Selective oxidation of pyrogallol-type catechins with unripe fruit homogenate of Citrus unshiu and structural revision of oolongtheanins

In our previous chemical study of the production mechanism of black tea polyphenols, we demonstrated that Japanese pear fruit homogenate oxidizes green tea catechins bearing pyrogallol-type and catechol-type B-rings to produce theaflavins and dehydrotheasinensins. In contrast, unripe fruit homogenate of Citrus unshiu selectively oxidizes pyrogallol-type catechins to yield only dehydrotheasinensins. The difference in the selectivity of the two homogenates is probably related to the lower redox potential of pyrogallol-type catechins. The oxidation of epigallocatechin with C. unshiu homogenate gave two new compounds, including an ethanol adduct of an oolongtheanin precursor and epigallocatechin 4′-O-rutinoside, together with theasinensin C, dehydrotheasinensin E, and desgalloyl oolongtheanin. The structure of desgalloyl oolongtheanin should be revised based on the spectroscopic and computational data collected in the current study, and a mechanism responsible for the production of oolongtheanins is also proposed.