Chemistry Reference
In-Depth Information
Organocatalytic [3+2] cycloaddition reactions
O
Me
Z
Z
20 mol % cat.*
N
N
O
Catalyst =
Me
Me
•HClO
4
N
(eq 1)
R
O
R
R
1
-
H
O
R
MeNO
2
, H
2
O, -20°C
Ph
CHO
70-98% yield
80:20-99:1
endo
:
exo
90-99% ee
Organocatalytic cyclopropanation
R
1
Me
R
20 mol % cat.*
Catalyst =
CO
2
H
(eq 2)
S
R
R
O
Me
H
-
CHCl
3
, -10°C
CHO
63-85% yield
6:1-72:1 dr
89-96% ee
Organocatalytic epoxidation
F
3
C
20 mol % cat.*
O
Me
O
N
R
O
R
O
Catalyst =
CF
3
CH
2
Cl
2
, AcOH
Oxidant, -30°C
(eq 3)
t
-Bu
or
H
H
72-95% yield
85-97% ee
OTMS
Ph
•HClO
4
F
3
C
CF
3
Scheme 2A.3.
Iminium - catalyzed cycloaddition reactions.
O
Me
O
N
20 mol % cat.*
OTMS
O
Catalyst =
Me
Me
CHO
t
-Bu
H
CHO
O
CH
2
Cl
2
, 0°C
Me
Me
Ph
•TFA
64% yield
89% ee
Scheme 2A.4.
Iminium - catalyzed [4 + 3] cycloaddition.
2A.2.2. 1,4 - Addition Reactions
The enantioselective Michael addition of a nucleophile to an α , β - unsaturated carbonyl
system represents a valuable method for the synthesis of enantioenriched products. In
this regard, the value of iminium catalysis was further demonstrated by the development
of enantioselective conjugate addition protocols that were previously unattainable using
conventional acid or metal catalysis. More specifi cally, Friedel-Crafts alkylation of
electron-rich aromatics [9-12] and conjugate addition of aryl trifl uoroborate salts [13]
were found to be particularly effective under the iminium catalysis manifold (Scheme
2A.5). The effectiveness of this methodology was further demonstrated by the concise
synthesis of a number of natural products and bioactive compounds [14].
