Biology Reference
In-Depth Information
The resulting product
4
consists of a mixture of regioisomeric
diphenylketals. Benzylation of the remaining free phenolic hydroxyl
functions afforded a fully protected system (not shown), which was then
submitted to a photolytic cleavage to release the nitrobenzyl protective
group at the anomeric center of the
D
-glucosyl unit. Acylation of this
position with the tribenzylated galloyl chloride
7
in the presence of
triethylamine (Et
3
N) gave fully protected variants of the target. Acidic
cleavage of the diphenylketals and the benzylidene acetal, followed by a
final hydrogenolytic cleavage of the benzyl groups, furnished the natural
product sanguiin H-5 (
6
, Fig. 5.2).
5.2.1.2
Construction of a 2,3-bridged ellagitannin scaffold
A copper-catalysed reductive Ullmann coupling of two methyl ether-
protected iodinated galloyl moieties linked to the appropriate positions of
a
D
-glucopyranosyl core system (
i.e.
, application of method
B
, see Fig.
5.1) was described by Dai and Martin (1998).
O
Ph
O
O
O
MeO
O
O
MeO
O
O
OMe
Ph
O
MeO
O
10
O
O
MeO
OMe
O
MeO
O
OMe
I
Cu
10
/
11
= 1:1.2 ratio
+
I
O
MeO
Ph
OMe
O
O
OMe
MeO
OMe
O
O
OMe
O
9
O
OMe
H
H
MeO
OMe
OMe
MeO
OMe
11
OMe
Fig. 5.3 Construction of a 2,3-HHDP-containing ellagitannin framework.
They could show that such coupling reaction conditions could be
applied to substrates in which galloyl-derived units are linked to the 2,3-
positions of a
D
-glucosyl unit (Fig. 5.3). However, the reductive coupling
of dihalide
9
not only produced the desired 2,3-hexamethoxydiphenoyl-
