Chemistry Reference
In-Depth Information
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.