Strategies for the Synthesis of Ellagitannins

Abstract

As a result of recent insights in the molecular structures and medicinal uses of the tannins, this class of natural products has enjoyed renewed scientific interest. This is reflected on the one hand in the enormously increased number of publications reporting on the isolation, characterization and biological activity of the tannins, and on the other hand in the steadily growing number of papers reporting tannin syntheses. In this review the most recent advances in the total synthesis of ellagitannins are discussed. In nature the ellagitannins occur usually only as the R-or only as the S-configured atropisomer, while the corresponding opposite atropisomers are seldom found. The configuration of the hexahydroxydiphenoyl units (HHDP's) found in ellagitannins is usually determined by the linkage position of the HHDP's with the polyol residues of the ellagitannins. Those ellagitannins of which the HHDP's are linked to the 2,3-or 4,6-positions of D-glucopyranose, have the S configuration; the corresponding 3,6-(HHDP)-ellagitannins are R-configured. Very few exceptions exist where both atropisomers are known. With one atropisomer showing the "normal" configuration, the other atropisomer is usually indicated in the literature as "unusual ellagitannin". This observed strict atropodiastereoselectivity in the biosynthesis of normally configured and unusual ellagitannins has led to the formulation of several hypotheses, which will be examined, in addition to the overview of effective concepts and strategies for the synthesis of ellagitannins.

Stimuli‐Responsive Tannin–FeIII Hybrid Microcapsules Demonstrated by the Active Release of an Anti‐Tuberculosis Agent

“…[3,[44][45][46] Ta nninsa re traditionally classifieda ccordingt ot heir structure and reactivity into hydrolysable, non-hydrolysable, and complex species. Different tannins, in fact, possess significantly different chemical structures:t heir similarity lies only in the content of ah igh amount of phenolic groups.…”

“…[1] Ta nnins are natural polyphenols found in most of the higher plants around the globe. [3] Tannins present a number of pharmacological activities, ranging from antioxidant and free radical scavenginga ctivity throught oa ntimicrobial, anticancer,a nd cardio-protective properties. [3] Tannins present a number of pharmacological activities, ranging from antioxidant and free radical scavenginga ctivity throught oa ntimicrobial, anticancer,a nd cardio-protective properties.…”

Oxidative Aryl‐Coupling Reactions in Synthesis

“…The ellagitannins [112,113], natural compounds belonging to the hydrolyzable tannin group of secondary plant metabolites, possess a hexahydroxydiphenoyl (HHDP) moiety 161 of (S) absolute configuration (Figure 4; for example, tellimagrandin II (162)). Therefore, they represent suitable targets for exploring oxidative coupling using this strategy.…”

Synthesis of All-Methylated Isorugosin B

“…Ellagitannins are a class of polyphenols that are suspected of being useful medically, as they are strong antioxidants [1][2][3] ; however, compared to other polyphenols, few synthetic studies of ellagitannins [4][5][6][7][8] have been conducted because of their chemical instability and structural intricacy. Within the ellagitannin family, compounds that contain valoneoyl groups are frequently found in natural products.…”