Biomedical Engineering Reference
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R
1
O
+
O
N
2
O
O
-
R
3
R
3
Rh(II)
O
O
R
1
R
1
N
N
+
O
N
R
3
-
O
R
2
O
R
2
R
2
5
6a
6b
SCHEME 13.2
(Scheme 13.2). These reactive dipoles can then be trapped with a variety of
dipolarophiles to give cycloadducts in high yield.
Interesting examples of intramolecular 1,3-dipolar cycloadditions of iso-
m
unchnones possessing an unactivated alkene have been reported to give rise to
complex azapolycyclic compounds in one step [4,8]. The isom
€
unchnones derived
from the Rh
2
(OAc)
4
-catalyzed reaction of acyclic diazoimides
7
-
10
were found to
undergo facile cycloaddition onto the tethered
€
-bond to provide polycyclic adducts
11
-
14
(Scheme 13.3). A notable feature of this cycloaddition is that only one
diastereomer is formed. The relative stereochemistry of cycloadduct
14
was deter-
mined by X-ray crystallography [7]. This confirmed the fact that addition of the olefin
took place
endo
with regard to the isom
p
unchnone dipole. Only low yields of
cycloadducts were observed when the deacylated diazoimides were subjected to the
cyclization-cycloaddition sequence. This result indicates that the reactivity of the
1,3-dipole is significantly diminished in the absence of the electron-withdrawing acyl
group and that alternative pathways then become competitive.
This method was further extended, leading to a significant increase in com-
plexity of the resulting polyheterocyclic systems, by employing a series of cyclic
diazoimides [6]. Treatment of cyclic diazoimides
15
-
17
with Rh
2
(OAc)
4
led to good
yields of cycloadducts
18
-
20
as single diastereomers (Scheme 13.4). Once again, the
stereochemical outcome is the result of an
endo
cyclization of the
€
p
-bond onto the
isomunchnone dipole, and this was confirmed by an X-ray crystallographic analysis
of cycloadduct
18
(Scheme 13.4) [6].
O
R
2
O
CH
3
Rh
2
(OAc)
4
O
H
O
R
1
N
2
N
CH
3
N
Bn
Bn
R
1
R
2
O
O
7
R
1
=R
2
=H
8
R
1
=H; R
2
=CH
3
9
R
1
=R
2
=CH
3
11
R
1
=R
2
=H
12
R
1
=H; R
2
=CH
3
13
R
1
=R
2
=CH
3
O
O
CH
3
Rh
2
(OAc)
4
O
O
N
2
N
N
CH
3
Bn
Bn
H
O
O
10
14
SCHEME 13.3
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