Chemistry Reference
In-Depth Information
HBM
1
(10 mol %)
O
H
2
O (30 mol %)
BuLi (10 mol %)
Ar
3
P(O) (10 mol %)
Ar = 2,6-dimethoxyphenyl
O
O
O
OEt
+
NC
OEt
R
H
R
CN
(1.2 equiv)
10 examples
88-99% yield
87-98% ee
THF, -78°C
Scheme 3.19.
HBM
4
(10 mol %)
O
OH
4-Bromophenol (10 mol %)
+
R'
NO
2
R'
R
H
THF/xylene, -40°C
R
NO
2
14 examples
3:1 to 22:1 anti selective
47-97% yield
72-92% ee
O
OH
ent-HBM
4
(10 mol %)
Me
4-Bromophenol (10 mol %)
+
Me
NO
2
H
THF/xylene, -40°C
NO
2
BnO
BnO
OH
Me
HN
X
BnO
X
Y
(-)-Ritodrine: X = H, Y = OH
β
3
-Agonist: X = Me, Y =OCh
2
CO
2
Et
Scheme 3.20.
syn
-selective nitroaldol products, respectively. To explain the enantiofacial selectivity,
they proposed two hypothetical transition state models (Scheme 3.21).
3.4. LBA CATALYSIS
The combination of Lewis acids and Brønsted acids gives LBA catalysts and provides
an opportunity to design a “unique proton.” The coordination of a Lewis acid to the
heteroatom of the Brønsted acid could increase the acidity of the latter [36].
In 1994, Yamamoto and others reported that a chiral LBA complex, generated
in
situ
by mixing chiral BINOL and SnCl
4
, was effective as a stoichiometric reagent for the
enantioselective protonation of silyl enol ethers [37]. Coordination of a Lewis acid to a
