Flavonoid unit

Fig. 3. The regular structure of a procyanidin-type condensed tannin showing characteristic 4,8 interflavonoid bonds linking the flavonoid units.

Reactions of anthocyanins and flavanols take place much faster in the presence of acetaldehyde that is present in wine as a result of yeast metabolism and can also be produced through ethanol oxidation, especially in the presence of phenolic compounds, or introduced by addition of spirit in Port wine technology. The third mechanism proposed involves nucleophilic addition of the flavanol onto protonated acetaldehyde, followed by protonation and dehydration of the resulting adduct and nucleophilic addition of a second flavonoid onto the carbocation thus formed. The resulting products are anthocyanin flavanol adducts in which the flavonoid units are linked in C6 or C8 position through a methyl-methine bond, often incorrectly called ethyl-link in the literature. 

Most reported anthocyanins are monomeric in nature however, new types of flavonoids consisting of an anthocyanin moiety covalently linked to another flavonoid unit have been reported in the period of this review. One class includes one anthocyanin unit and one flavone or flavonol unit attached covalently to each end of a common dicarboxylic acid. The other class involves one anthocyanin moiety covalently linked directly to a flavanol unit. 

Polymeric tannins have more complicated structures including esterified oligosaccharide chains for the hydrolyzable ones or further polymerization of the flavonoid units for condensed ones. Because of their diversity including small molecules and polymers, characterization of natural tannins is not simple. The first step in their characterization is isolation. This has been done using several procedures such as ultrafiltration and gel permeation chromatography [2]. 

Robustaflavone series consists of two flavonoid units linked by a (I-3, II-6) bond. This series was also subdivided into the following sets structures containing two flavone imits, (l-3, II-6)-biflavones (64-73) a flavone and a flavonol imit, (I-3, II-6)-flavone-flavonol (74) a flavanone and a flavone unit, (l-3, II-6)-flavanone-flavone (75-79) a flavone and a flavanone imit, (I-3, II-6)-flavone-flavanone (80-82) and finally, a set containing structures with two flavanone units or two flavanonol units, (l-3, II-6)-biflavanones and (1-3, 11-6)-biflavanonols (83-85). 

Agathisflavone series contains biflavonoids linked by a (1-6,11-8) bond. The series is further divided into structures with two flavone imits (100-110), two flavonol imits (111) flavanone-flavone dimers (112) and two flavanone units (113-114a). Succedaneaflavone series comprises (1-6, II-6)-biflavones (115-116b) (1-6, II-6)-biflavanones (117-118) and a (I-6,II-6)-benzofuran-naphthopyranobiflavone (119). Afzelone series includes (I-6,II-3)-biflavanone (120). Two flavonoid units linked by a (1-3,11-3) bond were included in the Chamaejasmin series. The subdivision of this series is as follow (1-3,11-3)-biflavones (121) a (I-3,II-3)-flavone-flavanone dimer (122), and (1-3,11-3)-biflavanones that have different stereochemistry (123-144). 

The (l-3,II-6)-biflavonoids were included in Stephaflavone series, which contains structures with two flavone units (145-151), and (I-3,II-6)-flavone-flavan dimers (152) that are formed by a mixture of two atropisomers. GB-Flavone series has structures with flavonoid units linked by a (1-3,11-8) bond. The series consists of the following structures (I-3,II-8)-biflavone (153) (I-... 

The nucleophilic centers on the A ring of a flavonoid unit tend to be more reactive than those found on the B ring. This is due to the vicinal hydroxyl substituents, which cause general activation in the B ring without any localized effects such as those found in the A ring. 

Among the polyphenols present in the tree barks, tannins are by far the most interesting oligomers (molecular weights of 1000-4000) in terms of their utilization as macromonomers for the crosslinking of proteins in leather (tanning) and for macromolecular syntheses. The two representative structures of the flavonoid units in tannins are shown in Fig. 1.8. The most salient aspects related to the sources, structures and production of tannins and to their exploitation in polymer modification and manufacture are given in Chapter 8. 

In condensed polyflavonoid tannin molecules, the A-rings of the constituent flavonoid units retain only one highly reactive nucleophilic centre, the remainder accommodating the interflavonoid bonds. Resorcinolic A-rings (wattle) show... 

PROAPfriiOCYAlNIDINS OR CONDENSED between flavonoid units, hydroxyation patterns, stereochem-... 

Characterization of hydrolyzable tannins can be accomplished by hydrolysis and identification of the fragments by standard techniques such as cochromatography on paper, TLC or HPLC and NMR, UV, and MS (Karchesy et al., 1989 Okuda et al., 1989a Porter, 1989). Condensed tannins can be cleaved in mild acidic solution to form a flavan-3-ol and a quinone methide. The quinone methide can be captured with a suitable nucleophile (e.g., -SCH2Ph) to form 4-thio-benzyl derivatives and permit determination of the constituent flavonoid units (Porter, 1989). The 4-thiobenzyl derivatives may be reduced with Raney-nickel to yield flavan-3-ols (Porter, 1989). 

Biflavonoids, biflavenyls dimers of flavonoid units. The interllavonoid linkage may be a carbon-carbon bond or an ether bond (other types of linkage in naturally occurring B. probably remain to be discovered). Ten types of B. have been reported, classified according to the sites and type of linkage ... 

Complex tannins, also called flavano-ETs or flavono-ETs depending on the flavonoid unit, are hybrid tannins composed of a C-glycosidic ET and a glycosidically linked flavan-3-ol (like catechin or epicatechin), anthocyanin or procyanidin. ... 

The biflavonyls and proanthocyanidins form two distinct groups of oligomeric flavonoid compounds whose structures are derived by combinations of the basic Cg.Cj.C flavonoid unit. The biflavonyls , such as hinokiflavone (59), ginkgetin (57) and... 

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