In the process of black tea production, various catechin dimers are produced from pyrogallol-type and catechol -type catechin by enzymatic oxidation. Dimerization of pyrogallol-type catechin affords dehydrotheasinensins stereoselectively. DFT optimization of dehydrotheasinensin C suggested that the selectivity was accounted for intramolecular π-π stacking between two A-rings in aqueous solution. It was supported by 1H NMR chemical shifts and NOE in D2O, as well as comparison of ECD spectra in water and CH3CN assisted by DFT calculations. On the other hand, oxidative coupling between pyrogallol-type and catechol-type catechins affords theaflavins with a benzotropolone chromophore. Plane structure of the chromophore is produced via a bicyclo[3.2.1]octane-type intermediate, and formation of the intermediate was non-stereoselective coupling between catechol quinone and pyrogallol, which was shown by trapping a pair of epimers of intermediates. In this study, a newly developed facile method for theaflavin preparation was applied. Furthermore, structure of goupiolone B was revised by biomimetic synthesis related to theaflavin production.
