Chemistry Reference
In-Depth Information
O
Bn
O
O
Bn
O
1. Grubbs II
2. H
3
O
+
NPhth
NPhth
O
8.358
8.359
8.360
OBn
OBn
O
O
Bn
BnO
NPhth
8.361
BnO
Grubbs II
8.362
1. Dess-Martin Oxidation
2. CrCl
2
, MeCOCBr
2
Me
3. (
R
)-CBS, catechol borane
4. MeNH
2
, MeOH
O
O
NPhth
NPhth
H
2
, Pd/C
O
O
OH
8.363
8.364
OH
Scheme 8.100
This strategy was employed in a short synthesis of dumetorine
8.371
(Scheme 8.102).
114
The triene
substrate
8.369
was assembled by employing
-allyl palladium chemistry (Section 9.2) to install the nitrogen
functionality. Treatment with the Grubbs first-generation catalyst, with the addition of titanium tetra(
iso
-
propoxide) to suppress chelation by the carbonyl group, resulted in rearrangement of the system to the
dumetorine skeleton
8.370
. The synthesis was completed by deprotection and reductive methylation of
nitrogen, followed by selective hydrogenation of the less-substituted alkene.
8.3.6 Asymmetric Metathesis
At first glace, it would appear odd that a reaction that principally involves the manipulation of sp
2
-hybridized
carbon atoms should lend itself to a useful asymmetric version. If the initially generated carbene is, however,
offered the choice of two enantiotopic alkenes, then a useful asymmetric reaction can be contemplated
(Scheme 8.103). Either enantiomer of the product can be formed depending on which of the two enantiotopic
alkenes is selected. Significant modification of the metathesis catalyst with incorporation of chirality can be
needed to achieve this aim.
[Ru]
z
x
x
z
y
y
8.365
8.366
[Ru]
x
z
x
z
y
y
8
.
8
3
Scheme 8.101
