Chemistry Reference
In-Depth Information
O
OTIPS
OTIPS
O
LLA
2
(10 mol %)
CH
2
Cl
2
, -78°C
OCH
2
CF
3
OCH
2
CF
3
+
n
H
n
n = 0, 1, or 2
97-99% yield
82:18-99:1
endo
selective
92-99% ee
O
OTBS
OTBS
O
LLA
2
(10 mol %)
CH
2
Cl
2
, -78°C
OCH
2
CF
3
OCH
2
CF
3
+
n
Me
n
n = 0, 1, or 2
91-99% yield
10:90-1:99
exo
selective
98% ee (
exo
)
O
OTBS
OCH
2
CF
3
O
n
Me
n
Me
n = 0, 1, or 2
Scheme 3.14.
O
O
O
i
-Pr
O
i
-Pr
Ti
Ti
O
Ti
O
O
O
O
O
Ti
i
-PrO
i
-PrO
O
O
LLA
3
Figure 3.2.
Since the fi rst report with LLA
3
, Maruoka and his colleagues extended the utility
of LLA
3
to other transformations. In 2005, it was reported that asymmetric 1,3-dipolar
cycloaddition of nitrones with acrolein is catalyzed by LLA
3
(Scheme 3.16 ) [28] . Inter-
estingly, the monomeric complexes from Ti(Oi-Pr)
4
/(S) - BINOL and ClTi(Oi - Pr)
3
provide the cycloadducts in low yields and moderate enantioselectivities, which
implies that Lewis acidity is enhanced by intramolecular activation of LLA
3
as shown
in Scheme 3.15 .
Subsequently, Maruoka et al. developed an enantioselective 1,3-dipolar cycloaddition
reaction between diazoacetates and
- substituted acroleins, which gives 2 - pyrazolines
with a quaternary carbon center in good yields and with excellent enantioselectivities
α
