Biology Reference
In-Depth Information
The second synthesis provided a much shorter route to the target
molecule (
67
, Feldman
et al.
, 1994). In the first step, the O-1 benzylated
D
-glucopyranosyl derivative
68
is fully acylated with the acid
8
(see Fig.
5.2) and desilylated using
n
-Bu
4
NF. The resulting
D
-glucopyranosyl
derivative
69
is then subjected to the Pb(OAc)
4
-mediated biaryl coupling
reaction. An amazing regioselectivity was observed during this coupling
reaction that took place only between the galloyl groups linked at the O-
4 and O-6 positions. Thus, apparently, the galloyl group at the O-2 and
O-3 positions could not adequately orient themselves relatively to each
other to enable coupling or could not express a sufficient reactivity
towards Pb(OAc)
4
to be oxidatively activated. Again, tellimagrandin I
(
67
) was finally released by hydrogenolysis (Fig. 5.13).
1.
8
, DCC, DMAP
2.
n
-Bu
4
NF, 90%
RO
HO
O
O
RO
HO
RO
OBn
HO
OBn
RO
HO
68
69
HO
1. Pb(OAc)
4
2. Pd/C, H
2,
29%
O
R =
O
Ph
Ph
O
67
: tellimagrandin I
Fig. 5.13 Second total synthesis of tellimagrandin I (
67
).
5.2.2.5.4 Total synthesis of tellimagrandin II
The natural product tellimagrandin II (
73
, Feldman and Sahasrabudhe,
1999) differs from tellimagrandin I (
67
) by the presence of one
additional galloyl unit at the anomeric center of the glucopyranosyl core.
Thus, the synthetic sequence for tellimagrandin II (
73
) is mostly
identical to that for the synthesis of tellimagrandin I (
67
). The only
difference is that the anomeric center of the glucopyranosyl intermediate
70
is protected with the photolabile
ortho
-nitrobenzyl group for the
synthesis of tellimagrandin II (
73
) instead of a benzyl group for the
synthesis of tellimagrandin I (
67
). This enables the selective deprotection
of the anomeric center, then allowing the acylation of that center with
3,4,5-tribenzylgalloyl chloride (TBGCl,
7
). Cleavage of the
ortho
-
nitrobenzyl protecting group at the anomeric center of the
D
-
