Chemistry Reference
In-Depth Information
Recently, tartrate-derived chiral PTCs, such as
125a
[138] and
125b
[139a] , were
developed by Shibasaki et al. in the conjugate addition of amino acid derivatives to dif-
ferent Michael acceptors (Scheme 8A.67 ).
O
Ph
2
CN
O
OBu
t
Ph
125a
(10 m ol %), Cs
2
CO
3
Chlorobenzene, -30
°
C
CO
2
Bu
t
+
N
OBn
Ph
CO
2
Bn
87%, 86% ee
C
6
H
13
5
O
O
O
125b
(10 mol %), Cs
2
CO
3
3-Fluorotoluene, -40
Ph
O
+
N
CO
2
Bn
Ph
C
6
H
13
°
C
5
Ph
N
Ph
BuO
2
C
84%, 82% ee
Scheme 8A.67.
In the conjugate addition of benzophenone imine-derived glycinate to enones with
high enantioselectivities (up to 97% ee), a signifi cant counteranion dependence of enan-
tioselectivity was observed in the employment of dimeric
Cinchona
alkaloid ammonium
salts
126
(X = Br, BF
4
, PF
6
) (Scheme 8A.68 ) [139a] .
O
Ph
N
126a/126b
(5 mol %)
Cs
2
CO
3
CH
2
Cl
2
,-78
OBu
t
Ph
+
O
Ph
N
°
CO
2
Bu
t
C
Ph
O
126a
X=BF
4
, 57%, 85/15 dr,
76% ee (major), 32% ee (minor)
126b
X=PF
6
,32%,93/7 dr,
97% ee (major),68%ee(minor)
Scheme 8A.68.
Chiral ammonium BINOL-derived catalyst
127
was used in the asymmetric conjugate
addition of 2 - naphthalen - 1 - yl - 2 - oxazoline - 4 - carboxylic acid
tert
-butyl ester to ethyl acry-
late, giving α-(hydroxymethyl)glutamic acid in 93% yield and 97% ee [140] (Scheme
8A.69 ).
Arai et al. developed a new chiral quaternary ammonium salt
128
derived from (
S
) -
BINOL in the conjugate addition of
N
- (diphenylmethylene)glycine
tert
- butyl ester to
electron-poor alkenes, affording low to moderate enantioselectivities (32-75% ee)
(Scheme 8A.70 ) [141] .
