The mixture of 0.4 g of methyl ester of trans-1-benzyl2-oxo-5-phenyl-1,2,3,5-tetrahydro-4,1-benzoxazepine-3-propionic acid obtained in Example 12, 0.16 g of sodium borohydride and 0.16 g of lithium chloride in 15 ml of tetrahydrofuran were stirred for 10 minutes at room temperature. To the resultant was added 30 ml of ethanol, which was stirred for 2 hours at 60° C. To the reaction mixture were added 100 ml of 1N hydrochloric acid and 150 ml of ethyl acetate. The ethyl acetate layer was washed with an aqueous solution of sodium hydrogencarbonate, which was dried over anhydrous magnesium sulfate, then the solvent was distilled off under reduced pressure. The residue was purified by means of a silica gel column chromatography (eluent, hexane:ethyl acetate=2:1) to give 0.19 g of trans-1-benzyl-3-(3-hydroxypropyl)-5-phenyl-1,5-dihydro-4,1-benzoxazepin-2(3H)-one as needles, m.p. 59°-62° C.
