PURPOSE: To obtain a benzothiazepine derivative, by reacting d-2-hydroxy-3-(2'- aminophenylthio)-2-(4"-methoxyphenyl)-propionic acid with acetyl chloride so that OH is acetylated and ring closure is done.

CONSTITUTION: 1mol compound shown by the formula I is reacted with 3mol (preferably 3W3.5mol) acetyl chloride in a solvent such as benzene, toluene, etc. at 80W180°C for 2W30hr to give a compound shown by the formula II.

USE: An intermediate for preparing d-2-(4'-methoxyphenyl-3-acetoxy-5-(N,N-dimethylaminoethyl)-2,3-dihydro-1,5-benzothiazepin-4(5H)-one having vasodilating action on coronary blood vessel and arousal action on electroencephalogram.