C-Glycosidic Ellagitannins and Their Influence on Wine Chemistry - Chemistry and Biology of Ellagitannins - page 333

stereoselective manner, with the newly formed C-C bond at C-1 being
always β-oriented. Indeed, this stereochemical preference is observed in
all known related compounds thus derived from vescalagin/castalagin
( 1 / 2 ) or stachyurin/casuarinin ( 3 / 4 ), including all of the flavano-
ellagitannins identified to date (Fig. 9.7). Such a selectivity can be
argued to result from an enzymatic control, but computer-aided
molecular modeling indicates that it can also strictly rely on the
stereoelectronic properties of a benzylic cation intermediate such as 27
(Quideau et al. , 2003, 2004, 2005). This rationale is discussed in greater
detail in Section 9.2.1.2.
9.2 Vescalagin versus Castalagin
9.2.1 Chemical reactivity dichotomy
Vescalagin ( 1 ) and castalagin ( 2 ), despite their quasi-identical structure,
express drastic differences in chemical reactivity, as well as in biological
activity (Quideau et al. , 2004, 2005). These differences are really striking
when one considers that the only structural difference between these two
epimers of relatively high molecular mass ( i.e. , 934 Da) is the orientation
of their small OH group at C-1. When we started to study their
chemistry, we soon realized that condensation reactions under mild acid-
catalyzed nucleophilic substitution conditions were possible only with
vescalagin ( 1 ) but not with castalagin ( 2 ). Such a refractory behavior of 2
had been previously documented but not fully rationalized (Yoshida et
al. , 1991, Viriot et al. , 1994, Vivas et al. , 1995, 2004). Moreover, the
more reactive vescalagin ( 1 ) participates in nucleophilic substitution
reactions with full retention of configuration at C-1. If the formation of a
stable benzylic cation intermediate such as 27 is involved in such
condensation reactions as suggested above (see Fig. 9.7), one might have
expected mixtures of diastereomeric products via an S N 1-type
mechanism. But no! Starting from vescalagin ( 1 ), the condensation
products we obtained exclusively display a β-orientation of the newly
formed bond at C-1, like in all of the related C -glycosidic ellagitannins
isolated to date (Quideau et al. , 2003, 2005). Intrigued by the reactivity