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1995, Tsai et al. , 1992, Han et al. , 1995, Chen et al. , 1995, Morimoto et
al. , 1986a/b, Saijo et al. , 1989). Access to pure ellagitannins by isolation
from natural sources is often cumbersome and yields only small
quantities of pure natural products (Okuda et al. , 1982a/b). For the
organic chemist, it is therefore a great challenge to provide synthetic
access to this type of natural product in order to provide sufficient
quantities of pure biologically active compounds and to optimise the
biological activity ( e.g. , lowering cytotoxicity, improving absorption or
improving selectivity) by derivatisation or structural modifications.
5.2 Strategies for the Synthesis of Ellagitannins
Up to now, only two different strategies have been developed for the
synthesis of monomeric and dimeric ellagitannins (Fig. 5.1), i.e. , A :
double esterification of a suitably protected hexahydroxydiphenic acid
with a diol or a tetrol derivative of D -glucopyranose and B : esterification
of a suitably protected gallic acid with a diol derivative of D -
glucopyranose, followed by coupling of the galloyl residues. The
coupling reaction of the 2,3- or 4,6-galloyl units occurred
diastereoselectively, using different reagents, to form the ( S )-HHDP unit.
The stereoselectivity is induced by a template effect brought about by
the chiral glucopyranose core (Feldman and Ensel, 1993, 1994a,
Feldman et al. , 1994, Feldman and Sambandam, 1995, Feldman and
Smith, 1996, Dai and Martin, 1998, Arisawa et al. , 1999). Method A
offers an alternative way for the synthesis of ellagitannins using
hexabenzyloxydiphenic acid as starting material.
Compared to method B , method A has several advantages. There is
no need for for orthogonal protection of the phenolic hydroxyl groups of
the hexahydroxydiphenic acid used. Consequently, there is no need for
any deprotection of multiple protective groups. The coupling step in
method B , which usually results in the formation of regioisomeric
mixtures that are difficult to separate by chromatography, is not required
in method A . Furthermore, method A can also be used for the synthesis
of ( R )-configured 2,3-HHDP ellagitannins (Itoh and Chika, 1995, Itoh
et al. , 1996, Khanbabaee and Lötzerich, 1998).
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