9A Anthocyanins and Anthocyanin-Derived Compounds 445
Mechanism.Two different mechanisms have been proposed for this type of reac-
tion leading to the anthocyanin-flavanol (A-F) and to the flavanol-anthocyanin (F-A)
products (Fig. 9A.3):
a)Anthocyanin-flavanol (A-F) direct condensation reaction.The mechanism of
reaction proposed for the A-F condensation reaction starts with the nucleophilic
attack of the C-8 o C-6 position of the flavanol to the electrophilic C-4 position
of the anthocyanin in the form of flavylium cation, giving rise either to a flavene,
that can be oxidized to the corresponding flavylium cation and finally dehydrated
to a xanthylium salt of yellow color (Jurd and Somers 1970; Somers 1971;
Baranowski and Nagel 1983; Liao et al. 1992; Santos-Buelga et al. 1999), or
to a colorless bicyclic condensation product (Fig. 9A.3b) first characterized in
model solutions containing malvidin-3,5-diglucoside and (+)-catechin (Bishop
and Nagel 1984).b)Flavanol-anthocyanin direct condensation reaction (F-A).In the case of F-A
condensation reaction, carbocations generated from the acid-catalyzed interfla-
vanic bond cleavage typical of the chemistry of procyanidins (Haslam 1980)
(Chapter 9B), act as electrophilic agents and react with the nucleophilic C-6 or
C-8 position of the anthocyanin in its hydrated hemiketal form, giving rise to a
colorless dimer that could be dehydrated to the corresponding red flavylium form
(Fig. 9A.3c).Evidence in wine.The products resulting from both mechanisms have been
detected in red wine fractions with the aid of thiolysis and mass spectrometry (Remy
et al. 2000). For F-A condensation products, the thiolysis results indicated that
malvidin-3-glucoside was linked through its C-6 or C-8 position to an (epi)catechin
moiety, generating the pigment (epi)catechin-malvidin-3-glucoside in flavylium
form (MW=781). The presence of this compound has been extensively reported
in wine (Vivar-Quintana et al. 2002; Monagas et al. 2003; Alcalde-Eon et al. 2004;
Alcalde-Eon et al. 2006) but its structure has been only recently confirmed as an
F-A product using ESI-MS, model solutions and a synthesized adduct identified as
catechin-(4α→8)-malvidin-3-glucoside by NMR (Salas et al. 2004a,b, 2005a,b).
F-A products of the entire series of anthocyanidin-3-glucosides and their acylated
derivates with (epi)catechin and (epi)gallocatechin have been also detected in wine
fractions (Alcalde-Eon et al. 2006; Boido et al. 2006).
In the case of A-F condensation products, the work of Remy et al. (2000) con-
firmed that the anthocyanin was linked through its C-4 position to the (epi)catechin
moeity forming colorless malvidin-3-glucoside-(epi)catechin adducts (MW=782)
that were first tentatively identified as adducts in flavene form (Santos-Buelga
et al. 1999) in model solutions containing catechin and malvidin-3-glucoside. How-
ever, the conversion of the flavene to the flavylium xanthylium previously described
in the literature (Somers 1971) could not be confirmed. Later on, the forma-
tion of these A-F colorless adducts was investigated by HPLC/ESI-MS and thi-
olysis in model solutions containing malvidin-3-glucoside, (–)-epicatechin and/or