To a solution of (2R)-2-(N-tert-butoxycarbonyl-N-methylamino)-3-(2-naphthyl)propionic acid (2.6 g, 7.9 mmol) in dichloromethane (20 ml) was added 1-hydroxy-7-azabenzotriazole (1.1 g, 8.0 mmol) and 1-ethyl-3-dimethylaminopropyl carbodiimide hydrochloride (1.6 g, 8.6 mmol) and the mixture was stirred at 30 min at room temperature. Then 2-benzyl-4-ethylsemicarbazide (1.3 g, 6.6 mmol) and diisopropylethylamine (1.5 ml, 8.6 mmol) in dichloromethane (20 ml) were added and the mixture was stirred overnight. The mixture was washed with saturated sodium bicarbonate (2×50 ml), dried (magnesium sulfate) and concentrated in vacuo. The obtained product was chromatographed on silica (100 g) with petrol ether/ethyl acetate 1:1 to give 3.0 g (90%) of 2-benzyl-1-[(2R)-2-(N-(tert-butoxycarbonyl)-N-methylamino)-3-(2-naphtyl)propionyl]-4-ethylsemicarbazide.
