Chemistry Reference
In-Depth Information
OCH
3
OH
N
OH
OH
N
NH
CF
3
N
N
N
S
N
H
N
H
98
99
100
CF
3
Figure 8A.6.
PTCs, for example,
Cinchona
alkaloid-derived chiral quaternary ammonium salts,
carbohydrate-derived azacrown ethers,
C
2
-symmetric ammonium catalysts, and so on,
were developed for catalytic asymmetric synthesis. Corey and Zhang reported that the
N
- (9 - anthracenylmethyl)cinchonine derivative
101
was employed in the sequential syn-
thesis of (
R
) - baclofen hydrochloride (Scheme 8A.54 ) [116] .
Br
OBn
NO
2
O
O
H
N
101
(1 mol
%)
+
CH
3
NO
2
N
H
CaF, tolunene
-40
°
C, 36 h
Cl
Cl
101
Scheme 8A.54.
Recently, Bakó et al. [117] investigated the conjugate addition of 2-nitropropane to
chalcones catalyzed by carbohydrate-derived azacrown ethers. In those studies, some of
D - glucose - and D - mannose - derived azacrown ethers such as
102
-
106
showed the best
results (Scheme 8A.55 ).
Other chiral PTCs such as
C
2
-symmetric ammonium catalysts (e.g., spirocyclic gua-
nidine
107
) were reported to be effective in the conjugate addition of 2-nitropropane to
chalcone, giving high yield and good enantioselectivity (Scheme 8A.56) [118].
N
- spiro
C
2
-symmetric chiral quaternary ammonium bromide
108
[119] and its deriva-
tives
109
[120] enabled the effi cient, highly stereoselective conjugate addition of nitroal-
kanes to alkylidenemalonates and cyclic enones, respectively. These reactions afforded
products in good yields and high enantioselectivities (Scheme 8A.57).
8A.3.4. Conjugate Addition of Silyl Enol Ether
MacMillan et al. reported the preparation of chiral
- butenolides with good
syn
selectiv-
ity (up to 92% de) and high ee's (84-99%) by using chiral imidazolidinones
110
as cata-
lyst (Scheme 8A.58 ) [121] .
γ
