PURPOSE: To prepare the title compound useful as an antiinflammatory agent, an analgesic agent and an antipyretic agent, under mild conditions, in a short time, in high yield, economically, from an easily available 2-thioxo-4-oxothiazolidine via a novel intermediate.

CONSTITUTION: The objective (+)-2-(6-methoxy-2-naphthyl)-propionic acid is prepared by (1) condensing a compound I with 2-acetyl-6-methoxy-naphthalene by dehydration to obtain a novel compound II, (2) hydrolyzing the compound II to a novel compound III, (3) desulfurizing the compound III to obtain a compound IV, (4) converting the compound IV to a racemic compound V by oxidative decarbonation, (5) optically resolving the racemic compound pref. by treating with L-tyrosine hydrazide or L-α-phenylethylamine in an alcoholic solvent with heating, and cooling the reaction mixture, and (6) decomposing the separated diastereomer salt with a strong acid.