Chemistry Reference
In-Depth Information
Ryu and Corey later found that different BLAs
2a
and
2b
, derived from protonation
with trifl imide (Tf
2
NH), have better stabilities than BLAs
1a
and
1b
[4]. It is interesting
that simple change of the counteranion from trifl ate ( CF S
3
−
) to trifl imide ((CF
3
SO
2
)
2
N
−
)
had a remarkably benefi cial effect on catalyst stability, with no loss of potency. These
new Lewis acids
2a
and
2b
are highly effective catalysts for a wide variety of enantiose-
lective D-A reactions that were previously beyond the reach of synthetic chemists
(Scheme 3.3 ).
The D-A reactions of unsymmetrical benzoquinones are particularly challenging due
to their multiple requirements of high enantioselectivity, orientational selectivity, and
site selectivity. The Corey group examined the D-A reactions of various unsymmetrical
O
O
H
BLA
2a
(20 mol %)
Toluene, -60°C
OEt
OEt
+
EtO
OEt
H
O
O
99% yield, >99% ee
O
H
O
BLA
2a
(20 mol %)
Toluene, -60°C
OEt
+
OCH
2
CF
3
98:2
endo
selective
97% yield, >99% ee
O
H
O
BLA
2b
(20 mol %)
Neat, -20°C
+
H
96% yield, 85% ee
O
O
H
BLA
2a
(20 mol %)
CH
2
Cl
2
, -95°C
+
H
O
99% yield, >99% ee
O
O
O
O
H
H
BLA
2a
(20 mol %)
+
+
CH
2
Cl
2
, -95°C
H
H
O
O
O
a
b
93% ee
99% yield, >99% ee
(
a
-isomer
:
b
-isomer = 62:38)
Scheme 3.3.
