Chemistry Reference
In-Depth Information
X
X
X
O
O
O
Y
N
O
N
O
N
P
P
P
Tf
Tf
Tf
O
O
O
H
H
H
X
X
X
PA
6a
: X = 2,4,6-(
i
-Pr)
3
C
6
H
2
PA
6b
: X = 4-(1-Ad)-2,6-(
i
-Pr)
2
C
6
H
2
PA
6c
: X = 9-phenanthryl
PA
7a
: Y = S, X = 2,4,6-(
i
-Pr)
3
C
6
H
2
PA
7b
: Y = S, X = 4-(
t
-Bu)-2,6-(
i
-Pr)
2
C
6
H
2
PA
8
: Y = Se, X = 2,4,6-(
i
-Pr)
3
C
6
H
2
PA
6d
: X = SiPh
3
PA
6e
: X = 4-(MeO)C
6
H
4
Figure 3.9.
Chiral
N
- trifl yl phosphoramides.
Me
O
OTIPS
COEt
PA
6a
(5 mol %)
Toluene, -78°C
R
1
Et
Me
TIPSO
12 h
R
1
7 examples
Up to 99% yield
Up to 92% ee
Scheme 3.49.
R
1
R
1
O
O
PA
6b
(5 mol %)
OEt
N
N
OEt
R
2
CHCl
3
, -40 to -55°C
R
2
H
16 examples
87:13 to 96:4
endo
selective
69-99% yield
56-92% ee for
endo
product
Scheme 3.50.
The Yamamoto group later prepared a new
N
- trifl yl phosphoramide PA
6b
, carrying
extremely bulky adamantyl group at the
para -
position of 3,3′ - substituents, to asymmetric
1,3-dipolar cycloaddition of diaryl nitrones with ethyl vinyl ether [85]. Whereas alumi-
num-based Lewis acid catalysts provided the products in high
exo
selectivity, the Brøn-
sted acid PA
6b
afforded
endo
product as the major diastereomer. It is explained that
the smaller acidic proton allows ethyl vinyl ether to approach in an
endo
selective way,
which is not favored in Lewis acid catalysis due to steric repulsion. Interestingly, intro-
duction of bulky substituents at the
para
- position of 3,3 ′ - substituents increased the
enantioselectivity as well as reactivity presumably by controlling the steric and electronic
properties of PA
6b
(Scheme 3.50 ).
Rueping and coworkers reported the Nazarov cyclization reaction catalyzed by
N
-
trifl yl phosphoramide PA
6c
[86]. Although phosphoric acids can catalyze the cyclization
