Proanthocyanidins definition

In addition to identification of flavan-3-ols and derivatives from natural sources (Table 11.3, Figure 11.3-Figure 11.5, Figure 11.7, and Figure 11.8), several synthetic studies and efforts at establishing absolute configuration have been reported. The modified Mosher method has been successfully applied to configurational definition of the flavan-3-ols and 4-arylflavan-3-ols, and the A-type proanthocyanidins. " The first stereoselective synthesis of a series of flavan-3-ol... 

We also need to point out the often improper use of proanthocyanidin nomenclature. In Ref. 104, both vitisinol (125) and amurensisin (126) were classified as procyanidins per definition they do not belong to this class of compounds (Figure 11.11). Vitisinol (125) is rather a member of the nonproanthocyanidin class with flavan or flavan-3-ol constituent units (see Section 11.3.3), while amurensisin (126) is simply a gallic acid derivative of epicatechin (see Section 11.3.1.2). 

Among polyphenolic compounds, two types of flavonoids, the anthocyanins and flavanols (i.e., catechins, proanthocyanidins, condensed tannins), are particularly relevant to the quality of red wines, as they are key compounds for color definition and astringency. Other flavonoids such as flavonols may have some influence on color and bitterness, although they are present in red wines in much lower amounts. Phenolic acids and hydrolysable tannins, released from barrel wood, may also have an influence on wine taste and color, and hydroxycinnamoyl derivatives from grape must are involved in the oxidative browning of white wines together with flavanols. Besides, some of these perceptions may be modified by other sensory characteristics (e.g. sourness, sweetness) related to other wine components (Preys et al. 2006). 

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... 

The protocol described here should thus contribute substantially towards a straightforward chemically orientated structural definition of the A-class proanthocyanidins. 

Although earlier reviews by Haslam (134-136) discuss the many problems that have arisen about the systematic classification and nomenclature of proanthocyanidins, it seems useful to repeat the definition of the term proanthocyanidin as initially established by Freudenberg and Weinges (116) - the proanthocyanidins are all the colorless substances isolated from plants which, when treated with acid, form anthocyanidins. Thus the leucoanthocyanidins (now restricted to the flavan-3,4-diols), the condensed tannins (Sect. 7.7), and any other colorless flavonoid derivatives that produce anthocyanidins on heating with acid should be grouped under this class of compounds. 

Studies of the reactions of flavan-3-ols and particularly those of catechin have been central to the elucidation of the structure and development of uses for the condensed tannins. This work, initiated by Freudenberg and his colleagues at Heidelberg in the 1920s [see Weinges et al. (377) for a thorough review], continues to be an important aspect of condensed tannin chemistry. A wide range of electrophilic aromatic substitution reactions has been examined to obtain definitive evidence for the location of substitution (i.e. C-6 or C-8) of proanthocyanidins and to establish the influence of steric hindrance on the relative reactivity of these nucleophilic centers in flavan-3-ols. 

The significance of flavan-3,4-diols in plants rests primarily on their probable role as precursors of the polymeric proanthocyanidins. Co-occurrence of the 5-deoxy compounds - i.e., quibourtacacidins, mollisacacidins, and robinetinidins - with the related proanthocyanidins in Acacia species and the ready synthesis of naturally occurring proanthocyanidins from reactions of these flavan-3,4-diols with catechin under mild acidic conditions constitutes heavy but not definitive evidence for this thesis (31, 315-317). 

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