Chemistry Reference
In-Depth Information
A highly enantioselective [3 + 2] cycloaddition reaction of several benzoquinones
with 2,3-dihydrofuran using a chiral BLA
2a
as a catalyst was developed, which allows
rapid access to a variety of chiral phenolic tricycles with high enantioselectivities [12].
The utility of this new methodology was demonstrated in a short, enantioselective total
synthesis of afl atoxin B
2
. By a trapping experiment using a large excess of 2,3-dihydro-
furan, the reaction pathway was elucidated as shown in Scheme 3.7.
OH
OH
O
R
2
R
1
R
1
BLA
2a
(20 mol %)
CH
2
Cl
2
: CH
3
CN
(1:1)
H
H
R
1
O
R
2
R
2
O
O
O
O
H
O
H
5 examples
32% yield
90% ee
(R
1
= OMe, and R
2
= H)
65-75% yield
92-95% ee
OH
O
O
O
MeO
MeO
O
ent-BLA
2a
(20 mol %
)
CH
2
Cl
2
: CH
3
CN
(1:1)
H
H
O
O
O
H
O
O
O
MeO
H
-78°C, 2 h
65% yield, 92% ee
Aflatoxin B
2
H
cat*
O
Me
H
N
O
B
H
H
MeO
O
Tf
2
N
H
O
O
H
MeO
O
O
H
O
OH
OH
MeO
MeO
H
H
H
O
O
H
O
H
O
O
H
Scheme 3.7.
Later, the application of BLAs was extended to the Michael additions of silyl
ketene acetals to cyclic and acyclic α , β - unsaturated ketones [13] . Interestingly, they
found that a catalytic amount of Ph
3
P=O increased the enantioselectivity because
it could serve as a trap for any electrophilic Me
3
Si species formed during the reaction.
