Structure elucidation and tentative formation pathway of a red colored enzymatic oxidation product of caffeic acid

A red colored caffeic acid trimer was synthesized in model solution by enzymatic oxidation. The structure was elucidated by mass spectrometry and NMR spectroscopy and consisted of four carbon rings and two unsaturated acyl side chains. A formation pathway is suggested which includes the formation of a caffeic acid ortho-quinone followed by a Michael-type reaction, and radical coupling of a semiquinone of the formed dimer and a third caffeic acid molecule. The final structure is formed by decarboxylation and cyclisation. Results of ESI-MSn analysis can be explained by the presence of two carboxylic acid groups and a tetracyclic structure. The relative configuration of the central chiral carbons was deduced by NOESY experiments.