Chemistry Reference
In-Depth Information
Scheme 2.3
A further method to induce chirality in the pyridoxamine-mediated transamination
reactions was developed by Kuzuhara et al. [13]. They synthesized optically resolved
pyridinophanes (
21
,
22
) having a nonbranched “ansa chain“ between the 2
0
- and 5
0
-
positions of pyridoxamine. With the five-carbon chain in
21
and
22
, the two isomers do
not interconvert readily. In the presence of zinc(
II
) in organic solvents such as metha-
nol, tert-butanol, acetonitrile, and nitromethane, they observed stereoselective transa-
mination between pyridinophanes and keto acids. The highest ee%s are 95% for
D-
and
L-
leucine by reaction of the corresponding
-keto acid with (S)- and (R)-
22
, respec-
tively. On the basis of kinetic analysis of the transamination reactions, Kuzuhara et al.
originally proposed a mechanism for the asymmetric induction through kinetically
controlled stereoselective protonation to the carboanion attached to an octahedral
Zn(
II
) chelate intermediate. However, they subsequently raised some questions about
this proposal [14].
a
2.2.2
Pyridoxamines with a Binding Site - Artificial Enzymes
An important feature of the enzymatic systems is the presence of a binding site. Thus
it is attractive to learn how to construct vitamin B6-dependent enzyme models that can
provide a substrate binding site and performmolecular recognition. The first example
was a catalyst (
23
) in which pyridoxamine was linked to the primary face of
b
-cyclo-
dextrin (
b
-CD) through a sulfur atom [15]. Catalyst
23
could transform
a
-keto acids into
a
-amino acids, as pyridoxamine does, but with selectivity. That is, phenylpyruvic acid
