5-Hydroxy-7-chloro-1-[2-methoxy-4-(2-methylbenzoylamino)benzoyl]-2,3,4,5-tetrahydro-1H-benzazepine (0.7 g), dimethylaminopyridine (0.83 g) and dimethylaminopyridine hydrochloride (0.72 g) are dissolved in chloroform (15 ml), and thereto are added N-tert-butoxycarbonyl-L-methionine (0.56 g) and dicyclohexylcarbodiimide (0.93 g), and the mixture is stirred at room temperature for 3 hours. To the mixture are added methanol (3 ml) and acetic acid (0.7 ml), and the mixture is stirred at room temperature for 30 minutes. The insoluble materials are removed by filtration, and to the filtrate is added 5% aqueous sodium hydrogen sulfate solution, and the mixture is extracted with dichloromethane. The dichloromethane layer is washed successively with saturated aqueous sodium hydrogen carbonate solution and saturated brine, and dried over magnesium sulfate. The solvent is evaporated and the resulting residue is purified by silica gel column chromatography (eluent; dichloromethane:methanol=150:1) to give 5-(N-tert-butoxycarbonyl-L-methionyloxy)-7-chloro-1-[2-methoxy-4-(2-methylbenzoylamino)benzoyl]-2,3,4,5-tetrahydro-1H-benzazepine (1.27 g).
