Chemistry Reference
In-Depth Information
O
O
O
BLA
3
(1 eq)
H
H
2
CCO
H
2
CCO
Bu
3
SnOTf (1 eq)
R
H
R
CH
2
Cl
2
, -78°C
6 examples
62-78% yield
65-84% ee
Proposed mechanism
Ph
H
Ph
Ph
H
Ph
O
N
TfO
B
O
N
H
Bu
3
SnOTf
B
O
O
OSnBu
3
H
O
BLA
3
H
2
CCO
Ph
H
Ph
TfO
Ph
H
O
H
R
Ph
O
N
B
O
O
O
TfO
H
H
N
B
CH
2
OSnBu
3
O
CH
2
H
O
R
O
O
OSnBu
3
Actual catalyst
Scheme 3.9.
containing medium- to large-sized rings in good to excellent diastereo- and enantiose-
lectivities. Interestingly, introducing
o
- fl uorophenyl at boron in oxazaborolidine pro-
vided TADA products in signifi cantly higher enantioselectivity than corresponding
o
- tolyl substituted BLA
1a
(90% ee vs. 10% ee). Additionally, this methodology was
successfully applied to the synthesis of lactone
B
, which could potentially serve as a
precursor to a number of sesquiterpene targets (Scheme 3.11).
The synthetic power of these excellent BLAs has also been well demonstrated by
their applications to the enantioselective syntheses of biologically important molecules.
Corey and others applied the catalytic enantioselective D-A reactions by BLAs as key
steps for the synthesis of (+)-estrone [17], georgyone [18], palominol [19], and tamifl u
[20]. An enantioselective total synthesis of (−)-rasfonin has been reported by Boeckman
et al. using chiral BLA-catalyzed vinylogous Mukaiyama aldol reaction as the key step
[21]. Recently, Yamamoto et al. utilized regioselective D-A reaction of 1- and 2-substi-
tuted CPs catalyzed by BLA
4
for their synthesis of plastansimycin [22].
