Chemistry Reference
In-Depth Information
O
O
O
O
Grubbs I
8.252
8.253
E
:
Z
= 4.7:1
Scheme 8.68
EtO
2
C
CO
2
Et
EtO
2
C
CO
2
Et
Grubbs II
8.254
8.255
Scheme 8.69
more-active catalysts can provide a solution to this problem in many cases (Scheme 8.69). The diene
8.254
failed to undergo RCM with the Grubbs I catalyst, but did so rapidly with Grubbs II.
80
Another way to solve the problem is by relay metathesis (Scheme 8.70). Given that formation of the desired
carbene by the intermolecular reaction between the catalyst and the substrate
8.256
is not feasible, the concept
is to attach a more available alkene, which can react with the catalyst to form a carbene
8.259
, and then acts
as a relay to transfer the ruthenium to the desired site by a facile RCM, leading to formation of the desired
product
8.257
.
Relay metathesis was employed to synthesize a building block
8.262
for peluroside (Scheme 8.71).
81
While
the silicon-tethered diene
8.261
gave the desired product, the yield was low even with a very high catalyst
desired reaction
prevented by
steric hindrance
Y
Y
X
Z
X
Z
"normal" substrate
8.256
8.257
Y
Y
[Ru]
[Ru]
X
Z
X
Z
"relay" substrate
8.258
8.259
Y
Y
"relay"-RCM
desired RCM
X
Z
[Ru]
X
Z
8.260
8.257
Scheme 8.70
