Example 13, benzyloxycarbonyl-L-aspartic acid [1-benzyl ester (2 g, 5.6 mmol), hydroxybenzotriazole (756 mg, 5.6 mmol), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (1.07 g, 5.6 mmol), 1-amino-3,3-diethoxypropane (784 mg, 5.3 mmol) and N,N-diisopropylethylamine (0.986 mL, 5.6 mmol) were reacted in anhydrous tetrahydrofuran (30 ml) at room temperature for 16 hours, to provide benzyl (3S)-3-{[(benzyloxy)carbonyl]amino}-4-[(3,3-diethoxypropyl)amino]-4-oxobutanoate (2.81 g, 100%) as a pale yellow waxy solid.
