Biomedical Engineering Reference
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O
2
N
S
O
H
CO
2
Et
CO
2
Et
Rh(II)
180°C
Xylene
S
N
C
2
H
5
O
O
2
N
N
2
N
C
2
H
5
72
73
S
Ph
-
+
O
S
S
H
O
BrCH
2
COCl
Et
3
N
N
NH
N
C
2
H
5
C
2
H
5
Ph
Ph
C
2
H
5
H
74
75
SCHEME 13.16
76
OMe
OMe
H
NH
2
MeO
N
MeO
N
N
O
RO
2
C
O
Isoschizogamine
80
79
OMe
OMe
-
NO
2
+
H
S
H
H
O
S
N
RO
2
C
Ar
N
O
RO
2
C
O
77
(Ar =
o
-NO
2
C
6
H
2
(OMe)
2
)
78
SCHEME 13.17
extrusion of sulfur from cycloadduct
78
, followed by reduction of both the nitro and
keto groups and a subsequent dehydration. It was reasoned that the key cycloadduct
78
would be accessible from an intramolecular dipolar cycloaddition of the thioi-
som
€
unchnone dipole
77
(Scheme 13.17).
13.2.3. Cross-Conjugated Heteroaromatic Betaines
To test the feasibility of the retrosynthetic strategy outlined in Scheme 13.17, efforts
were initially focused on model substrates. Unfortunately, it was found that the
presence of an
ortho
substituent on the aromatic ring of the dipole-derived thioamide
77
twists the thioisomunchnone far enough away from the tethered
cis
-alkenyl
substituent in the preferred transition state, thereby preventing the desired intramo-
lecular cycloaddition. In order to avoid this difficulty, cycloaddition of the related
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