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OAc
N
N
N
N
N
N
O
O
AcO
AcO
AcO
OAc
O
AcO
AcO
OAc
OAc
S
S
S
13
14
OAc
OAc
N
N
N
N
O
AcO
AcO
OAc
O
AcO
OAc
OAc
S
S
15
Fig. 8 Structures of sugar-thiourea anion receptors 13-15.
OH
OH
HO
Me
2
PO
4
Na
PhPO
4
Na
2
O
OH
K
ass1:1
K
ass1:1
/K
ass1:2
NH
X
nd
3.8
13
3.1/0
40/8.8
60/20
OMe
OH
16
X = O
17
NH
X = S
18
X = NBn·HCl
O
H
N
H
N
O
H
HO
OH
OH
OH
X
Fig. 9 Structures of anion receptors 16-18 and binding constants for phosphate anions
measured at 298 K in D
2
O.
with thiourea derivative 14 (18140 M
1
), which reflects an extra energy
needed to disrupt the helical structure of 15. The free energy associated
to the helical folding was estimated at 60 J
mol
1
.
Upon benzoate binding intramolecular hydrogen bonds are disrupted
and this binding event leads to helix uncoiling. This behavior was in
contrast to most of the foldamers described, where anion binding in-
duces helicity
22
or causes helicity inversion.
23
The same group designed alternating monosaccharide-thiourea anion
receptors 16-18 that can bind phosphates in water (Fig. 9).
24
A water
soluble anion receptors bearing urea, thiourea and guanidinium as
binding motifs were synthesized. Receptor 17 bound dimethyl phosphate
(K
a
=
3.8 M
1
) as well as phenyl phosphate (40 M
1
) as determined by
1
H NMR titration. Interestingly, comparison of the binding constants for
17 and 18 showed only a small difference in anity for both phosphates.
As in the case of electrically charged guanidinium derivative, the inter-
action with phosphate is supposed to be further reinforced by Coulombic
attraction. The explanation for this is that guanidinium cation is much
more eciently solvated in water than thiourea receptor, so interaction
of the former receptor with phosphate anions is greatly diminished. The
less acidic and highly solvated glycosylurea derivative 16 formed com-
plexes with phosphates with weak anities. It is worth to notice, that
binding of phosphate can be additionally stabilized by hydrogen bonds
with sugar hydroxy groups, which were deprotected in 16-18.
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