Biomedical Engineering Reference
In-Depth Information
7.5.
EPOXIDATION AND AZIRIDINATION
The catalytic asymmetric epoxidation of electron-deficient olefins, particularly
a
-unsaturated ketones, has been the subject of numerous investigations and a
number of useful methods have been elaborated. Among these, the method utilizing
chiral phase-transfer catalysis occupies a unique place featuring its practical advan-
tages and allowing highly enantioselective epoxidation of
trans
-
,
b
,
-unsaturated
a
b
ketones, particularly chalcone (Table 7.4) [59].
Chiral aziridines have been used as chiral auxiliaries, chiral ligands for
transition metals, and chiral building blocks for preparation of biologically active
species, such as amino acids,
-lactams, and alkaloids. Murugan and Siva developed a
new procedure for asymmetric aziridination reactions to achieve excellent level of
enantioselectivity using new chiral phase-transfer catalysts
1e
and
4c
derived from
cinchonine and cinchonidine, respectively (Scheme 7.24) [66].
b
7.6. STRECKER REACTION
The catalytic asymmetric cyanation of imines, the Strecker reaction, represents
one of the most direct and viable methods for the asymmetric synthesis of
-amino
acids and their derivatives. Numerous recent efforts in this field have resulted in
the establishment of highly efficient and general protocols, although the use of
alkyl metal cyanide or anhydrous hydrogen cyanide generally at low temperature
is inevitable. In this regard, we disclose the first example of phase-transfer-
catalyzed, highly enantioselective Strecker reaction of aldimines using
aqueous KCN based on the molecular design of chiral quaternary ammonium
salts
56
bearing the tetranaphthyl backbone as a remarkably efficient catalyst
(Scheme 7.25) [67].
a
1e
or
4c
(10 mol%)
O
CO
2
t
-Bu
*
CO
2
t
-Bu
OH
+
t
-Bu
N
N
20% NaOH aq
Toluene
Ph
Ph
with
1e
: 79% (ee = 94%,
S
)
with
4c
: 56% (ee = 88%,
R
)
Me
_
+
O
2
S
H
N
Br
O
_
Me
+
N
Br
O
O
2
S
Me
N
HO
N
H
CHO
HO
1e
4c
CHO
Me
SCHEME 7.24
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