A mixture of (2R,3S)-3-(4-methoxyphenyl)-2,3-epoxypropionamide (3.86 g) and chlorobenzene (77 ml) is refluxed with heating under nitrogen atmosphere. When the reflux is started, a solution of 2-aminothiophenol (2.75 g) and ferric chloride hexahydrate (0.54 mg) in methanol (0.1 ml) is added immediately into the reaction mixture, and the mixture is reacted at the same temperature for 5 minutes. The reaction mixture is subjected to HPLC analysis* to confirm the production of 3-(2-aminophenylthio)-2-hydroxy-3-(4-methoxyphenyl)propionamide (6.09 g) [(2S,3S)/(2S,3R)=91.5/8.5]. To the reaction mixture is added p-toluenesulfonic acid monohydrate (0.76 g), and the mixture is refluxed for 32 hours, and concentrated under reduced pressure to remove the chlorobenzene. To the residue is added methanol (25 ml), and the mixture is refluxed for one hour, and cooled with ice overnight. The precipitated crystals are collected by filtration, washed with methanol, and dried at 50° C. to give (2S,3S)-2,3-dihydro-3-hydroxy-2-(4-methoxyphenyl)-1,5-benzothiazepin-4(5H)-one (4.76 g).
