A mixture of 30.1 g of cis-(+)-2-(4-methoxyphenyl)-3-hydroxy-2,3-dihydro-1,5-benzothiazepine-4(5H)-one, 17.2 g of 2-(dimethylamino)ethyl chloride hydrochloride, 31.8 g of potassium carbonate and 240 ml of acetone is refluxed for 30 minutes after stirring. Then, 10 ml of water are added to the mixture, and the aqueous mixture is further refluxed for 3 hours under stirring. After the reaction is completed, the mixture is evaporated under reduced pressure to remove acetone. The residue is dissolved in toluene, and the solution is washed twice with water. The toluene layer is collected and then evaporated under reduced pressure to remove toluene. The residue is dissolved in ethanol. After cooling the solution, said solution is acidified with 10% HCl/ethanol and then stirred at a temperature below 5° C. for 5 hours. The crystalline precipitates are collected by filtration, washed with ethanol and then dried. 37.1 g of cis-(+)-2-(4-methoxyphenyl)-3-hydroxy-5-[2-(dimethylamino)ethyl]-2,3-dihydro-1,5-benzothiazepine-4(5H)-one hydrochloride are thereby obtained. Yield: 90.7%
