Process for the preparation of (2RS,3RS)-3-(2'-aminophenylthio)-2-hydroxy-3-(4''-methoxyphenyl)-propionic acid methyl ester

(2RS,3RS)-3-(2'-aminophenylthio)-2-hydroxy-3-(4''-methoxyphenyl)-propi onic acid methyl ester, an important precursor for the preparation of the pharmaceutical active substance diltiazem, is obtained in a particularly high stereoselectivity and yield by addition of o-aminothiophenol onto 3-(4'-methoxyphenyl)-glycidic acid methyl ester if the reaction is carried out in the presence of alkali metal salts of weak acids in the presence of catalytic amounts of iron compounds. (2RS,3RS)-3-(2'-aminophenylthio)-2-hydroxy-3-(4''-methoxyphenyl)-propionic acid methyl ester, an important precursor for the preparation of the pharmaceutical active substance diltiazem, is obtained in a particularly high stereoselectivity and yield by addition of o-aminothiophenol onto 3-(4'-methoxyphenyl)-glycidic acid methyl ester if the reaction is carried out in the presence of alkali metal salts of weak acids in the presence of catalytic amounts of iron compounds.\!