合成路线1

Aldehyde (II), prepared by ozonolysis of cyclohexene (I), was ketalized with (S,S)-2,4-pentanediol (III) to afford dioxane (IV). Titanium chloride-mediated coupling of acetal (IV) with the ketene acetal (V) afforded diastereoselectively adduct (VI), which was subsequently hydrolyzed to carboxylic acid (VII) by means of trifluoroacetic acid. Removal of the pentanediol moiety to furnish the (R)-alcohol (IX) was accomplished via Jones oxidation of the secondary alcohol (VII) to ketone (VIII), followed by beta-elimination in the presence of piperidinium acetate. Reduction of the free carboxyl group by borane-tetrahydrofuran complex gave diol (X), which was further converted to dimesylate (XI). Disulfide displacement of the mesylate groups provided (+)-lipoic acid isopropyl ester (XII), which was finally hydrolyzed to the title acid using K2CO3 in MeOH/H2O.