[29] Preparation and separation of S-oxides of α-lipoic acid

This chapter discusses the preparation and separation of S-oxides of α-lipoic acid. When lipoic acid (I) is isolated from beef liver, it is accompanied by a monooxidation product. This product, originally called “β-3-lipoic acid,” has two structural possibilities, (II) or (III), depending upon which S atom is oxidized. In addition, (II) and (III) each consists of a stereoisomeric pair (IIa), (IIb) and (IIIa), (IIIb), respectively. In this early work, it has been not possible to determine the monooxidation product that has been formed or whether the material has been naturally occurring or produced in the work-up. The oxidation of lipoic acid has been accomplished by several chemical reagents. The chapter describes the development of a technique for determining the true number of oxidation products formed under various oxidation conditions. The technique combines the use of thin-layer chromatography (TLC) and the use of nuclear magnetic resonance (NMR) chemical shift reagents. The TLC technique permits the determination of the relative amounts of isomers formed, but the specific structural assignments within thiolsulfinate and thiolsulfonate groups must await the results of ongoing 13C NMR studies.