PURPOSE: To obtain easily the titled substance which is an intermediate for preparing compounds having coronary vasodilator action, etc., by reacting d-2- hydroxy-3-(2'-aminophenylthio)-3-(4"-methoxyphenyl)-propionic acid as a starting material with thionyl chloride, etc.

CONSTITUTION: A compound of formula I is reacted with preferably 2mol or slight excess acetyl chloride in the presence or absence of a base in a solvent, e.g. chloroform or toluene, at 0°CW the refluxing temperature to give d-2-acetoxy-3-(2'-N-acetylaminophynylthio)-3-(4"-methoxyphenyl)-propionic acid of formula II, which is then reacted with an inorganic dehydration reagent, e.g. thionyl chloride, to afford a compound of formula III. The resultant compound of formula III is then reacted with an amine, e.g. methylamine or dimethylamine, to give the aimed compound of formula IV. The above-mentioned inorganic dehydration reagent is preferably used in an equimolar or slight excess amount based on the compound of formula II.