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of eugenigrandin A is electron-richer and more highly susceptible to
oxidation than the catechol B-ring of acutissimin A. After oxidation of
the glucose C-1-linked pyrogallol ring, addition of the gallocatechin
pyrogallol B-ring to the carbonyl group of the intermediate
cyclopentenedione moiety (see route B in Fig. 4.19), followed by further
oxidation of the B-ring, results in the generation of psiguavin.
4.4 Conclusion
All ellagitannins are biosynthesized via galloyl glucoses. Since most of
the ellagitannins have a fully acylated glucose core, pentagalloylglucose
(PGG) is a key intermediate in ellagitannin biosynthesis. If the tannins
are produced for plant defense, then the use of pentagalloylglucose
would seem ideal, because it strongly interacts with proteins and has a
severely bitter and astringent taste. However, no plants were found to
accumulate this particular tannin as a major polyphenol, even though
there are many plants that accumulate proanthocyanidins and
ellagitannins. The most inconvenient feature of pentagalloylglucose may
be its poor water solubility. In addition, in
Paeonia
sp., a large amount of
paeoniflorin (see Fig. 4.4), which bears a hydrophobic benzoyl group,
was found to solubilize tannins by hydrophobic association.
Furthermore, the oxidative metabolism of pentagalloylglucose into
ellagitannins also increases water solubility, and this mechanism may
enable many plants to accumulate tannins in higher concentration. The
dehydroellagitannins are sometimes accumulated in large amounts in
plant tissue. For example, the geraniin contents in the dried leaves of
some
Geranium
and
Euphorbia
sp. were reported to range between 4 and
12% by weight of dry material (Okuda
et al.
, 1980a). The reason why
geraniin is accumulated in such high concentrations is still unclear;
however, it is strongly suggested that its characteristic structure and
interesting reactivity, as described in this chapter, have some biological
significance.
