Total Synthesis of Cercidinin A

Formation of Dehydrohexahydroxydiphenoyl Esters by Oxidative Coupling of Galloyl Esters in an Aqueous Medium Involved in Ellagitannin Biosynthesis

Hexahydroxydiphenoyl (HHDP) and dehydrohexahydroxydiphenoyl (DHHDP) groups are the major acyl components of ellagitannins, which are polyphenols whose biosynthesis have attracted considerable attention; however, the mechanisms of the production of HHDP and DHHDP in the ellagitannin biosynthesis have not been clarified. With the aim of elucidating such a mechanism, this study investigates the CuCl₂ -mediated oxidation of simple galloyl derivatives in an aqueous medium. It is shown that the oxidation of methyl gallate affords a DHHDP-type dimer, whose reduction with Na₂ S₂ O₄ yields an HHDP-type dimer. However, the oxidation of the HHDP-type product over CuCl₂ does not afford the parent DHHDP ester. The oxidation of 1,4-butanediol digallate under the same conditions produces a DHHDP-type product via the intramolecular coupling of galloyl groups. These results strongly suggest that the DHHDP group is the initial product of the oxidative coupling of two galloyl groups in the ellagitannin biosynthesis, and subsequent reductive metabolism affords HHDP esters.

Synthesis of an Ellagitannin Component, the Macaranoyl Group with a Tetra-ortho-Substituted Diaryl Ether Structure

Herein, a practical synthesis of the macaranoyl group contained in ellagitannins, i.e., a C-O digallate structure with a tetra-ortho-substituted diaryl ether bond, is described. The methodology involved an oxa-Michael addition/elimination reaction between a brominated ortho-quinone monoketal and a phenol with a hexahydroxydiphenoyl moiety in the presence of 18-crown-6 under dark conditions, followed by reductive aromatization. The existence of rotamers originating from the constructed ether moiety is discussed as well.

Recent Advances in the Production and Applications of Ellagic Acid and Its Derivatives. A Review

Ellagitannins (ETs), characterized by their diversity and chemical complexity, belong to the class of hydrolysable tannins that, via hydrolysis under acidic or alkaline conditions, can yield ellagic acid (EA). They are mostly found as a part of extractives in angiosperms. As known antioxidants and chelators, EA and EA derivatives are drawing an increasing interest towards extensive technical and biomedical applications. The latter ones include possible antibacterial, antifungal, antiviral, anti-inflammatory, hepato- and cardioprotective, chemopreventive, neuroprotective, anti-diabetic, gastroprotective, antihyperlipidemic, and antidepressant-like activities, among others. EA’s synthesis and production challenges prompt further research on new methods and alternative sources. Conventional and prospective methods and raw materials for the production of EA and its derivatives are reviewed. Among the potential sources of EA, the residues and industrial streams of the pulp industry have been highlighted and considered as an alluring alternative in terms of commercial exploitation.

Total Synthesis of Phyllanemblinin B

A total synthesis was developed of phyllanemblinin B, a natural ellagitannin containing 1-O-galloyl and 2,4-O-hexahydroxydiphenoyl groups on a d-glucose scaffold. The use of a μ-hydroxo copper(II) complex resulted in a completely stereoselective oxidative coupling of the galloyl groups on the open-chain glucose moiety and facilitated the first total synthesis of phyllanemblinin B.

Oenothein B, dimeric hydrolysable tannin inhibiting HCV invasion from Oenothera erythrosepala

The envelope proteins of the hepatitis C virus (HCV), E1 and E2, have been revealed to be essential for invasion of HCV. Thus, we were engaged in the search for the inhibitors against HCV invasion through the assay system using the model virus expressing recombinant HCV envelopes, E1 and E2. Now, we disclosed dimeric hydrolysable tannin oenothein B (1) from MeOH extract of Oenothera erythrosepala as an active principle for inhibition of HCV invasion and its potency was almost the same as that of monomeric hydrolysable tannin, tellimagrandin I (2). Furthermore, by use of stereoselectively prepared 1-β- and 1-α-O-methyl tellimagrandin Is (4 and 5), the introduction of methyl moiety into 1-hydroxy group of 2 was clarified to result in slightly reduction of activity and β-isomer was revealed to exhibit a little stronger activity than α-one.

Structural Revisions in Natural Ellagitannins

Ellagitannins are literally a class of tannins. Triggered by the oxidation of the phenolic parts on β-pentagalloyl-d-glucose, ellagitannins are generated through various structural conversions, such as the coupling of the phenolic parts, oxidation to highly complex structures, and the formation of dimer and lager analogs, which expand the structural diversity. To date, more than 1000 natural ellagitannins have been identified. Since these phenolic compounds exhibit a variety of biological activities, ellagitannins have potential applications in medicine and health enhancement. Within the context of identifying suitable applications, considerations need to be based on correct structural features. This review describes the structural revisions of 32 natural ellagitannins, namely alnusiin; alnusnin A and B; castalagin; castalin; casuarinin; cercidinin A and B; chebulagic acid; chebulinic acid; corilagin; geraniin; isoterchebin; nobotanin B, C, E, G, H, I, J, and K; punicalagin; punicalin; punigluconin; roxbin B; sanguiin H-2, H-3, and H-6; stachyurin; terchebin; vescalagin; and vescalin. The major focus is on the outline of the initial structural determination, on the processes to find the errors in the structure, and on the methods for the revision of the structure.

Bioinspired Total Synthesis of (-)-Vescalin: A Nonahydroxytriphenoylated C-Glucosidic Ellagitannin

The first total synthesis of the 2,3,5-O-(S,R)-nonahydroxytriphenoylated (NHTP) C-glucosidic ellagitannin (-)-vescalin was accomplished through a series of transformations mimicking the sequence of events leading to its biogenesis. The key steps of this synthesis encompass a Wittig-mediated ring opening of a glucopyranosic hemiacetal, a C-glucosidation event through a phenolic aldol-type reaction, and a Wynberg-Feringa-Yamada-type oxidative phenolic coupling, which forged the NHTP unit of (-)-vescalin.

Fundamental Methods in Ellagitannin Synthesis

This account describes methods for synthesizing natural ellagitannins. The ellagitannins, a class of polyphenols, has a wide variety of chemical structures and biological activities. Here, we focus on three topics, which are the synthesis of the hexahydroxydiphenoyl group, construction of the hexahydroxydiphenoyl bridge, and synthesis of C–O digallates. The hexahydroxydiphenoyl group and the C–O digallates are C–C and C–O coupled galloyl groups, respectively, both group of which are the two major components of ellagitannins. By combining methods described in this account, many ellagitannins might be synthesized.