Biology Reference
In-Depth Information
1.3.6 Structural transformations of ellagitannin oligomers
Although ellagitannins are generally stable compounds, some oligomers
do undergo structural transformations under rather mild conditions.
1.3.6.1
Isomerization of dimers via Smiles rearrangement
Hirtellin C is a macrocyclic dimer possessing both the
m-
and
p-
GOG
units that was isolated from
Reaumuria hirtella
(
Tamaricaceae
). It is
readily isomerized into isohirtellin C in hot water at 95 °C via a Smiles
rearrangement that enables the conversion of the
p
-GOG unit into a
m
-
GOG unit. This rearrangement is presumably facilitated by the release of
steric effects originating from hydroxyl groups in the
p-
GOG linking
unit. Even at pH 7.4 and at only 40 °C, this isomerization reaction was
almost complete within 30 min (Yoshida
et al
., 1993a).
Application of this isomerization reaction was used to confirm the
structure of tamarixinins B (a macrocyclic dimer) and C, both isolated
from
Tamarix pakistanica
(
Tamaricaceae
). Indeed, their Smiles
rearrangement led to analogs of established structures, tamarixinin B
being thus converted into hirtellin C, and tamarixinin C into hirtellin A
(Fig. 1.6, Yoshida,
et al
., 1993b).
The tergalloyl group in eucalbanin C, a
p
-DOG-type dimer isolated
from
Eucalyptus alba
(
Myrtaceae
), was quantitatively converted into
eucalbanin B of the valoneoyl
m
-DOG-type in a phosphate buffer (pH
7.4) at room temperature (Fig. 1.7). The dilactonized tergalloyl group in
eucalbanin A was also isomerized to the dilactonized valoneoyl group in
cornusiin B (Yoshida
et al
., 1992c).
1.3.6.2
Hydrolysis of ellagitannin oligomers into monomers
Partial hydrolysis of oligomers in boiling water or weak acids affords
monomers. The linking unit usually remains attached onto one of the
monomers thus released from the dimeric structure. This type of
transformation, or partial degradation, is a useful tool to gather important
informations in the course of the elucidation of an oligomeric structure.
