Chemistry Reference
In-Depth Information
Shi and coworkers successfully utilized the ketone-catalyzed method in an enantiose-
lective synthesis of 2-arylcyclopentanones (Scheme 11.42) [75]. In the presence of
34a
and oxone, benzylidenecyclobutanes underwent epoxidation with 86-96% ee, and the
subsequent epoxide rearrangement using Lewis acid catalysts such as Et
2
AlCl and LiI
gave the enantio-enriched 2-arylcyclopentanones [76]. The high enantiomeric excesses
of the epoxides were generally maintained in the rearranged products. Choice of the
Lewis acids determines the stereochemical course of the reaction, and the rearrange-
ment with Et
2
AlCl proceeds via a concerted mechanism with inversion of the confi gura-
tion. On the other hand, the reactions with LiI go through a stepwise process with double
inversion to yield the retention products. Both enantiomers can be synthesized with the
single epoxidation catalyst.
O
Et
2
AlCl
toluene
O
Ar
O
O
N
O
34a
(20 mol
%
)
Ar
+ Oxone
Ar
O
O
O
O
LiI
CH
2
Cl
2
Ar
34a
Scheme 11.42.
While oxone is the usual oxidant to generate active dioxiranes in the ketone-catalyzed
epoxidation, Shi and Shu reported an alternative oxidant, a combination of hydrogen
peroxide and acetonitrile (Scheme 11.43) [77]. In the reaction with ketone
31
, a variety
of epoxides were obtained in good yields with comparable enantioselectivity.
N
- Aryl -
substituted, oxazolidinone - containing ketone
34
was also shown to promote asymmetric
epoxidation with high enantioselectivity with hydrogen peroxide.
R
2
R
2
31
(10-30 mol
%)
O
R
3
+ 30% H
2
O
2
R
3
R
1
R
1
CH
3
CN, K
2
CO
3
89-99% ee
Scheme 11.43.
Acetonitrile, which is usually employed as cosolvent, reacts with hydrogen peroxide
to generate peroxyimidic acid and then reacts with the ketone to give the active dioxi-
rane (Scheme 11.44). Under the conditions, a stoichiometric amount of the amide is
produced as a by - product.
Percarboxylic acids such as peracetic acid and
m
CPBA are common oxidants for
olefi n epoxidation in organic transformations. Although the utilization of chiral perac-
ids, even as stoichiometric oxidants, has been studied, methods for achieving high
enantioselectivity had not been reported. In 2007, however, Miller and coworkers dem-
onstrated a unique acid/peracid catalytic cycle for asymmetric epoxidation (Scheme
11.45) [78]. At the initial stage of their study, the authors employed benzyl
N
- Boc
L - aspartate
36
as catalyst for constructing the hypothesized acid/peracid shuttle system
and applied carbodiimide activation technique in peptide synthesis to the purpose. Acid
36
is activated by carbodiimide, and the subsequent reaction with hydrogen peroxide
