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CO 2 Me
CO 2 Me
MeO 2 C
MeO 2 C
MeO 2 C
CO 2 Me
Ni(COD) 2 /dppb
or
Pd(dba) 2 /PPh 3
H
+
CO, MeOH
MeO 2 C
96
95
MeO 2 C
O
X
94
Scheme 10.27 Ni(0) or Pd(0) synthesis of fused cyclopentenones.
-
methylenecyclopentenones fused to five-membered rings via a palladium-catalyzed car-
bonylative cyclization of enynes (Scheme 10.28). 59
Another
interesting
variation
of
this
reaction
is
the
synthesis
of
α
MeO 2 C
CO 2 Me
CO 2 Me
MeO 2 C
Pd(dba) 2 /PPh 3
LiCl, CO
43% yield
THF, H 2 O, 70 ° C
AcO
97
98
O
Scheme 10.28 Synthesis of
α
-methylenecyclopentenones.
10.5 Metal Carbine Strategies for the Synthesis of Cyclopentenones
10.5.1 Cyclopropylcarbene Chromium Complexes
The reactions of cyclopropylcarbene chromium complexes with alkynes have attracted
significant attention from several research groups. For example, Herndon and co-workers
reported that cyclopropylchromium carbine 99 reacts with diphenylacetylene 72a afford-
ing a mixture of cyclopentenones 100 and 101 with a ratio of 1:20, 60-64 as depicted in
Scheme 10.29. The major product of the reaction is the thermodynamically stable cy-
clopentenone 101 , in which the trans stereochemistry of the original alkyne substituents is
retained in the product. In the commonly accepted mechanism for this reaction, the products
are believed to be formed via “ in situ ” reduction of the cyclopentadienone moiety to the
corresponding cyclopentenone derivatives under the reaction conditions. 64
O
O
Ph
Cr(CO) 5
OMe
Ph
Ph
Ph
Ph
OMe
OMe
Ph
99
72a
79% yield
101
4% yield
100
Scheme 10.29 Cylopentenone synthesis from,cyclopropyl chromium complexes reported by
Herndon.
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