Chemistry Reference
In-Depth Information
O
H
O
18a
(10 mol %)
C
3
H
7
C
3
H
7
+
CO
2
Et
H
CO
2
Et
CH
2
Cl
2
, 25
°
C
92% ee (96%)
E
/
Z
96:4
OH
O
18
(10 mol %)
CO
2
Et
*
+
H
CO
2
Et
CH
2
Cl
2
, 25
°
C
18a
: 98% ee (
S
) (95%)
anti
/
syn
86/14
18b
: 94% ee (
R
) (70%)
anti
/
syn
95/5
Scheme 8D.22.
The glyoxylate-ene reactions by treatment of anhydrous box Cu(II) complexes
18
can be extended to the keto ester-ene reactions (Scheme 8D.23) [42]. Methyl pyruvate
is examined as a carbonyl enophile to give low yield of the corresponding ene product
bearing quaternary chiral center with 1,1-disubstituted olefi n. The addition of a large
excess of methylenecyclo-hexane or -pentane (10 or 5 equiv) under heated conditions
(40°C) is necessary to provide high yield and enantioselectivity using 20 or 5 mol % of
the Cu catalyst (Scheme 8D.23 ).
O
18a
(X mol %)
Me
OH
+
Me
CO
2
Me
CH
2
Cl
2
, T
°
C, 48 h
CO
2
Me
n
n
5 eq.
10 eq.
5 eq.
1 eq.
1 eq.
1 eq.
n = 1, X = 10, T = 25
n = 1, X = 20, T = 40
n = 0, X = 5, T = 40
99% ee (35%)
98% ee (84%)
98% ee (98%)
Scheme 8D.23.
Evans and Wu also designed chiral
C
2
- symmetric trivalent pybox - Sc complexes
19
and applied them to the asymmetric carbonyl-ene reactions with
N
- phenyl glyox-
amide (Scheme 8D.24 ) [48] . When unsymmetrical 1,1,2 - trisubstituted olefi ns
20a
and
20b
are used with the treatment of Sc complex
19
, the ene products are obtained
in excellent diastereo- and enantioselectivity, respectively. The regioisomeric products
are not observed in both reactions. It is suggested that the major product is obtained
via proton transfer from the
-
cis
substituent through an
exo
- transition state. In
contrast, a general preference for
endo
-transition states is observed in the box-Cu
complexes
18
in glyoxylate-ene reactions with cyclohexene as an enophile (see Section
8D.2.8 ).
β
