Biology Reference
In-Depth Information
However, the silylation of the free phenolic hydroxyl groups within
the mixture
93
led to another mixture consisting of four discrete isomers
including atropisomer (
R
)-
94
(40% of the crude mixture of isomers) and
(
S
)-
94
(36% of the crude mixture of isomers), which were isolated by
column chromatography. The mixture
94
was then subjected to
Ac
2
O/H
2
SO
4
to furnish a mixture of the atropisomers (
R
)-
95
and (
S
)-
95
,
and four other undefined isomers. To verify these results, the separated
compounds (
R
)-
94
and (S)-
94
were individually exposed to
Ac
2
O/H
2
SO
4
.
The exposure of (
R
)-
94
to Ac
2
O/H
2
SO
4
afforded a 1:1 mixture of the
ring-opened diacetate (
R
)-
95
and a second but structurally unknown
isomer, while the pure (
S
)-atropisomer (
S
)-
94
furnished a 1:1 mixture of
the ring-opened diacetate (
S
)-
95
and a second also structurally unknown
isomer. It is worth noting that these reactions largely proceeded with
inversion of stereochemistry at C-1 to furnish the corresponding axial
acetates (
R
)-
95
and (
S
)-
95
.
Cleavage of the silyl ether bonds of (
R
)-
95
and (
S
)-
95
by desylilation
resulted in a mixture of at least 6 isomers, from which (
R
)-
96
and (
S
)-
96
could be isolated by chromatography. Feldman
et al.
observed that (
R
)-
96
and (
S
)-
96
equilibrate over the course of 48 h in acetone-
d
6
solutions.
Hydrogenolytic removal of the diphenylketals of (
R
)-
96
gave the single
isomer (
R
)-
97
and that of (
S
)-
96
delivered the single isomer (
S
)-
97
,
which also underwent facile atropisomerisation over 24 h (Feldman
et al.
, 2003).
5.2.5 3,6-HHDP-containing ellagitannins
5.2.5.1
Synthesis of O-permethylcorilagin
The first construction of the methyl ether-protected corilagin
108
, a 3,6-
HHDP-containning ellagitannin featuring a glucopyranose ring in its
1
C
4
-
conformation, has been achieved by the group of Yamada (Ikeda
et al.
,
2004). The synthesis of nonamethylcorilagin (
108
) started from phenyl
1-thio-β-
D
-glucopyranoside
98
, which possesses four free hydroxyl
groups at C-2, C-3, C-4 and C-6 of the sugar in a
4
C
1
-conformation (Fig.
