Biomedical Engineering Reference
In-Depth Information
O
OSiMe
3
OSiMe
3
HC(OMe)
3
mesitylene, 220°
C
Me
3
SiOTf
Et
3
N, PhMe
O
OMe
OMe
O
OMe
OMe
O
OMe
OMe
99%
70%
H
MeO
OMe
O
H
O
O
HO
Me
3
SiO
Eremopetasidione
SCHEME 8.2
Synthesis of eremopetasidione.
In Schemes 8.2-8.4, the key steps leading to the total synthesis of some selected
natural products are shown, highlighting the critical importance of the Cope
rearrangement that was used in a number of cascade protocols with other [3,3]-
sigmatropic rearrangements.
Isolated from rhizomes of
Petasites japonicus
, eremopetasidione is a natural
product used in popular Chinese medicine for the treatment of poisonous snake bites
and contusions. As shown in Scheme 8.2, the decalin ring systemof eremopetasidione
is prepared via a Cope rearrangement of a 1:1 mixture of
E
- and
Z
-silyl enol ethers,
providing a decalin intermediate that evolved
in situ
by double bond migration to the
thermodynamically more stable tetrasubstituted enol ether. The target molecule was
eventually isolated after acid hydrolysis followed by a series of simple and routine
transformations [25].
Gravelliferone is a typical diprenylated coumarin isolated from
R. graveolens
that was synthesized from 2,4-diprenylated benzaldehyde in 10% overall yield. The
synthesis featured both a Wittig reaction and a fascinating series of well planned and
executed cascade [3,3]-sigmatropic rearrangements, including two Claisen rearran-
gements and three Cope rearrangements (Scheme 8.3) [26].
Ph
3
P=CHCO
2
Et
PhOPh, reflux, 4 h
O
O
O
O
O
O
[3,3]
CO
2
Et
10%
CHO
CO
2
Et
O
O
O
O
O
O
O
O
[3,3]
CO
2
Et
[3,3]
HO
O
O
O
O
O
[3,3]
[3,3]
Gravelliferone
SCHEME 8.3
Synthesis of gravelliferone.
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