On-line liquid chromatography and circular dichroism detection of stereo-isomers of alpha-tocopherol derivatives generated by an electrochemical reaction.

The electrochemical oxidation of (+/-)-alpha-tocopherol on a porous graphite electrode was performed in the presence of methanol, and successive separation and detection of the products were performed by an on-line liquid chromatography/mass spectrometry system. Three products were identified, one of which was determined to be alpha-tocopheryl quinone, because its m/z was 469 [M+Na](+). The other two products showed identical mass and UV spectra, and were suspected to be diastereomers of 9-methoxy-alpha-tocopheron, because their molecular weights were m/z 483 [M+Na](+), and also because it is known that the chemical oxidation of alpha-tocopherol by benzoyl peroxide or N-bromosuccinimide in the presence of methanol should provide 9-methoxy-alpha-tocopheron. To confirm that these two compounds were diastereomers, a circular dichroism detector was used. The signs of both peaks detected by the circular dichroism detector at 230 nm were opposite. In addition to observations of identical mass and ultraviolet spectra, these results indicated that the two products were diastereomers of 9-methoxy-alpha-tocopheron, whose stereochemistry is different at the newly generated chiral center of the 9-position. The on-line use of a circular dichroism detector with an electrochemical cell/liquid chromatography system may expand the utility of the system to study the metabolism of a chiral drug.