Biology Reference
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1993, 1994a/b, Itoh and Chika, 1995, Itoh
et al.
, 1996, Dai and Martin,
1998, Arisawa
et al.
, 1999, Ikeda
et al.
, 2004). Application of known
methods for the cleavage of methyl ether groups,
e.g.
, using boron
tribromide-etherate, resulted in product decomposition, as observed in
the case of attempted syntheses of gallotannins (Khanbabaee and
Lötzerich, 1997a). Therefore, the appropriate selection of suitable
protecting groups became a decisive pre-requisite for a successful
ellagitannin synthesis. In our own work, we chose benzyl protective
groups for the phenolic hydroxyl functions, as these groups are stable
under the reaction conditions of all the different steps of our synthetic
sequence, and can be cleaved mildly at the end of the synthesis. Our
strategy for the synthesis of ellagitannins is then also based on the
concept of kinetic racemate resolution, thus using racemic
hexabenzyloxydiphenic acid
rac
-
16
(see Fig. 5.4, Schmidt
et al.
, 1954)
in a double esterification reaction with a suitable diol derivative of
D
-
glucopyranose (
i.e.
, method
A
). This method enabled us to synthesise a
series of natural and non-natural ellagitannins (Khanbabaee
et al.
, 1997,
1999, Khanbabaee and Lötzerich, 1997b/c, 1998, 1999, Khanbabaee and
Großer, 2002, 2003).
5.2.1 2,3-HHDP-containing ellagitannins
5.2.1.1
Total synthesis of sanguiin H-5
The first total synthesis of the natural product sanguiin H-5 (
6
), also
known as isostrictinin (Okuda, 1982e), was reported by Feldman and
Sambandam in 1995 (Fig. 5.2). This is the first instance of a coupling of
galloyl residues linked to the 2,3-positions of a
D
-glucosyl unit to form
the 2,3-HHDP-containing intermediate
4
by means of Pb(OAc)
4
(
i.e.
,
Wessely oxidation). Two problems had to be solved in order to
synthesise a stereochemically pure product. Firstly, concerning the
oxidative coupling to be achieved between the neighbouring C-2 and C-3
galloyl units on the
D
-glucosyl unit
3
, only the (
S
)-atropisomer was to be
formed. Secondly, concerning the galloylation of the anomeric position
of the
D
-glucosyl unit, only the
β
anomer was to be generated. In the first
