NEW MATERIAL:(+)-(2R*,3R*)-3-(2-aminophenylthio)-2-hydrox-y-3-(4-met-hoxy- phenyl)propionic acid derivative of formula I (R is H or 1W4C lower alkyl).

USE: An intermediate of filtiazem hydrochloride useful as a coronary vasodilator.

PREPARATION: The compound of formula I can be prepared according to the reaction formula, by (1) reacting the compound of formula III (M is alkali metal) with a mineral acid salt of an optically active organic amine, (2) decomposing the resultant diastereomer salt to the compound of formula IV (A1⊕ is conjugated acid of organic base) taking advantage of the solubility difference in a solvent, (3) treating the compound with a base (e.g. alkali metal alkoxide, etc.), (4) esterifying the resultant compound of formula V (A2 is alkali metal) to the compound of formula VI, (5) reacting the compound of formula VI with 2-aminothiophenol, and if necessary, (6) hydrolyzing the product.