Biology Reference
In-Depth Information
2.1.2 Dehydroellagitannins
The dehydrohexahydroxydiphenoyl (DHHDP) group of tannins
belonging to the dehydroellagitannin subclass features a partially
hydrated triketone moiety that forms a hemiacetal group with a nearby
phenolic hydroxyl group, thus stabilizing the molecule. In aqueous or
alcoholic solutions, this DHHDP hemiacetal group usually equilibrates
between a six- and a five-membered ring forms. The first compound
identified in this class, geraniin (
19
), a keystone compound, was shown
to display the hemiacetal group under its six-membered ring form in its
crystalline state (see Chapter 1). In addition to dehydrogeraniin (see
Chapter 1) and amariin (
20
, from
Phyllanthus amarus
, Foo, 1993)
having a 3,6-DHHDP group, and mallotusinic acid having a 3,6-
valoneoyl group (from
Mallotus
species, see Chapter 1), macarinin A
(
21
) and macaranin C (
22
) (from
Macaranga
species), which possess a
tergalloyl and a macaranoyl group, respectively, in place of the 3,6-
HHDP group, are also examples of dehydroellagitannins.
Most dehydroellagitannins are obtained as amorphous powders
consisting of mixtures of the six- and five-membered hemiacetal forms
of their DHHDP group, as found for dehydrogeraniin and amariin (
20
),
which exist as mixtures of four isomers because of the presence of two
DHHDP units in each molecule. Other dehydroellagitannins include
hellioscopinin A (
23
, from
Euphorbia helioscopia
, Lee
et al
., 1990a),
carpinusin (
24
, from
Carpinus
laxiflora
, Nonaka
et al.
, 1992) (1,6-
HHDP; 2,4-DHHDP-type), terchebin (
25
, from
Terminalia chebula
,
Okuda
et al.
, 1980a), and supinanin (
26,
from
Euphorbia spina
, Lee
et
