Biomedical Engineering Reference
In-Depth Information
OTBS
MeO
2
C
Rh
2
(
R
-PTAD)
4
(0.5 mol%)
Hexane, -26°C
65%
CO
2
Me
OTBS
+
(ee = 90%)
O
O
CO
2
Me
O
O
Claisen
OTBS
52%
MOMO
MOMO
O
O
O
O
Rh
2
(
R
-PTAD)
4
=
N
O
Rh
O
O
R
Rh
O
H
O
(-)-5-
epi
-Vibsanin E
4
R = adamantyl
SCHEME 8.4
Synthesis of (
)-5-
epi
-vibsanin E.
)-5-
epi
-vibsanin E, isolated from
Viburnum
odoratissimum
, was achieved in 18 steps, featuring a synthetic sequence that
incorporated a rhodium-mediated [4
An elegant total synthesis of (
3]-cycloaddition between a vinyl carbenoid
and a diene to yield a divinylcyclopropane, followed by Cope rearrangement to form
the key cycloheptadiene. A series of standard transformations was then used, one of
which being a microwave irradiation-promoted Claisen rearrangement, to eventually
afford the targeted natural product (Scheme 8.4) [27].
The success and flexibility of the Cope rearrangement is evident in view of the
number of variations that have been investigated, such as the oxy-Cope, anionic
oxy-Cope (see below) [5], aza-Cope [28,29], aza-Cope-Mannich [30], 2-oxonia
Cope [31], or oxaza Cope [32] rearrangements exemplified in Schemes 8.5-8.8.
The aza-Cope (also called the amino-Claisen rearrangement) is a [3,3]-
sigmatropic rearrangement of an
N
-allyl enamine. Whereas neutral allylic enamines
þ
O
TsN
CHIRACEL OD-H
NTs
TsN
+
N
Ts
(
S
)
(
R
)
PdCl
2
(PhCN)
2
(cat.)
CH
2
Cl
2
, rt
87%
CO
2
H
HO
2
C
(+)-Kainic acid
HN
TsN
(-)-Kainic acid
(dr > 98%; ee > 98%)
SCHEME 8.5
Synthesis of (
)-kainic acid.
Search WWH ::
Custom Search