Highly Oxidized Ellagitannins of Carpinus japonica and Their Oxidation-Reduction Disproportionation.

In the research on ellagitannin metabolism, two unique dehydroellagitannins, carpinins E (1) and F (2), bearing dehydrohexahydroxydiphenoyl (DHHDP) and hydrated biscyclohexenetrione dicarboxyl ester (HBCHT) groups, were isolated from young leaves of Carpinus japonica. Upon heating in H2O or treatment with pH 6 buffer at room temperature, 1 and 2 afforded the reduction product 3, isocarpinin A, with an (R)-hexahydroxydiphenoyl (HHDP) group, suggesting the occurrence of redox disproportionation of the (S)-DHHDP group. This was supported by the increase in production of 3 in the pH 6 buffer solution by coexistence of epigallocatechin-3-O-gallate (15), accompanied by oxidation of 15. In contrast, treatment of 1 and 2 with ascorbic acid yielded 4, carpinin A, with an (S)-HHDP group. Upon heating with ascorbic acid, the HBCHT group was also reduced to an (S)-HHDP group, and 2 was converted to 2,3;4,6-bis(S)-HHDP glucose. In leaves of C. japonica, the tannins 1 and 2 are dominant in young spring leaves, but compounds 3 and 4 become the major components of tannins in mature leaves. These results suggest that, in ellagitannin biosynthesis, oxidative coupling of the two galloyl groups first generates a DHHDP group, and subsequent reduction of DHHDP esters produces HHDP esters.

Production of Ellagitannin Hexahydroxydiphenoyl Ester by Spontaneous Reduction of Dehydrohexa-hydroxydiphenoyl Ester.

Amariin is an ellagitannin with two dehydrohexahydroxydiphenoyl (DHHDP) moieties connecting glucose 2,4- and 3,6-hydroxy groups. This tannin is predominant in the young leaves of Triadica sebifera and Carpinus japonica. However, as the leaves grow, the 3,6-DHHDP is converted to its reduced form, the hexahydroxydiphenoyl (HHDP) group, to generate geraniin, a predominant ellagitannin of the matured leaves. The purified amariin is unstable in aqueous solution, and the 3,6-(R)-DHHDP is spontaneously degraded to give HHDP, whereas 2,4-(R)-DHHDP is stable. The driving force of the selective reduction of the 3,6-DHHDP of amariin is shown to be the conformational change of glucose from O,3B to 1C4. Heating geraniin with pyridine affords 2,4-(R)-DHHDP reduction products. Furthermore, the acid hydrolysis of geraniin yields two equivalents of ellagic acid. Although the reaction mechanism is still ambiguous, these results propose an alternative biosynthetic route of the ellagitannin HHDP groups.

Ellagitannins and Related Compounds from Penthorum chinense.

Four new ellagitannin metabolites, penthorumnins A-D (1-3 and 5), were isolated from the dried stem of Penthorum chinense. The structures were determined using spectroscopic and chemical analysis as well as using computations that revealed the following: (1) the acyl group of penthorumnin A (1) has a unique cyclopentane carboxylic acid structure that is derived from a hexahydroxydiphenoyl (HHDP) group; (2) penthorumnin B (2) has a 2-carboxymethyl-2,3-dihydro-3-oxo-1 H-indene-1-carboxylic acid structure that originates from the acyl group of penthorumnin A; (3) penthorumnin C (3) is a glucoside of trihydroxyacetophenone with an acyl group that is oxidatively derived from the HHDP group. This acyl group is closely related to that of balanophotannin F (4), which has been previously isolated from Balanophora japonica and whose absolute configuration has been revised using the DFT method; and (4) penthorumnin D (5) is defined as 2',4',6'-trihydroxyacetophenone 4'- O-[4,6-( S)-dehydrohexahydroxydiphenoyl]-ß-glucoside. The variety of acyl groups in these ellagitannins is indicative of the occurrence of a unique metabolism in this plant involving the HHDP ester.

Diastereomeric Ellagitannin Isomers from Penthorum chinense.

From the dried stem of Penthorum chinense (Penthoraceae), 1-O-galloyl-4,6-(R)-hexahydroxydiphenoyl (HHDP)-ß-D-glucose and 2',4',6'-trihydroxyacetophenone 4'-O-[4,6-(R)-HHDP]-ß-D-glucoside were isolated together with their (S)-HHDP isomers. Ellagitannins with a 4,6-(S)-HHDP-glucose moiety are widely distributed in the plant kingdom; however, 4,6-(R)-HHDP glucoses are extremely rare. Lowest-energy conformers of 1-O-galloyl-(S)- and (R)-HHDP-glucopyranoses were derived by density functional theory calculations, and the calculated (1)H and (13)C NMR chemical shifts and the (1)H-(1)H coupling constants were in agreement with the experimental values. The results revealed a conformational difference of the diastereomeric macrocyclic ester rings. In addition, a new compound, 1',3',5'-trihydroxybenzene 1'-O-[4,6-(S)-HHDP]-ß-D-glucoside, was also isolated.

A New Flavonoid from Camellia sinensis Fermented Tea.

A new flavonoid, quercetin 3-0-α-L-rhamnopyranosyl-(l-6)-(2-E-p-coumaroyl)-D-D-glucopyranoside-4'-O-α-L-rhamnopyranoside, was isolated from the fermented tea of Camellia sinensis L. (Pu-erh tea) from China. The chemical structure was elucidated based on analyses of the spectroscopic data.