Tannins configuration

In chemical terms, tannins are relatively bulky phenol molecules, produced by the polymerization of elementary molecules with phenolic functions. Their configuration affects their reactivity (Section 6.3.4). They must be sufficiently bulky to produce stable combinations with proteins, but if they are too bulky, they are likely to be too far from the active protein sites. The molecular weights of active tannins range approximately from 600 to... 

In conclusion, flavanols, procyanidins and, consequently, condensed tannins react more-or-less easily with free radicals, according to their configuration. These chain reactions produce brown polymers with varying structures that precipitate. In wine, these phenomena depend on the phenol content. The oxidation kinetics are apparently slower than in a model medium, probably due to the presence of even more easily oxidizable compounds, also involved in the oxidation of procyanidins. 

The potential of this development for the structural elucidation of the proanthocyanidin condensed tannins, especially the 5-deoxy analogues, from important commercial sources is clear. In addition, the method facilitates the ready definition of the absolute configuration of the chainterminating flavan-3-ol moiety in 5-deoxyoligoflavanoids, especially in... 

These acid-catalyzed rearrangement reactions were also extended to a study in which pecan (Caraya illinoensis) nut pith tannins were reacted with phloroglucinol and acetic acid at lOO C for extended periods of time (727). Besides the formation of the anticipated 4-arylflavan-3-ols, these conditions also catalyze the formation of a unique series of novel [l]-benzofuro[2,3- ]chromenes, e.g. (121), and spirobidihydro[l]benzo-furans, e.g (122). The formation of products of type (121) demonstrates the susceptibility of the C(3)-hydroxyl group in 4-arylflavan-3-ols to inversion of configuration under acidic conditions, while that of the spiro compounds of type (122) is a further manifestation of the lability of the C(4)-C(10) bond in condensed tannin constituent unit. 

Roux, D.G., and D. Ferreira The Direct Biomimetic Synthesis, Structure and Absolute Configuration of Angular and Linear Condensed Tannins. Fortschr. Chem. organ. Naturstoffe, 41, 47 (1982). 

Botha, J.J., D. Ferreira, and D.G. Roux Condensed Tannins Direct Synthesis, Structure, and Absolute Configuration of Four Biflavanoids from Black Wattle Bark ( Mimosa ) Extract. J. Chem. Soc., Chem. Commun., 700 (1978) Synthesis of Condensed Tannins. Part 4. A Direct Biomimetic Approach to [4,6]- and [4,8]-Biflavanoids. J. Chem. Soc., Perkin Trans. 1, 1235 (1981). 

Contents E. HASLAM The Metabolism of Gallic Acid and Hexahydroxydi-phenic Acid in Higher Plants. — D. G. ROUX and D. FERREIRA The Direct Biomimetic Synthesis, Structure and Absolute Configuration of Angular and Linear Condensed Tannins. — ST. J. GOULD and ST. M. WEINREB Streptonigrin. — D. J. ROBINS The Pyrrolizidine Alkaloids. — J. W. DALY Alkaloids of Neotropical Poison Frogs (Dendrobatidae). 

Chalcones (e.g., 9) are key intermediates in the formation of several major groups of flavonoids (see Fig. 11.2). In some plants, they are converted into aurones (such as 11). In other instances, chalcones undergo reduction of the exocy-clic double bond to produce dihydrochalcones (such as 12). Chalcones and flavanones (e.g., 10) exist in equilibrium in in vitro systems. However, as only flavanones with a (25)-configuration are known to occur naturally, this interconversion appears to be enzymatically controlled and does not involve racemization. Desaturation of flavanones can yield flavones (such as 4 and 5), whereas introduction of oxygen at the 3-position gives dihydroflavonols (flavanonols) (such as 13) which, in turn, are the precursors of flavonols (e.g., 6-8), anthocyanins (e.g., 1-3), and condensed tannin precursors (Ebel and H lbrock, 1982 Hahlbrock, 1981 Haslam 1974) (Figs. 11.7 and 11.8). 

Botha J J, Ferreira D, Roux D G 1978 Condensed tannins Circular dichroism method of assessing the absolute configuration at C-4 of 4-arylflavan-3-ols and stereochemistry of their formation from flavan-3,4-diols. J Chem Soc Chem Commun 698-700... 

Roux D G, Ferreira D 1982 The direct biomimetic synthesis structure and absolute configuration of angular and linear condensed tannins. Fortschr Chem Org Naturst 4147-4176... 

Steenkamp J A, Ferreira D, Roux D G, Hull WE 1983 Synthesis of condensed tannins. Part 8. The first branched (4,6 8 4,6)-tetraflavanoid. Coupling sequence and absolute configuration. J Chem Soc Perkin Trans I 23-28... 

The Direct Biomimetic Synthesis, Structure and Absolute Configuration of Angular and Linear Condensed Tannins... 

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