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O
R 2
SiR 1 3
Ph
1)
R
OCH 3
R 1 3 Si
41
OCH 3
(OC) 5 Cr
R 2 CHN 2
2)
R
43a-f
42
40
(OC) 3 Cr
43a R=Me R 1 =TIPS R 2 =H 84% yield
43b R=n-Bu R 1 =TIPS R 2 =H 83% yield
43c R=Ph R 1 =TIPS R 2 =H 72% yield
43d R=Ph R 1 =TBS R 2 =H 70% yie ld
43e R=Ph R 1 =TIPS R 2 =Ph 73% yield
43f R=Ph R 1 =TBS R 2 =Ph 76% yield
Scheme 10.13 Synthesis of cyclopentenones reported by Moses.
A more detailed mechanism for the addition of the carbenoid to the silyl vinylketene
is outlined in Scheme 10.14. Initially, there is a stereoselective addition of the carbenoid
reagent to the ketene to provide the (Z)-enolate. Subsequent ionization of the leaving group
generates the 2-oxidopentadienyl cation 46 as the sterically favored intermediate. The last
step is a concerted 4
electrocyclic ring closure that preserves the original alkene geometry
and provides the observed diastereomer of the cyclopentenone. It should be noted that in
previous studies, Danheiser and co-workers 23 suggested a different mechanism that cannot
be discounted. However, kinetic experiments are in concordance with the mechanism
depicted in Scheme 10.14.
R 1 3
Si
R
SiR 1 3
Ph
1)
C
OCH 3
O
41
OCH 3
(OC) 5 Cr
R
44
40
R 2
(OC) 3 Cr
H
N 2
42
R 2
R 2
- O
O
conrotatory 4 π
electrocyclic
ring closure
R 2
N 2
R
- O
R
R 1 3 Si
-N 2
R
OCH 3
R 1 3 Si
OCH 3
R 1 3 Si
OCH 3
(OC) 3 Cr
43
46
(OC) 3 Cr
(OC) 3 Cr
45
Scheme 10.14 Mechanism for the [4
+
1] synthesis of cyclopentenones reported by Moses.
 
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