Biology Reference
In-Depth Information
starting material also reacted with the sugar derivative
23
, but in an
open-chain manner to give polar products, which were easily separated
by silica gel chromatography. The subsequent removal of the photolabile
ortho
-nitrobenzyl ether at the anomeric center of
24
furnished the O-1
deprotected
D
-glucopyranose derivative
25
as an anomer mixture (
i.e.
,
α/β, 85:15). Acylation of
25
with protected galloyl chloride
7
yielded the
required
β
-anomer
26
. Finally, the natural product strictinin (
27
) was
obtained by removal of all protecting groups of
26
by hydrogenolysis
under standard conditions (Fig. 5.6, Khanbabaee
et al.
, 1997).
5.2.2.2
Total syntheses of gemin D and hippomanin A
The isolation of gemin D (
37
) from the leaves of
Geum japonicum
(
Rosaceae
) and from the flowers of
Camellia japonica
(
Theaceae
) was
first reported in 1982 (Yoshida
et al.
, 1982). In 1985, the Okuda group
reported the complete structural determination of this ellagitannin
(Yoshida
et al.
, 1985). The isolation of hippomanin A (
38
) from the
aerial parts of
Hippomane mancinella
L. (N. O.
Euphorbiaceae
) had
already been reported in 1974 (Rao, 1974), but again, the structural
elucidation of this regioisomeric analogue of gemin D (
37
) was reported
only three years later (Rao, 1977). Both gemin D (
37
) and hippomanin A
(
38
) possess a (
S
)-configured HHDP unit linked to the 4,6-positions of
D
-glucopyranose. The only difference between gemin D (
37
) and
hippomanin A (
38
) is the presence of a galloyl group at either the 3- or
the 2-position of the glucosyl core of these ellagitannins. Gemin D (
37
)
exhibits both anticancer (Miyamoto
et al.
, 1987) and anti-HIV (Vlietinck
et al.
, 1998) activities. The extracts of
Geum japonicum
and
Camellia
japonica
have long been used in Japan and China as diuretics, astringent
and haemostatic agents (Yoshida
et al.
, 1985).
Apart from the previously described protecting group strategies and
diastereoselective double acylation of the 4,6-positions of the
D
-
glucopyranose core, the selective monofunctionalisation of the 2-OH or
3-OH group of the sugar unit is decisive for a successful synthesis of
both natural products. It is known that the C-2 hydroxyl function of a
D
-
glucopyranose protected at the anomeric position is the most reactive
secondary hydroxyl groups. This effect is often more pronounced in α-
