Tert-butyl[6-(aminothiocarbonyl)-4-butoxy-2-isobutyl-1-oxo-1,2-dihydro-3-isoquinolinyl]methylcarbamate (Example 189 (1)) (1.38 g, 3 mmol) was dissolved in a solution of 4N hydrogen chloride in ethyl acetate (10 ml) and the mixture was stirred at room temperature for 1 h. The reaction mixture was concentrated under reduced pressure and the residue was suspended in tetrahydrofuran (20 ml). To the obtained mixture were added triethylamine (0.84 ml, 6 mmol) and 9-fluorenylmethylchloroformate (1.16 g, 4.5 mmol) and the mixture was stirred at 0° C. for 1 h. The reaction mixture was poured into water and extracted with ethyl acetate. The extract was washed with brine, dried over anhydrous magnesium sulfate and concentrated under reduced pressure. The residue was purified by silica gel column chromatography to give 9H-fluoren-9-ylmethyl [6-(aminocarbothioyl)-4-butoxy-2-isobutyl-1-oxo-1,2-dihydro-3-isoquinolinyl]methylcarbamate (1.52 g, 86.9%) as an amorphous.
