In 250 ml of dimethylformamide is dissolved 13.9 g of trans-3-amino-2-phenyl-2,3-dihydro-1,5-benzothiazepin-4(5H)-one, and 2.3 g of 60% sodium hydride is added to the solution. After the mixture is stirred at room temperature for 20 minutes, 80 ml of a solution containing 10 g of tert-butyl bromoacetate in dimethylformamide is added thereto dropwise. The mixture is stirred at room temperature for 3 hours and then concentrated under reduced pressure. Water is added to the residue, and the organic substance is extracted with ethyl acetate. The ethyl acetate layer is washed with water and dried. Thereafter the solvent is distilled off under reduced pressure to give 18.6 g of tert-butyl trans-3-amino- 4-oxo-2-phenyl-2,3,4,5-tetrahydro-1,5-benzothiazepine-5-acetate in the form of white crystals, m.p. 172°-174° C.
