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esterified with the sugar core (Fig. 5.18). For the total synthesis of
pedunculagin (
87
) via method
B
, an important issue was the question of
whether an existing chiral biaryl unit in the molecule would influence the
stereochemical outcome of an oxidative coupling of neighbouring galloyl
units. In fact, it was found that the sequence of the galloyl-galloyl
coupling is decisive. If the 4,6-positioned galloyl units are coupled first,
no subsequent coupling between the 2,3-positioned galloyl units is
possible. Therefore, the C-2 and C-3 galloyl units must be coupled first
and then the C-4 and C-6 galloyl units. Hence, the Feldman synthesis of
87
commenced with the acylation of the 2,3-positioned hydroxyl groups
of the
D
-glucopyranosyl derivative
28
with the 3,4-
O
-diphenylketal
gallic acid derivative
8
(see Fig. 5.2 and Fig. 5.17).
O
Ph
O
O
OBn
1. diphenylketal
8
,
DCC, DMAP,
DMAP.HCl
2. n-Bu
4
NF
O
O
2
3
O
O
O
O
O
Ph
O
S
O
85
O
O
3. Pb(OAc)
4
4. BnBr, K
2
CO
3,
KI, 28%
O
OBn
HO
O
HO
28
Bn, CPh
2
1. HCl, MeOH
2. diphenylketal
8
,
DCC, DMAP,
DMAP.HCl
3. n-Bu
4
NF
4. Pb(OAc)
4
,51%
HO
O
OH
O
HO
O
O
HO
Pd/C, H
2
S
O
O
32%
O
O
6
6
O
HO
O
4
O
O
O
O
4
OH
O
HO
OBn
HO
2
O
O
3
O
O
O
2
1
3
HO
O
O
O
O
O
HO
87
O
S
86
HO
OH
O
O
HO
O
Bn, CPh
2
Fig. 5.17
Total synthesis of pedunculagin (
87
) via the biaryl coupling strategy.
Selective desilylation was followed by Pb(OAc)
4
-mediated coupling
of the two galloyl units at C-2 and C-3 of the
D
-glucopyranosyl core to
give the 2,3-coupled product
85
. After removal of the benzylidene
