Biology Reference
In-Depth Information
speculate that
R. chiliantha
have specific enzymes that selectively
oxidize the self-associated galloylpedunculagin dimeric complex.
Additionally, if an oxidative C-O coupling occurred between one of the
phenolic
meta
-hydroxyl group of the O-1 galloyl group of one molecule
and the 4,6-HHDP pyrogallol ring connected to the 4-position of the
glucose core of another molecule in the same self-associated complex
(see coupling mode B in Fig. 4.8), a sanguisorboyl group would be
formed. This group is one of the characteristic interlinking units in
ellagitannin oligomers, such as lambertianin A (Tanaka
et al
., 1993b),
especially in those ellagitannins isolated from
Sanguisorba
and
Rubus
species (Tanaka
et al.
, 1985, Li
et al.
, 2007a). Since plants biosynthesize
dimeric ellagitannins that are characteristic of their species and/or family
to which they belong (see Chapters 1 and 2), regioselectivity and
chemoselectivitity (
i.e.
, C-C
versus
C-O coupling) in oligomerizing
oxidative coupling, even if facilitated by preliminary formations of self-
associated and pre-arranged complexes, is probably under enzymatic
control.
4.3 Biomimetic Transformation of Ellagitannins
4.3.1 Dehydroellagitannins and related compounds
Dehydroellagitannins constitute a group of ellagitannins that feature the
dehydrohexahydroxydiphenoyl (DHHDP) bisester unit (Schmidt
et al.
,
1967a and b, Haddock
et al.
, 1982). This bisacyl group usually exists as
an equilibrium mixture of two hydrated forms of a cyclohexenetrione
structure (Fig. 4.9), displays a chiral methine carbon, and both of its
enantiomeric variants are known (Lee
et al.
, 1991). It is biosynthesized
by oxidation of the HHDP group, although the details of its production
mechanism have not yet been clarified. In many dehydroellagitannins,
the DHHDP group is bridged between the 2- and 4-positions of a
glucopyranose core in its
1
C
4
-conformation. This unusual conformation
of the sugar moiety and the bisester connection of the DHHDP unit with
axially-oriented hydroxyl groups constitute additional characteristic
features of this sub-class of ellagitannins.
