Xanthylium salt formation

Es-Safi, N.E. et al., Xanthylium salts formation involved in wine colour changes. Int. J. Food Sci. Technol. 35, 63, 2000. 

Figure 6 Mechanism of xanthylium salts formation from ( )-catechin, via the colorless and the xanthene derivatives. (Reproduced with permission from reference 39. Copyright 1999 American Chemical Society.)...

The mechanism postulated above for xanthylium salts formation was also found to occur more generally. Thus yellow xanthylium salts showing absorption maxima at 440 nm were detected when glyoxylic acid was replaced by furfural and HMF. Using LC/ESI-MS chromatography these confounds were detected at m/z 637 and 667 amu for furfural and HNff respectively. The corresponding... 

Jurd, L. and Somers, T.C., The formation of xanthylium salts from proanthocyanidins. Phytochemistry 9, 419, 1970. 

Similar processes have been observed with other aldehydes, such as glyoxylic acid. However, the carboxymethine-linked oligomers resulting from reaction with glyoxylic acid proceeded to xanthylium salts rather than to larger polymers (Es-Safi et al. 1999b). The postulated pathway involves dehydration and cyclisation of the carboxymethine dimer (Eig. 9B.6(4)) followed by oxidation of the resulting xanthene (Eig. 9B.6(5)) that was also detected in the medium. Formation of xanthylium salts was also shown in the case of furfural and hydroxymethylfurfural (Es-Safi et al. 2000). 

The formation of xanthylium pigments may also result from non enzymatic reactions of colorless precursors such as flavanols as previously reported (/yellow derivatives by various oxidation processes. In the presence of tartaric acid and iron or copper ions, yellow pigments with absorption maxima at 440 and 460 nm were formed along with the colorless (+)-catechin carboxymethine bridged dimers 46, 47). These pigments were analysed by ID and 2D NMR and identified as non esterified and esterified xanthylium salts respectively 33, 38, 39, 42) (Figure 5). 

From a mechanistic point of view, the formation of the identified xanthylium salts involve dehydration of the first colorless dimers giving a xanthene derivative followed by an oxidation process. Detection of the xanthene adducts was also achieved through LC/MS analysis. The esterification was shown to occiu before the dehydration process as shown in Figure 6. 

Fig. 3.7 Reaction between catechins and aldehydes formation of xanthylium salts.

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