Biology Reference
In-Depth Information
The dehydrotheasinensin A has the same partial structure as that of
the DHHDP group. This dimeric product is unstable and gradually
decomposed under neutral conditions to yield theasinensins A and D as
major products, which are diastereomers differing only by the
atropisomerism of the biphenyl bond. The reaction also produced
oxidation products, including a product having two carboxyl groups.
Therefore, the decomposition reaction is an oxido-reductive dismutation.
The oxidation product has a carbon skeleton similar to that of the
chebuloyl and dehydrochebuloyl groups (Fig. 4.10). Despite the
structural similarities between dehydrotheasinensin A and the DHHDP
group, the production of the ellagitannin DHHDP unit probably proceeds
via a different mechanism. Under similar conditions, PGG (
8
) did not
yield the products derived from quinone intermediates, although the
production of a very small amount of ellagic acid was observed. In the
oxidation of epigallocatechin-3-
O
-gallate, only a limited amount of the
galloyl group was oxidized, and only coupling reactions between the
galloyl group and the B-ring were observed, because the redox potential
of the galloyl group is higher than that of the pyrogallol B-ring (Li
et al.
,
2007b).
4.3.3 Reactions of dehydroellagitannins and biomimetic conversions
The DHHDP group exists as hydrated forms of its cyclohexenetrione
moiety. Therefore, the dehydroellagitannins are susceptible to suffer
from alkaline degradation (Tanaka
et al.
, 1990). In fact, after a brief
treatment with a basic aqueous solution, the DHHDP group of geraniin
was converted to both dehydrochebuloyl and brevifolin carboxyl groups
(see Figs 4.10 and 4.13). Also, heating in pyridine afforded the
tetrahydroxydibenzofuran dicarboxyl group via an oxido-reductive
dismutation. The reaction with triethylamine gave the brevifolin carboxyl
group, and the aromatic ester of the DHHDP group was selectively
hydrolyzed during the reaction. This reaction was used to determine the
location of the DHHDP group, and it was shown that the
cyclohexenetrione ring of the (
S
)-DHHDP group in granatins A and B
(Okuda
et al.
, 1980) and in hellioscopinin A (Lee
et al.
, 1990) was
