Condensation with flavanols

Francia-Aricha, E.M. et al.. New anthocyanin pigments formed after condensation with flavanols, J. Agric. Food Chem., 45, 2262, 1997. 

Francia-Aricha EM, Guerra MT, Rivas-Gonzalo JC, Santos-Buelga C (1997) New antho-cyanin pigments formed after condensation with flavanols. J Agric Food Chem 45 2262-2266... 

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 reaction mechanism proposed by Timberlake and Bridle (1976), suggests that acetaldehyde, in the form of a carbo-cation, reacts with flavanol (tarmin) at position C-6 or C-8 (Figure 1 (1)). This, via several condensation reactions, gives rise to tannin-ethyl-anthocyanin derivatives (Figure 1 (2)) such as malvidin-3-glucoside-ethyl-(epi)catechin and malvidin-3-(6-/7-coumaroyl)-glucoside-ethyl-(epi)catechin. 

Indirect reaction condensation with an ethyl cross-bond. In an acid medium, ethanal forms a carbocation that initially reacts with the negative nodes (4 and 8) of the flavanols (procyanidin... 

Fig. 57.16 Products proposed for the condensation of flavanols with glyoxylic acid, lactone [204] and xanthylium structures [200]...

Condensed tannins or proanthocyanidins are oligomeric to polymeric compounds formed from the condensation of flavanols or flavandiols, with degrees of... 

In 1908, while working at University of Heidelberg, Auwers and Muller described the transformation of 4-methyl-2-cumaranone (3) to flavanol 6. Thus aldol condensation of 3 with benzaldehyde gave benzylidene derivative 4, which was brominated to give dibromide 5. Subsequent treatment of 5 with alcoholic KOH then furnished 2-methylflavonol 6. In the following years, Auwers published more extensively on the scope and limitations of this reaction. ... 

The extraction of the flavanols (catechol and 4-ethylcatechol), from steam-treated green coffee beans36 (Figure 2), can be correlated with the presence of the so-called condensed tannins.37... 

Flavan-3-ols orflavanols have a saturated three-carbon chain with a hydroxyl group in the C3 position. In foods they are present as monomers or as proanthocyanidins, which are polymeric flavanols (4 to 11 units) known also as condensed tannins. In foods they are never glycosylated. 

Flavanols and procyanidins Flavanols, or flavan-3-ols, are synthesized via two routes, with (+) catechins formed from flavan-3,4-diols via leucoanthocyanidin reductase (LAR), and (—) epicatechins from anthocyanidins via anthocyanidin reductase (ANR) (see Fig. 5.4). These flavan-3-ol molecules are then polymerized to condensed tannins (proanthocyanidins or procyanidins), widely varying in the number and nature of their component monomers and linkages (Aron and Kennedy 2008 Deluc and others 2008). It is still not known whether these polymerization reactions happen spontaneously, are enzyme catalyzed, or result from a mixture of both. 

Fig. 2.103. Structures of all the compounds found in fraction B (a) anthocyanins and acylated anthocyanins direct condensation products between flavanols and anthocyanins (c) dimers resulting from the condensation mediated by acetaldehyde between anthocyanins and flavanols. Reprinted with permission from C. Alcalde-Eon el al. [236].

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