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containing product 10 , but also the non-coupled and reduced product 11
in a 10 / 11 ratio of 1:1.2. As in the case of the Feldman coupling, this
reaction was also ( S )-atropodiastereoselective, as a consequence of
conformational preferences in D -glucopyranose.
5.2.1.3 Total syntheses of (S)-configured ellagitannins praecoxin B and
pterocaryanin C, and (R)-configured artificial ellagitannins mahtabin A
and pariin M
The isolation and full characterisation of the 2,3-HHDP-bearing
ellagitannin, praecoxin B ( 15 ), from the aerial parts of Stachyurus
praecox SIEB. et ZUCC. ( Stachyuraceae ), a plant known for its high
tannin content, was first reported in 1983, and then, in 1991 by Okuda
and co-workers (Okuda et al. , 1983a, Hatano et al. , 1991). This
ellagitannin was also found in the bark of Mallotus japonicus (THUNB.)
MUELLER-ARG. ( Euphorbiaceae ) by Nishioka and co-workers (Saijo
et al. , 1989) and in the leaves of Tibouchina semidecandra COGN by
Okuda, Yoshida and co-workers (Yoshida et al. , 1991c). The tannin-
containing extracts from these plants are used in Asian folk medicine as
diuretics, for the treatment of stomach and duodenal tumours, and also as
natural medicines against diarrhoea, dysentery, fluor genitalis and
various skin conditions, and as astringents and haemostatic agents.
It is important to point out that the HHDP units of nearly all
naturally occurring 2,3- and 4,6-HHDP-containing ellagitannins are ( S )-
configured (Khanbabaee and van Ree, 2001a). To our knowledge, up to
now, only a handful unusual ellagitannins have been described as
exceptions to this rule (Khanbabaee and van Ree, 2001b). For example,
cercidinins A and B were reported as two unusual ellagitannins bearing a
2,3-linked ( R )-configured HHDP unit (Nonaka et al. , 1989b).
Structurally, cercidinins A and B would be the diastereoisomeric
counterparts of pterocaryanin C ( 14 ) and praecoxin B ( 15 ), respectively.
We synthesized all four diastereoisomers, compared their physical
properties and hence found out that the structural determination of
cercidinins A and B was wrong (Khanbabaee and Lötzerich, 1997b,
1998). We thus named the corresponding synthetic ellagitannins
mahtabin A ( 12 ) instead of cercidinin A and pariin M ( 13 ) instead of
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