Chemistry Reference
In-Depth Information
Cp
2
TiCl
2
2 MeLi
O
O
O
Cp
2
TiMe
2
8
1
9
8
0
Scheme 8.55
Cp
2
TiCl
2
2
Li
Cp
2
Ti
O
O
O
8
8
8
1
9
1
Scheme 8.56
Cp
2
TiCl
2
2
MgBr
O
•
Cp
2
Ti
8
.
9
2
8
.
1
9
3
Scheme 8.57
An outstanding application of Tebbe's reagent in organic synthesis is in a synthesis of capnellene
8.205
b.
58
The synthesis started with the
-dimethyllactone
8.194
. Partial reduction, Horner-Wadsworth-Emmons
olefination and an
in situ
oxa-Michael addition gave the tetrahydrofuran
8.195
(Scheme 8.58). This was ring
opened and trapped in that form using LDA and tosyl chloride. Treatment of the tosylate
8.196
with the
magnesium derivative of cyclopentadiene resulted in alkylation. The corresponding sodium and potassium
derivatives could not be used due to the formation of by-products through Michael addition. Warming the
alkylation product
8.197
at 75
◦
C gave the Diels-Alder adduct
8.199
. This is formed by, first, a 1,5-sigmatropic
shift, then the cycloaddition.
The tricyclic structure
8.199
contains two of the rings required for capnellene, it also contains the in-
herent ring strain to drive metathesis chemistry forwards. Treatment of the Diels-Alder adduct
8.199
with
Tebbe's reagent in the presence of DMAP yielded the sensitive cyclobutene
8.200
(Scheme 8.59). This is
formed by initial addition of Tebbe's reagent to the alkene with the bulkier Cp
2
Ti moiety avoiding the more
hindered terminus. Fragmentation of the metallcyclobutane
8.201
generates a new carbene
8.202
, which
undergoes a metathesis reaction with the ester carbonyl to give the cyclobutene
8.200
and an oxa-titanium
by-product.
,
