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In-Depth Information
(
R
)-
16
could be assigned by comparison of their specific rotation with
values published by Schmidt and Demmler (1952, 1954). A
characteristic of both (
S
)-
16
and (
R
)-
16
is their highly stable
configuration. Schmidt and Demmler (1952, 1954) also showed that
even under the most extreme reaction conditions such as boiling in acetic
acid or melting, no racemisation takes place.
In the next reaction step, the benzylidene acetal protecting groups in
(
S
)-
17
and (
R
)-
17
were cleaved by acidic hydrolysis to yield the
corresponding diols (
S
)-
18
and (
R
)-
18
. The subsequent acylation of these
diols with 3,4,5-tri-
O
-benzylgallic acid (TBGA,
22
) via the Steglich
method (Neises and Steglich, 1978, Höfle
et al.
, 1978) produced the
fully protected intermediates (
S
)-
19
and (
R
)-
19
. Finally, the natural
product praecoxin B (
15
) and pariin M (
13
) could be synthesised by
removal of all protecting groups from (
S
)-
19
and (
R
)-
19
, respectively, in
two additional steps via irradiation to cleave the photolabile
ortho
-
nitrobenzyl group, thus furnishing α,β-anomeric mixtures of (
S
)-
20
and
(
R
)-
20
, and via hydrogenolysis using palladium on activated charcoal
and hydrogen to cleave all twelve benzyl ethers of the aromatic rings
(Fig. 5.4). A small variation of this strategy enabled the synthesis of
pterocaryanin C (
14
) and mahtabin A (
12
) from the α,β-anomeric
mixtures of (
S
)-
20
and (
R
)-
20
, each consisting of 80% of α-anomer and
20% of β-anomer. Esterification of these anomeric mixtures with 3,4,5-
tri-
O
-benzylgalloyl chloride (TBGCl,
7
) in the presence of Et
3
N gave the
corresponding β-anomers (
S
)-
21
and (
R
)-
21
, respectively. No
corresponding α anomer could be detected by thin layer chromatography
and NMR analysis of the reaction mixtures in each case. The total
syntheses of pterocaryanin C (
14
) and mahtabin A (
12
) were then
completed by hydrogenolysis of their respective precursors (
S
)-
21
and
(
R
)-
21
using palladium on activated charcoal and hydrogen (Fig. 5.4).
The comparison of NMR data and specific rotations of the synthetic
ellagitannins
12
and
13
(Khanbabaee and Lötzerich, 1998) with the data
sets for cercidinin A and cercidinin B (Nonaka
et al.
, 1989b) clearly
showed several differences. This is why we first presumed that the
postulated chemical structures for cercidinins A and B were in fact not
corresponding to those of the synthetic 1-
O
-galloyl-4,6-di-
O
-galloyl-2,3-
O
-(
R
)-hexahydroxydiphenoyl-β-
D
-glucopyranose (
12
) and 4,6-di-
O
-
