Vescalagin

This complex tannin is formed during the aging process of red wine, whereby the catechin unit originates from the grapes, and the ellagitannin, in this case vescalagin, originates from the oak barrels. 

Other electrophiles include anthocyanin flavylium cations. The intermediate cation generated, in mildly acidic conditions, from vescalagin, an ellagitannin present in wines after barrel aging or addition of oak chips and tannin extracts, has also been shown to participate to such reactions (Quideau et al. 2003). 

Again this involves nucleophilic substitution of a flavonoid at its nucleophilic C8 or C6 centre on an electrophilic center, which may be the C4 of an anthocyanin flavylium or the carbocation generated by protonation and dehydration of vescalagin. 

Addition of catechin or epicatechin onto vescalagin yield complex tannin structures called acutissimin and epiacutissimin (Quideau et al. 2003) that have been isolated earlier from Quercus acutissirm. It is worth noting that the isomer of vescalagin (i.e. castalagin) fails to undergo this reaction. 

Analysis of toasted oak extracts shows a breakdown of the ellagitannins, especially after medium toasting (Table 13.19). This is related to the fusion temperatures of a blend of vescalagin and castala-gin (163°C) and gallic acid (250°C). Ellagic acid only reacts at higher temperatures (F > 450°C), but it is not very soluble in dilute alcohol solutions. 

The C-glucosidic ellagitannin subclass also includes complex tannins known as flavano-ellagitannins, which are hydrolysable tannin hybrids with a C-glucosidic ellagitannin moiety such as stachyurin (37) or vescalagin (39) and a flavan-3-ol... 

Fig. 64.13 Hemisynthesis of acutissiimns 53,54 and epiacutissiimns 64,65 from vescalagin (39) and either (+)-catechin (62) or (—)-epicatechin (63) respectively, in acidic media...

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