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N
N
Zn
O
O
OH
OH
O
O
HO
HO
O
O
OH
HO
HO
29
OH
Fig. 17 A water soluble Zn-salophen anion receptor 29.
Increasing water content in DMSO to 5% caused expected decrease in
binding constants but chiral recognition ability of receptor 27 remained
roughly at the same level (K
R
/K
S
=
2.01). Titrations of mandelic acid with
derivative 28 gave no pronounced chiral recognition. Binding constants
were identical within experimental error for both enantiomers. In an ex-
tension of this work, chiral recognition ability of receptor 27 was probed
with three N-Boc protected amino acid anions derived from tryptophane,
valine and phenylalanine. These amino acids were recognized by receptor
27 with good enantioselectivities (K
D
/K
L
) 2.57, 2.42, and 2.40, respectively.
A chiral recognition of amino acid zwitter ions by Zn-salophen decor-
ated with
D
-glucose 29, was investigated by UV/VIS spectrophotometric
titrations in water
31
(Fig. 17). The binding constants of amino acids were
reduced when compared with achiral acetate. This decrease in binding
constants was explained by unfavorable steric interactions of amino acids
with sugar rings. The binding constants for
L
- and
D
- amino acids were
different with enantioselectivity up to 9.6 for phenylalanine. This high
chiral recognition was explained by two interactions of receptor 29 with
amino acids. Carboxylate anion is bound to Zn-salophen while ammo-
nium cation can interact by hydrogen bonds with hydroxy groups of
glucose.
4 Conclusion
In this chapter we have, in our opinion, presented the most important
motifs that have been used to construct anion receptors effective in chiral
recognition. The carbohydrates have been successfully applied for
modification of binding pocket of anion receptors as well as chiral bar-
riers modulating discrimination of enantiomers. The works described in
this chapter illustrate that in the design of new enantioselective recep-
tors, important are not only individual anion binding motifs but also
their arrangement in the space.
Another important conclusion is that the field of chiral recognition by
sugar decorated anion receptors is a highly unexplored area of supra-
molecular chemistry. Anion receptors presented in this review contain
mainly O-protected sugars and some examples of free carbohydrates.
Finally, the synthesis of the vast majority of anion receptors described
here is relatively straightforward so we decided not include it in this
Chapter. It is our hope that the reader will appreciate the opportunities
sugars offer in context of chiral recognition.
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