Biology Reference
In-Depth Information
configuration of the HHDP group at the O-3~O-6 positions, such as in
geraniin, is
R
. The absolute configuration at the methine carbon of the
DHHDP group at the O-2~O-4 positions in geraniin, terchebin and
mallotusinic acid (Okuda and Seno, 1981) is
R
, whereas this
configuration is
S
in isoterchebin (see structure
27
in Chapter 2) (Okuda
et al
., 1982e/f). The methine carbon of the chebuloyl group in the
molecules of chebulinic acid and chebulagic acid, which are
biogenetically derived from geraniin, retains the stereochemical features
of the DHHDP group at the O-2~O-4 positions of geraniin
(Yoshida
et al
., 1980, 1982).
HO
OH
HO
OH
HO
OH
OH
HO
OH
O
H
HOOC
O
H
HO
OH
HO
O
O
O
O
HO
OH
H
O
OH
O
R
-HHDP
S
-HHDP
Chebuloyl
1.1.4 Condensation of dehydroellagitannins with other substances
Tannins are, in general, capable of interacting with co-existing
substances, and are often bound to basic compounds, proteins and other
high molecular mass compounds, as well as metallic ions. Besides the
binding activities indexed by the aforementioned RA and RMB values,
dehydroellagitannins also express structure-specific reactivity in
condensation reactions with certain co-existing substances under mild
conditions.
For example, a condensation product derived from geraniin and
ascorbic acid, ascorgeraniin (or elaeocarpusin) was isolated from
Geranium thunbergii
and also from
Acer nikoense
and
Elaeocarpus
sylvestris
. This compound also co-exists with geraniin in some other
Acer
,
Rhus
and
Cercidiphyllum
species. It has been prepared by
condensation of geraniin with ascorbic acid in a moderately acidic
aqueous or a methanolic aqueous solution at room temperature, thus
demonstrating that it could be produced in the plant without any enzyme
intervention (Okuda
et al.
, 1986a/b). An analog, putranjivain A, was
isolated from several euphorbiaceous plants (Lin
et al
., 1990).
