Biology Reference
In-Depth Information
Interestingly, vescalagin (
1
) is almost systematically found in lower
amounts than castalagin (
2
) in plant extracts. This intriguing observation
could be a consequence of the lack of chemical reactivity at C-1 of
castalagin (
2
). Thus, on the sole basis of this chemical reactivity
difference between
1
and
2
, one can argue that vescalagin (
1
) is the
preferred precursor of the
in vivo
formation of
C
-glycosidic ellagitannin
oligomers and C-1 conjugates such as the flavano-ellagitannins and the
lyxose/xylose-bearing conjugates. For example, considering the
structures of the
C
-glycosidic ellagitannins typically found in significant
amounts in fagaceous wood species (Fig. 9.2), grandinin and the roburins
A-E would be, all six of them, derived from a nucleophilic substitution at
C-1 of vescalagin (
1
). Overall, the construction of these six compounds
would require nine equivalents of
1
for only one equivalent of castalagin
(
2
), which would be involved in the formation of roburin D (
6
) via a
nucleophilic attack of its 4,6-HHBP group onto the C-1 position of a
vescalagin unit.
9.2.1.2
Diastereofacial differentiation of the vescalagin-derived benzylic
cation
Starting from vescalagin (
1
), all of the condensation reactions we
performed using various carbon-, oxygen- and sulfur-based nucleophiles
occurred with retention of configuration at C-1,
i.e.
, the newly formed
bond is still β-oriented (see Sections 9.2.2 and 9.3). This β-orientation is
exclusively observed in all of the
C
-glycosidic ellagitannin-derived
oligomers and conjugates, including flavano-ellagitannins, which have
been so far isolated from plants. Since chemical reactions thus appear to
follow the same stereochemistry path than biochemical transformations,
the rationale for such a high diastereoselectivity cannot be invoked to
rely upon an enzymatic control. Moreover, as stated above, this
diastereoselectivity can at first appear surprising if one claims a passage
via a benzylic cation intermediate such as
27
in the context of an S
N
1-
type mechanism (Fig. 9.7). However, an examination of
27
by
computational means has revealed stereoelectronic effects that can
explain the diastereofacial selection imposed on its electrophilic C-1
cationic center.
