Production of theasinensins A and D, epigallocatechin gallate dimers of black tea, by oxidation–reduction dismutation of dehydrotheasinensin A

Biomimetic One-Pot Preparation of a Black Tea Polyphenol Theasinensin A from Epigallocatechin Gallate by Treatment with Copper(II) Chloride and Ascorbic Acid

“…Standard samples of theasinensins A-C were synthesized from 1 and epigallocatechin by enzymatic oxidation. 9,10) Oxidation of 1 in Small Scale Tea catechin mixture (5 mg) or 1 (5 mg, 0.01 mmol) was added to a 30% MeOH (2.0 ml) solution containing CuCl 2 or other metal salts (0.01 mmol) and shaken vigorously at room temperature for 24 h. was ascorbic acid (50 mg) was added to the solution and heated at 80-85°C for 15 min.…”

“…9) Addition of imidazole, ethylenediamine, and oxalic acid as ligands was not effective. The initial oxidation step of the reaction was carried out at room temperature and elevation of the reaction temperature increased byproducts.…”

“…8) In vitro model experiments to mimic black tea production revealed that the enzymes oxidize 1 to ortho-quinone and subsequent dimerization of the quinone affords an unstable dimer, named dehydrotheasinensin A (2). 9) On heating and drying of the tea leaves in the final stage of black tea production, 2 undergoes redox dismutation to give reduction products 3 and its atropisomer, along with a complex mixture of oxidation products, including compounds produced by oxidative cleavage of aromatic rings 9) (Chart 1). It is also known that reduction of 2 with reducing agents, such as mercaptoethanol and ascorbic acid, afford only 3, with an R-biphenyl bond.…”

“…17,18) As for theasinensins, the yields of chemical and enzymatic preparations from tea catechins are low. 9,19) Therefore, in this study we developed a facile and biomimetic method to prepare 3 and its analogs.…”

Structures of Epicatechin Gallate Trimer and Tetramer Produced by Enzymatic Oxidation

“…12) In our previous studies concerning the enzymatic oxidation of tea catechins, oxidative coupling reactions of catechin B-rings were demonstrated. [13][14][15][16][17][18][19][20][21][22] On the other hand, 60-80% of the total tea catechins possess galloyl esters located at the C-3 hydroxy group and although oxidation of galloyl groups must be important in the formation of black tea polyphenols, 23) only limited examples of oxidative coupling of galloyl groups has been reported. 22,[24][25][26] Our previous in vitro experiments showed that enzymes preferentially oxidize the catechol B-rings of epicatechin and the resulting quinone, which is a potential oxidizing reagent, subsequently oxidizes pyrogallol rings, the redox potential of which is lower than that of the catechol ring.…”