STUDIES ON THE OXIDATION PRODUCTS OF LIPOIC ACID

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Abstract
When lipoic acid in chloroform was oxidized with 5 molar ratio of t-butyl hydro-peroxide for 1 day at 40", an unidentified lipoic acid derivative was detected at Rf 0.90 on the paper chromatogram using 2.8% aqueous NHg- saturated n-butanol as a developing agent. It was positive to KCN-nitroprusside reagent and an inhibited Streptococcus faecalis 10 Cl. The infrared spectrum indicates that the compound is t-butyl ester of lipoic acid sulfone. The free acid form (X) was obtained by hydrolysis. The infrared spectrum of X was essentially Identical with that of [alpha]-lipoic acid except that the peaks assignable to a sulfoxide group were observed at 1,310 cm-1 (vas SO5) and 1140 cm (vs SO2). Its elementary analysis value corresponded to that of monosulfone. The X inhibited Sfaecalis 10 Cl with a 50% inhibition index of about 100o DL-[alpha]-Lipoic acid was oxidized with 2 molar ratio of H0O2 in aqueous acetone for 1 day at room temperature. An unidentified lipoic acid-active product produced in addition to [beta]-lipoic acid and unreacted [alpha]-lipoic acid. It showed almost the same activity and elementary analysis value as [beta]-Hpoic acid. The characteristic peaks in the infrared as well as ultraviolet spectra showed some differences. The oxidation product may be an isomer of [beta]- lipoic acid whose positon of a sulfoxide group is different or a conformer based on different steric positions of a sulfoxide group and a valeric acid side-chain attached to the 1,2-dithiolane ring. Oxidation of [alpha]-lipoic acid with a 10 molar ratio of H2O2 in glacial acetic acid resulted in the formation of a white precipitate (mp 108-110[degree]). Characteristic peaks due to a sulfoxide group were observed in the infrared spectrum,, Its elementary analysis value was consistent with C8H14O6S2. The product may be a disulfone derivative of lipoic acid.

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