Coupling reactions between flavylium ions and catechin

In order to model natural polymeric pigments present in old red wines, new covalent adducts have been synthesized upon condensation of synthetic flavylium ions (models of anthocyanins) with catechin (model of tannins) in the presence and in the absence of acetaldehyde. These new pigments have been investigated by 1D and 2D NMR, HPLC, FAB-mass and UV-visible spectroscopies and molecular modelling. The two flavylium salts used in this work (3,4′-dimethoxy-7-hydroxyflavylium chloride and 5,7-dihydroxy-3,4′-dimethoxyflavylium chloride) display quite different reactivities toward catechin. The electronic donating effect of the catechin moiety and the formation of noncovalent dimers in acidic aqueous or methanolic solution should be mainly responsible for the improved stability of the flavylium chromophore in the new pigments.