To obtain the subject compound useful as a precursor for the production of diltiazem known as a pharmaceutically active substance in high stereoselectivity and yield.

The objective (2RS,3RS)-3-(2'-aminophenylthio)-2-hydroxy-3-(4''- methoxyphenyl)-propionic acid methyl ester can be produced by adding o- aminothiophenol of formula III to 3-(4'-methoxyphenyl)-grlycidic acid methyl ester of formula II in the presence of a weak acid alkali metal salt and a catalytic amount of an iron compound. The iron compound to be used in the reaction is e.g. an organic acid iron(III) sulfate and its amount of preferably 1-20ppm. The reaction temperature is preferably 90-120°C and the molar ratio of the compound of formula II to the compound of formula III is preferably 1:(0.9-1.1).