Chemistry Reference
In-Depth Information
Grubbs II
+
OAc
AcO
OAc
O
O
8.322
8.323
8.324
Scheme 8.90
SiMe
3
N
N
O
O
Grubbs II
Me
3
Si
O
8.325
8.326
O
OTr
OTr
Scheme 8.91
Br
i
-Pr
O
n
-Bu
i
-Pr
OTBS
Ar =
N
N
Mo
Ph
O
n
-Bu
catalyst
8.328
Z
:
E
>98:2
Br
OAr
catalyst
8.327
OAc
N
N
OAc
AcO
Ru
O
catalyst
t
-BuCO
2
i
-Pr
8.329
Z
:
E
=
ca
8:1
catalyst
Scheme 8.92
Application of these principles even allows selectivity when one component has two alkenes (schemes
8.90, 8.91).
104
Using the Grubbs second-generation catalyst, the product of cross-metathesis is the
E
-isomer. Some
specialized
Z
-selective catalysts have been designed (Scheme 8.92).
105
A simple example of the use of cross-metathesis is in a synthesis of Diospongin A
8.336
in which an
isolated alkene was converted into a Michael acceptor by cross-metathesis (Scheme 8.93).
106
The starting ho-
moallylic alcohol
8.330
was converted to the corresponding protected epoxide
8.333
with
syn
-stereochemistry
via an iodolactonization protocol. Ring opening with a vinyl Grignard reagent then gave the cross-metathesis
substrate
8.334
. Cross-metathesis with phenyl vinyl ketone converted the relatively unreactive isolated vinyl
group to a good Michael acceptor
8.335
, so that, upon removal of the silyl protecting group, cycliza-
tion occurred to give the natural product
8.336
. Another synthesis of this natural product may be found
in Scheme 6.29.
