To a solution of (3R)-2-oxo-3-butoxycarbonylamino-5-(2-cyclohexen-1-yl)-1,3,4,5-tetrahydro-2H-1,5-benzodiazepin e (12.87 g) in xylene (200 mL), nitrobenzene (22.16 g) and 10% palladium carbon (6 g) were added, followed by refluxing for 1 hour and 30 minutes. The reaction mixture was allowed to cool down and then filtered. The filtrate was concentrated under reduced pressure. The residue was dissolved in ethanol (30mL), and a 4 N hydrochloric acid-dioxane solution (20 mL) was added, followed by stirring at 50° C. for 1 hour. After the reaction mixture was allowed to cool down, crystals so precipitated were collected by filtration and washed with 2-propanol, whereby the hydrochloride of the title compound was obtained. The hydrochloride was then dissolved under heat in a mixed solution of methanol and water. After the reaction mixture was allowed to cool down, a saturated aqueous solution of sodium hydrogencarbonate was added to the reaction mixture for neutralization. Crystals so precipitated were collected by filtration, washed with water and then dried, whereby the title compound (5.55 g) was obtained.
