Chemistry Reference
In-Depth Information
ceeded in improving the enantioselectivity by attaching Merrifi eld resin on the hydroxy
moiety of cinchonidine-derived catalyst possessing the 9-anthracenylmethyl group on
nitrogen (
17b
) [24] .
Itsuno and others reported a new type of immobilization method onto a polymer
through ionic bond to sulfonate groups. Polymer-supported chiral quaternary ammo-
nium salts
18a
and
18b
were easily prepared by reaction of the sulfonated polymer with
a corresponding chiral ammonium salt. The polymeric catalysts could be applied to
alkylation of
2a
and reused without loss of reactivity and selectivity (Scheme 2C.9) [25].
O
O
Catalyst
Ph
2
CN
Ph
2
CN
*
+
PhCH
2
Br
O
t-
Bu
O
t-
Bu
50% KOH
aq
Toluene, 0°C
2a
H
Ph
3a
18a
(10 mol %): 73%, 95% ee (
S
)
18b
(1 mol %): 84%, 98% ee (
R
)
SO
3
-
+
R
4
N
Ar
+
H
N
+
Bu
Bu
+
R
4
N =
N
or
OH
N
Ar
18b
(Ar = 3,4,5-F
3
-C
6
H
2
)
18a
Scheme 2C.9.
A recyclable fl uorous chiral phase-transfer catalyst
19
has been developed in our
group and its high chiral effi ciency and reusability have been demonstrated in the asym-
metric alkylation of
2a
. After the reaction,
19
could be easily recovered by the simple
extraction with FC-72 (perfl uorohexanes) as a fl uorous solvent and could be used for
the next run without any loss of reactivity and selectivity (Scheme 2C.10) [26].
2C.3.2. Asymmetric Synthesis of
α
,
α
- Dialkyl -
α
- Amino Acids
Nonproteinogenic, chiral
-amino acids possessing stereochemically stable
quaternary carbon centers have been signifi cant synthetic targets not only because they
are often effective enzyme inhibitors but also they are indispensable for the elucidation
of enzymatic mechanisms. Accordingly, numerous studies have been conducted to
develop truly effi cient methods for their preparation [27], and phase-transfer catalysis
has made unique contributions.
α
,
α
- dialkyl -
α
