Chemistry Reference
In-Depth Information
was transaminated ca. 100 times as rapidly as was pyruvic acid by
23
, while simple
pyridoxamine shows no such selectivity. Compound
23
is selective because of binding
of the phenyl group into the
-CD cavity. As expected from this, a better binding p-tert-
butylphenyl group has an even larger effect: from competition studies, the preference
for 4-tert-butylphenylalanine formation from its keto acid exceeds 15 000 times that for
the amination of pyruvic acid [16].
Another pyridoxamine analog (
24
), prepared with the pyridoxamine attached to the
secondary face of
b
-CD [17], showed similar properties to those of the primary deri-
vative. As another comparison, an artificial macrocyclic binding group (i.e. tetraa-
za[1.7.1.7]paracyclophane ring) was used in place of
b
b
-CD [18]. The resulting catalyst
(
25
) accelerated the conversion of aromatic
a
-keto acids bearing good hydrophobic
segments into the corresponding
-amino acids by more than one order of magnitude
as compared with the rates for reactions catalyzed by pyridoxamine. To fix pyridoxa-
mine to the cyclodextrin with even better defined geometry, a set of four compounds
[
26-(1
-
4)
] was made by reacting a pyridoxaminedithiol with
a
b
-CD-6A,6B-diiodide [16].
As molecular models suggested, isomers
26-(1)
and
26-(2)
, with the pyridoxamine held
over the
-CD cavity, preferred para-substituted phenylpyruvic acids, while the other
pair held the pyridoxamine to the side and preferred meta-substituted phenylpyruvic
acid substrates.
b
