C-Glycosidic Ellagitannins and Their Influence on Wine Chemistry - Chemistry and Biology of Ellagitannins - page 342

not contain any detectable amounts of acutissimins. On the other hand,
red wines typically contain (+)-catechin and (-)-epicatechin at
concentrations ranging from 115 mg/L to 190 mg/L for the former, and
around 80 mg/L for the latter (Carando and Teissedre, 1999, Cabanis et
al. , 1998). As a mildly acidic hydroalcoholic solution ( i.e. , ~ 12%
aqueous ethanol at pH ~ 3-4), red wine gathers all of the prerequiste
physico-chemical factors to potentially mediate the formation of
acutissimins. To a wine model solution, consisting of a 12% (v/v)
ethanolic aqueous solution with 5 g/L of tartaric acid at pH 3.2, was
added vescalagin ( 1 ) and (+)-catechin in order to confirm this
potentiality. The mixture was allowed to react at room temperature
for 25 days, after which time both acutissimins A/B ( 14 / 15 ) were UV-
detected as major products, together with a third major component
resulting from the nucleophilic substitution of the OH-1 group of 1
by ethanol. This compound was also generated in a high isolated
yield of 94% from 1 and ethanol in THF containing 1.5% (v/v) of
TFA after 5 h at 60 °C, and characterized by mass spectrometry and
NMR spectroscopy (Quideau et al. , 2005). Its structure was thus
unambiguously determined as corresponding to β-1- O -ethylvescalagin
( 31 ) (Fig. 9.11). Once again, this ethyl ether of vescalagin was generated
with retention of configuration at C-1. Of particular note is the isolation
by Okuda's group of the methyl analog of 31 from the leaves of
Tibouchina semidecandra ( Melastomataceae ). This compound (see 32
in Fig. 9.11) was however probably produced as an artefact during the
liquid chromatographic separation procedure relying on the use of
methanol (Yoshida et al. , 1991).
Another interesting example of the implication of such simple
alcohols in the chemistry of C -glycosidic ellagitannins has been
described by Puech and associates in the course of their investigations on
the aging of spirits in oak barrels. Both vescalagin ( 1 ) and castalagin ( 2 ),
and conceivably other oak-extracted ellagitannins, are transformed into
ethyl-based hemiketal and ketal derivatives after prolonged exposure
to 40 and 70% (v/v) ethanolic aqueous solutions (Fig. 9.11). These
transformations imply an oxidation of the NHTP pyrogallol I-ring of
these ellagitannins prior to the nucleophilic addition of ethanol onto the
resulting α-hydroxy- ortho -quinone (Puech et al. , 1999b).