Biomedical Engineering Reference
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O
O
CH
3
O
Rh
2
(OAc)
4
O
N
2
N
N
CH
3
n
H
n
O
O
15
(
n
=1)
16
(
n
=2)
17
(
n
=3)
18
(
n
=1, 88%)
19
(
n
=2, 86%)
20
(
n
=3, 83%)
SCHEME 13.4
Lengthening the alkenyl tether by one carbon atom was observed to have no
effect on the ability of the isomunchnone to cycloadd across the olefinic
p
-bond. This
was shown in a study of the cycloaddition behavior of diazoimide
21
, which afforded
cycloadduct
22
in 86% yield as a single diastereomer (Scheme 13.5) [6].
The generality of the method was further demonstrated by synthesizing cyclic
diazoimides
23
and
24
in which the alkenyl tether was placed alpha to the nitrogen
atom (Scheme 13.6). Thus, when these diazoimides were treated with a catalytic
amount of Rh
2
(OAc)
4
, the tandem cyclization-cycloaddition process gave polycycles
25
and
26
in 69% and 76% yields, respectively. With both of these systems, the length
of the alkenyl tether proved to be crucial for the intramolecular cycloaddition reaction
across the isom
unchnone dipole. Only when the tether was a butenyl group was
cycloaddition observed. If the length of the tether was increased or decreased by one
methylene unit, no products derived from intramolecular cycloaddition were
encountered [6].
The 1,3-dipolar cycloaddition of isom
€
-diazoimides
of type
27
provides a uniquely functionalized cycloadduct (i.e.,
28
) containing a
“
masked
”
N
-acyliminium ion (Scheme 13.7). By incorporating an internal
nucleophile on the tether, annulation of the original dipolar cycloadduct
28
would
allow the construction of a more complex nitrogen heterocyclic system, particularly
€
unchnones derived from
a
O
CH
3
O
O
Rh
2
(OAc)
4
O
N
2
N
CH
3
N
H
O
O
21
22
SCHEME 13.5
O
CH
3
O
O
n
Rh
2
(OAc)
4
N
O
H
N
O
n
O
N
2
CH
3
23
(
n
=1)
24
(
n
=2)
25
(
n
=1, 69%)
26
(
n
=2, 76%)
SCHEME 13.6
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