The oxidation of caffeic acid derivatives as model reaction for the formation of potent gonadotropin inhibitors in plant extracts.

Synthetic caffeic acid derivatives, substoichiometrically oxidized with KMnO4, exhibit antigonadotropic activity against pregnant mare serum gonadotropin (PMSG) to a greater degree than caffeic acid itself. Inhibitory compounds, formed after an oxidation of caffeic acid and its derivatives are bound to PMSG dependent on their concentration to result in hormone-inhibitor complexes. These PMSG-inhibitor complexes exhibited little or no biological activity, depending on the structure of the inhibitor. The substoichiometric oxidation with KMnO4 led to the corresponding unstable o-quinones as first products. The complete oxidation reaction could be divided into an initial KMnO4-dependent step followed by a manganese-catalyzed autoxidation, which was accompanied by a pronounced oxygen uptake from the solution. The HPLC analysis after an oxidation of caffeic acid derivatives led to product patterns with strong similarities to those of caffeic acid in the respective product UV spectra, suggesting the formation of compounds with similar structures.