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substituent is present instead of aromatic one, poor enantioselectivity
was observed. The effect of size of the protecting group on chiral recog-
nition ability of receptor 23 was also studied. Binding constants for en-
antiomers of N-formyl-, N-acetyl-, and N-pivaloyl-protected phenylglycine
were measured. Receptor 23 showed the highest degree of enantio-
differentiation for the bulky (Boc, Piv) protecting groups and almost no
differentiation for enantiomers of N-formyl- protected phenylglycine was
observed.
More recently, the anion binding anities of three urea-glucopyranose
derived anion receptors 24-26 were investigated (Fig. 14).
29
The use of
three various aromatic platform enabled construction of anion receptors
with varying geometry of the binding pocket: the smallest for naphtha-
lene derivative 25, medium for benzene 24, and the largest one for an-
thracene derivative 26. Preliminary binding studies using receptors 24-26
were conducted by titration of chiral receptors by model achiral carb-
oxylates - acetate and benzoate, in DMSO-water (99.5 : 0.5 v/v) solution. In
all cases the smaller acetate anion has the higher binding constant.
Interestingly, the relative ratio of association constants K
acetate
/K
benzoate
substantially depends on the size of binding pocket. The anion receptors
26 binds acetate with constant about 10-times higher than benzoate. The
ratio for 24 and 25 was found to be 3 and 5, respectively.
Next, the chiral recognition of enantiomeric anions of mandelic acid
and N-Boc-protected tryptophane by three receptors 24-26 was also in-
vestigated. The receptors 24 and 25 showed no noticeable chiral recog-
nition. On the other hand, association constants measured for 26 were
343 (
D
-Trp-COO
) and 190 M
1
(
L
-Trp-COO
). Additionally, for their
AcO
OAc
OAc
AcO
AcO
OA c
AcO
OA
c
OAc
OAc
O
O
AcO
AcO
O
AcO
O
OAc
AcO
NH
HN
NH
HN
OAc
NH
HN
O
NH
HN
O
O
O
24
25
OAc
AcO
AcO
AcO
OAc
O
O
OAc
AcO
AcO
NH
HN
O
NH
HN
O
26
Fig. 14 Structures of anion receptors 24-26.
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