Artificial receptors inspired by crystal structures of complexes formed between acyclic receptors and monosaccharides: Design, syntheses, and binding properties

Article abstract of DOI:10.1021/jo400933q

The binding motifs found in the crystal structures of complexes formed between artificial receptors and monosaccharides, reported previously by our group, have inspired us to design new macrocyclic and acyclic receptors, which were expected to form strong

Full text of DOI:10.1021/jo400933q

Article
pubs.acs.org/joc
Artificial Receptors Inspired by Crystal Structures of Complexes
Formed between Acyclic Receptors and Monosaccharides: Design,
Syntheses, and Binding Properties
Jan Lippe and Monika Mazik*
Institut fur Organische Chemie, Technische Universitat Bergakademie Freiberg, Leipziger Strasse 29, 09596 Freiberg, Germany
̈
̈
S
* Supporting Information
ABSTRACT: The binding motifs found in the crystal
structures of complexes formed between artificial receptors
and monosaccharides, reported previously by our group, have
inspired us to design new macrocyclic and acyclic receptors,
which were expected to form strong 1:1 complexes with
monosaccharides, in particular with β-glucosides, through
participation in the formation of CH-π interactions and
hydrogen bonds with the sugar substrate. As first representa-
tives of these compounds we have prepared the macrocycles
8−12 and the acyclic molecules 13−16, incorporating two
central triethylbenzene units. The new compounds had been
designed to bind monosaccharides via interactions of both
central benzene rings with the sugar CH groups. Initial binding studies have confirmed the expected favorable binding capabilities
of the macrocyclic compounds and indicated interesting binding properties of the acyclic analogues.
sugar binding proteins often one or two aromatic residues stack
on the sugar ring.1 The most common hydrogen bonding
scheme involving sugar OHs in natural complexes (such as
complex of galactose-binding protein (GBP) with D-glucose) is
(NH)n→OH→OC, where NH is a hydrogen bond donor
group and OC is a carbonyl or carboxylate acceptor.1b
INTRODUCTION
■
A large number of X-ray crystal structures of carbohydrate-
binding proteins bound to various sugar substrates have been
described in the literature;1 however, the crystalline complexes
between artificial receptors and sugars are largely unexplored.2 In
this context, the crystal structures of the complexes formed
between acyclic receptors and monosaccharides, reported
previously by our group,3 provide valuable model systems to
study the basic molecular features of carbohydrate recognition
(see Figure 1). The binding motifs found in the crystal structures
of the complexes between the aminopyridine-based receptor 1
and methyl β-glucopyranoside 3a and also between the
pyrimidine-based receptor 2 and octyl β-glucopyranoside 3b
(see Figure 1b) show remarkable similarity to the motifs
observed in the crystal structures of protein−carbohydrate
complexes.1b All OH groups and the ring oxygen atom of the
bound sugar 3a or 3b are involved in the formation of hydrogen
bonds, including cooperative and bidentate hydrogen bonds;
most of the hydrogen bonds exhibit nearly optimal geometry.
The typical hydrogen bonding scheme involving sugar OH
groups is NH→OH→N, where NH is the amine group and N
the pyridine or pyrimidine nitrogen atom of the receptor 1 or 2.
In addition, the CHs of the sugar molecule participate in the
formation of the CH···π interactions with the central benzene
ring of the receptor molecule. For example, in the case of the 2:1
receptor−sugar complex 2·3b, both sides of the pyranose ring are
involved in CH···π interactions; the 2-CH of 3b interacts with
the benzene ring of one receptor molecule, whereas the 5-CH
interacts with the central benzene ring of the other receptor, as
shown in Figure 1b. It should be noted that in the complexes of
The binding motifs found in the crystal structures4 of the
complexes 1·3a and 2·3b,3 in particular the participation of the
central benzene ring in CH···π interactions with the sugar CH-
groups, have inspired us to design new macrocyclic and acyclic
carbohydrate receptors of types I and II (see Figure 2a). As first
representatives of the two groups we have prepared the
macrocyclic compounds 8−12 and the acyclic molecules 13−
16 (see Figure 2b), consisting of two central triethylbenzene
units. The macrocyclic compounds of type I were expected to
have particularly favorable binding capabilities toward carbohy-
drates and to form 1:1 complexes with monosaccharides,
especially with β-glucosides, through participation in the
formation of hydrogen bonds and CH-π interactions.4 Both
triethylbenzene units of the prepared compounds were
anticipated to participate in CH-π interactions with the sugar
CH groups. As a result of the formation of 1:1 complexes, instead
of 2:1 receptor−sugar complexes as in the case of receptor 2
(both in the solid state and in solution3), the new compounds
were expected to be more effective carbohydrate receptors than
the previously studied molecules. Examples of binding
interactions, indicated by molecular modeling for complexes
Received: May 14, 2013
Published: September 3, 2013
© 2013 American Chemical Society
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Figure 1. (a) Structures of the previously described pyridine- and pyrimidine-based receptors 1 and 2 as well as sugars used for the binding studies. (b)
Schematic representation of the binding motifs observed in the crystal structure of the 2:1 complex between pyrimidine-based receptor 2 and octyl β-D-
glucopyranoside (3b).3
Figure 2. (a) Structures of the receptors of types I and II. (b) Structures of the prepared macrocyclic compounds 8−12 (receptors of type I) and acyclic
derivatives 13−16 (type II).
with β-glucosides, are shown in Figure 3a and b for the complex
9·3a. First binding studies with selected monosaccharides (see
below, Tables 1 and 2) have confirmed the expected favorable
binding capabilities of the macrocyclic compounds and indicated
interesting binding properties of the acyclic analogues.
interactions in the stabilization of receptor−sugar complexes
not only in the crystalline state3 but also in solutions.6
Particularly interesting results showing the importance of CH-
π interactions in the molecular recognition of carbohydrates by
artificial receptors have been reported in excellent works of Davis
et al.7
It should be noted that a number of our previous studies with
artificial receptors showed the important role of CH-π
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a,b
Table 2. Examples of Association Constants for Receptors
8/9 and Sugars 3b, 4b, and 5b
d
receptor−sugar
K12
β12 = K11K12
[M−2
c
complex
solvent
CDCl3
K11 [M−1
]
[M−1
]
]
e
8·3b
>100000
16900
e
5% DMSO-d6/
260
4.39 × 106
CDCl3
8·4b
8·5b
9·3b
CDCl3
CDCl3
CDCl3
11000
12000
210
930
e
2.31 × 106
1.11 × 107
e
e
>100000
>100000
5% DMSO-d6/
CDCl3
e
10% DMSO-d6/
CDCl3
22540
180
4.06 × 106
9·4b
9·5b
13·3b
14·3b
CDCl3
CDCl3
CDCl3
CDCl3
13000
16500
280
1100
1160
e
3.64 × 106
1.81 × 107
64400
e
>100000
a
b
Average Ka values from multiple titrations. Errors in Ka are less than
c
10%. CDCl3 was stored over activated molecular sieves and
d
deacidified with Al2O3. K12 corresponds to 1:2 receptor−sugar
e
association constant. Calculation program indicated “mixed” 1:1 and
1:2 receptor−sugar binding model with K11 > 100000 M−1; the
binding constants were too large to be accurately determined by the
NMR method.
Figure 3. Energy-minimized structure of the 1:1 complex formed
between receptor 9 and α-glucoside 3a (a) and between 9 and α-
mannoside 7 (c) [MacroModel V.8.5, OPLS 2001 force field, MCMM,
50000 steps. Color code: receptor N, blue; receptor C, gray; the sugar
molecule is highlighted in orange]. (b, d) Examples of binding motifs
indicated by molecular modeling for the 1:1 complexes 9·3a and 9·7.
of 21a previously reported by our group, compound 21b could
be obtained in better yield of 60% by improving the workup. In
contrast to 21a/b compounds 29a/b could be easily synthesized
in a one-step reaction by stirring 28a/b in a 7 N solution of
ammonia in methanol. The resulting raw products were purified
by column chromatography with a methanol/chloroform
mixture as a mobile phase to yield 29a in 78% and 29b in 87%.
Condensation of the corresponding carbaldehyde, such as
isophtalaldehyde (22a), pyridine-2,6-dicarbaldehyde (22b),
and 2-hydroxyisophtalaldehyde (22c) in dry ethanol, provided
the insoluble imines (to increase the yield molecular sieves and a
catalytic amount of acetic acid were used). In the case of the
cyclic receptors 8−12 precipitation of the imines 23−27 could
be observed almost immediately. For the acyclic receptors 13−
16 the reaction time had to be doubled to obtain the desired
imines 30−33. The imines were filtered off and reduced without
further purification with sodium borohydride.
The binding properties of compounds 8−12 were first
evaluated in two-phase systems through extractions of methyl
pyranosides from the solid state into a 1 mM CDCl3 solution of
the corresponding macrocyclic compound. Monosaccharides
such as β-glucoside 3a, α-glucoside 4a, β-galactoside 5a, α-
galactoside 6 and α-mannoside 7 were selected as substrates for
these experiments. The liquid−solid extractions indicated the
expected favorable interactions between the binding partners and
provided evidence for stronger complexation of the β-anomers
RESULTS AND DISCUSSION
■
The syntheses of the macrocycles 8−12 and the acyclic
compounds 13−16 are summarized in Schemes 1 and 2,
respectively. The basis for the syntheses of compounds 8−16 was
1,3,5-tris(bromomethyl)-2,4,6-triethylbenzene (17), which
could be easily obtained8 from commercially available 1,3,5-
triethylbenzene. The reaction of 17 with 2-amino-4,6-dimethyl-
pyridine (18a) gave the mono- and disubstituted products 19a
and 28a, respectively. In the case of the syntheses of 19b and 28b,
the lower reactivity of 2-amino-4,6-dimethylpyrimidine (18b)
could be compensated by raising the reaction temperature from
room temperature to 55 °C; by doing so the yields of 19b and
28b were doubled (a further increase of temperature showed no
additional formation of the desired products). Exchange of the
solvent from CH3CN/THF to DMF and the base from K2CO3 to
NaOH led almost exclusively to the entirely substituted 1,3,5-
tris[(4,6-dimethyl-pyrimidin-2-yl)-aminomethyl]-2,4,6-triethyl-
benzene.
The conversion of 19a to the corresponding bis-amine 21a
was carried out via Gabriel synthesis. Compared to the synthesis
a
Table 1. Solubilization of Sugars in CDCl3 by Receptors 8−12 (1 mM Solutions)
receptor
β-glucoside 3a
β-galactoside 5a
α-glucoside 4a
α-galactoside 6
α-mannoside 7
8
1.12
1.40
1.23
1.28
0.43
1.10
1.25
1.15
1.05
0.48
0.46
0.41
0.44
0.36
0.19
0.50
0.82
0.44
0.62
0.25
0.14
0.23
0.18
0.15
0.14
9
10
11
12
a
1
Molar ratios sugar/receptor occurring in solution. The H NMR signals of the corresponding sugar were integrated with respect to the receptor’s
signals to provide the sugar-receptor ratio; control experiments were performed in the absence of the receptor.
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a
Scheme 1. Synthesis of Compounds 8−12
a
Reagents and conditions: (a) CH3CN/THF, K2CO3; (b) potassium phthalimide, DMSO; (c) N2H4, EtOH/toluene; (d) EtOH, AcOH (catalytic
amount); (e) NaBH4, MeOH; (f) H2O.
a
Scheme 2. Synthesis of Compounds 13−16
a
Reagents and conditions: (a) CH3CN/THF, K2CO3; (b) NH3/MeOH (c) EtOH, AcOH (catalytic amount); (d) NaBH4, MeOH; (f) H2O.
3a and 5a (see Table 1). The preference of 8 and 9 for β- versus
α-glucoside indicated by liquid−solid extractions (see also Figure
bonds with the hydroxy groups of 7 (for comparison, see Figure
3b and d).
1
4) was further confirmed by H NMR spectroscopic titrations
It should be also noted that in the case of compounds 8−11
more than the stoichiometric amount of β-glucoside 3a and/or
β-galactoside 5a was extracted from the solid, suggesting the
occurrence of complexes of stoichiometry higher than 1:1.
The properties of macrocycles 8 and 9 were also analyzed
through extraction of methyl glycosides from aqueous solution
into nonpolar solvent (liquid−liquid extractions), using the
procedure described by Davis et al.9 Studies of the extraction of
β-glucoside 3a, β-galactoside 5a, α-glucoside 4a, and α-
galactoside 6 from aqueous solution into chloroform revealed
(see below), which showed a particularly high affinity of 8 and 9
for β-glucoside. Among the tested monosaccharides, α-manno-
side 7 was the least extracted substrate (see Table 1). Weaker
binding of 7 in comparison to 3 was also indicated by molecular
modeling calculations, as shown in Figure 3c and d for the
complex 9·7. According to the calculations, the binding mode of
α-mannoside 7 is quite different from that of β-glucoside 3a. For
example, in contrast to the binding of 3a by 9, the pyrimidine
nitrogens of 9 do not participate in the formation of hydrogen
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complexes (K11 > 100000 M−1; see Table 1). After the addition of
5% DMSO-d6 the binding constants for 9·3b were still too strong
to be determined by the NMR method (K11 > 100000 M−1; see
Table 2), whereas those for 8·3b were determined to be 16900
(K11) and 260 M−1 (K12), indicating weaker interactions of β-
glucoside 3b with 8 compared to those with the receptor 9.
Studies performed with β-glucoside 3b and compound 9 in 10%
DMSO-d6 in CDCl3 revealed K11 = 22540 M−1 and K12 = 180
M−1. The observed complexation-induced shifts of the receptor
or sugar signals, depending on the titration conditions, revealed
that both hydrogen bonds and CH-π interactions contribute to
the stabilization of the receptor-sugar complex (for examples of
spectral changes observed during the titrations, see Figures S1−
S8 and Table S1 in Supporting Information).
The spectral changes observed during the titrations of 9 with
3b in methanol/chloroform mixture (5% CD3OD in CDCl3; see
Table 7 in Supporting Information) were less substantial than
those observed during the titrations in dimethyl sulfoxide-
containing chloroform solutions. The curve fitting of the titration
data obtained in the presence of 5% CD3OD indicated the
formation of weak complexes with 1:1 receptor−sugar
stoichiometry (K11 ≈ 300 M−1). As expected, the interactions
between 9 and 3b in a more polar solvent such as CD3OD/
CDCl3 mixture are weaker than those observed in CDCl3 and in
DMSO-d6/CDCl3 mixtures, but it should be noted that the
decrease of the binding constant is particularly drastic.
Detailed analyses of the different interactions contributing to
complex stability for compounds 8/9 and other prepared
compounds are the subject of current work. Binding constants
obtained for β-glucoside 3b and the compounds 8/9 in CDCl3
were significantly higher than those determined for α-glucoside
4b and β-galactoside 5b (see Table 1). Thus, as in the case of the
receptor systems reported by Davis et al.,7 the compounds 8 and
9 show preference for β-glucoside, i.e., for a substrate with an all-
equatorial substitution pattern. As in the case of the macrocycles
8 and 9, the acyclic aminopyrimidine-based compound 14 was
shown to be a more effective receptor for β-glucoside 3b (K11 >
100000 M−1; see Table 2) than the aminopyridine-based
analogue 13. The binding studies revealed that 1:1 complexes
predominate in the solution; however, the presence of weaker 1:2
Figure 4. Solubilization of sugars 3a, 4a, 5a, 6, and 7 in CDCl3 by
macrocyclic compounds 8 and 9 (1 mM CDCl3 solutions).
that compound 9 (1 mM chloroform solution) is able to extract
about 0.50 equiv of β-glucoside 3a, 0.4 equiv of β-galactoside 5a,
and 0.09 equiv α-galactoside 6 from 1 M aqueous solutions,
whereas 8 is able to extract about 0.40 equiv of 3a, 0.36 equiv of
5a, 0.15 equiv of 4a, and 0.06 equiv of 6 (control experiments
were performed in the absence of the corresponding receptor).
As mentioned above, the binding properties of 8 and 9 toward
1
selected monosaccharides were further studied by H NMR
spectroscopic titrations. The interactions of the both compounds
with carbohydrates were investigated in CDCl3 and DMSO-d6/
CDCl3 mixtures (5:95 and 10:90 v/v) by adding increasing
amounts of the carbohydrate to a solution of 8 or 9 as well as by
inverse titrations, in which the concentration of sugar was held
constant and that of the corresponding receptor was varied.
Octyl glycosides such as β-glucoside 3b, α-glucoside 4b, and β-
galactoside 5b were selected as substrates for the initial titration
1
experiments. The H NMR titration data (for examples, see
Figure 5 and Figures S1−S5 in Supporting Information) were
analyzed using the EQNMR program;10 the binding constants
are summarized in Table 1.
In the case of β-glucoside 3b the interactions with both
receptors 8 and 9 in CDCl3 were too strong to be accurately
analyzed by the NMR method; the analysis of the titration data
indicated the formation of very strong 1:1 receptor−sugar
Figure 5. (a, b) Mole ratio plots: (a) titration of β-glucoside 3b with receptor 9 (analysis of the complexation-induced shift of the CH signal of 3b;
inverse titration) and (b) titration of receptor 9 with β-galactoside 5b in CDCl3 (analysis of the complexation-induced shift of the pyrimidine CH signal
of 9). (c) Partial 1H NMR spectra (400 MHz) of receptor 9 ([9] = 1.01 mM) after addition of 0.00−4.52 equiv of 5b in CDCl3.
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receptor−sugar complexes were also detected in the CDCl3 or
CDCl3/DMSO-d6 mixture.
EXPERIMENTAL SECTION
■
Analytical TLC was carried out on silica gel 60 F254 plates; column
chromatography was carried out on silica gel. Melting points are
uncorrected. The mass analyzer used for the HRMS measurements was
Finnigan MAT 95 XLT (Orbitrap). The syntheses of compounds 19a,
20a, and 21a are described in ref 5m, whereas the syntheses of 28a and
29a are given in ref 5l. Compounds 22a and 22b are commercial
available and the synthesis of 22c is described in ref 12. For examples of
other triethylbenzene-based macrocyclic receptors, see ref 13.
General Procedure for the Synthesis of Macrocyclic Com-
pounds 8−12. To a solution of 21a or 21b (0.65 mmol) in dry EtOH/
MeOH (50:1 v/v) (10 mL) were added the corresponding aldehyde
(22a, 22b, or 22c, 0.65 mmol) and one drop of acetic acid, and the
mixture was heated to 70 °C for 12 h. After the mixture was cooled to
room temperature, the precipitate was filtered and washed with small
amounts of EtOH. The precipitate was solved in dry MeOH (10 mL),
NaBH4 (7.28 mmol) was slowly added, and the mixture stirred at room
temperature for 3 h. Afterward the solvent was evaporated, the residue
was suspended in a mixture of H2O/CHCl3 (3:1 v/v), and the resulting
mixture was allowed to stir for 3 h. The suspension was extracted with
CHCl3; all combined organic layers (100 mL) were washed with H2O
(50 mL), dried over MgSO4, and evaporated; and the residue was
purified by column chromatography (CHCl3/MeOH, 7:1).
CONCLUSION
■
We have presented here the successful synthesis of nine
representatives of new carbohydrate receptors of types I and II
shown in Figure 2 (compounds 8−16). The design of these
macrocyclic and acyclic compounds was inspired by the binding
motifs, especially CH-π interactions, observed in the crystal
structures of complexes formed between artificial receptors and
monosaccharides (complexes 1·3a and 2·3b), reported pre-
viously by our group.3 In contrast to the 2:1 receptor−sugar
binding, which was observed in the case of receptor 2 in the solid
state and in solution, the new compounds were expected to form
strong 1:1 complexes with monosaccharides, especially with β-
1
glucosides. Preliminary binding studies, including H NMR
spectroscopic titrations and binding studies in two-phase
systems, have confirmed the expected favorable binding
capabilities of the macrocyclic compounds and indicated
promising binding properties of the acyclic analogues.
1
1
In the case of the macrocyclic compounds 8 and 9, the H
Compound 8. Yield: 55% (160 mg). Mp: 138−139 °C; H NMR
(400 MHz, CDCl3): δ 1.13 (t, J = 7.4 Hz, 6H), 1.14 (t, J = 7.3 Hz, 12H),
2.21 (s, 6H), 2.34 (s, 6H), 2.71 (q, J = 7.3 Hz, 8H), 2.78 (q, J = 7.3 Hz,
8H), 3.74 (s, 8H), 3.91 (s, 8H), 4.19 (s, 2H), 4.35 (d, J = 4.0 Hz, 4H),
6.08 (s, 2H), 6.32 (s, 2H), 7.17 (t, J = 1.5 Hz, 2H), 7.19 (d, J = 1.3 Hz,
4H), 7.56 (s, 2H). 13C NMR (100 MHz, CDCl3): δ 16.7, 16.9, 22.9,
24.0, 29.7, 40.6, 47.4, 54.9, 103.7, 113.6, 127.2, 128.1, 132.4, 134.4,
142.5, 143.1, 146.1, 158.0. HR-MS (ESI): calcd for C60H81N8 913.65787
[M + H]+; found 913.65780. Rf = 0.10 (CHCl3/MeOH, 7:1 v/v).
NMR titrations revealed effective recognition of neutral
carbohydrates, β- versus α-anomer binding preferences in the
recognition of glycosides, high binding preference for β-
glucoside, i.e., for a substrate with an all-equatorial substitution
pattern (K11 > 100000 M−1 in CDCl3), and considerably
increased binding affinity toward the tested carbohydrates in
comparison with the previously described acyclic aminopyridine-
based receptor with triethylbenzene-derived core.11 Complex-
ation-induced upfield shifts of the sugar CH resonances,
observed upon addition of 8 or 9 to the β-glucoside 3b
(“inverse” titrations), clearly indicated the interactions of the CH
groups of 3b with the aromatic residues of the corresponding
receptor. Liquid−liquid extractions demonstrated the ability of 8
and 9 to extract monosaccharides from water into nonpolar
solvent.
1
Compound 9. Yield: 67% (200 mg). Mp: 142−143 °C. H NMR
(600 MHz, CDCl3): δ 1.12 (t, J = 7.4 Hz, 6H), 1.24 (t, J = 7.4 Hz, 3H),
2.29 (s, 12H), 2.69 (q, J = 7.2 Hz, 4H), 2.79 (q, J = 7.4 Hz, 2H), 3.74 (s,
8H), 3.90 (s, 8H), 4.55 (d, J = 4.3 Hz, 4H), 4.73 (t, J = 4.3 Hz, 2H), 6.32
(s, 2H), 7.22 (m, 6H), 7.55 (s, 2H). 13C NMR (100 MHz, CDCl3): δ
167.4, 161.8, 142.9, 142.5, 140.5, 134.4, 132.3, 128.0, 127.0, 127.0, 109.6,
54.9, 47.4, 39.9, 23.9, 22.8, 22.4, 16.8, 16.8. HR-MS (ESI): calcd for
C58H79N10 915.64836 [M + H]+; found 915.64840. Rf = 0.67 (CHCl3/
MeOH + 1% NH3, 10:1 v/v).
Compound 10. Yield: 30% (85 mg). Mp: 180 °C (decomposition).
1H NMR (500 MHz, CDCl3): δ 1.14 (t, J = 7.3 Hz, 6H), 1.23 (t, J = 7.46
Hz, 12H), 2.19 (s, 6H), 2.35 (s, 6H), 2.83 (q, J = 7.3 Hz, 8H), 2.93 (q, J =
7.3 Hz, 4H), 3.74 (s, 8H),), 4.02 (s, 8H), 4.37 (d, J = 3.4 Hz, 4H), 5.99
(s, 2H), 6.29 (s, 2H), 7.14 (d, J = 7.65 Hz, 4H), 7.60 (t, J = 7.6 Hz, 2H).
13C NMR (100 MHz, CDCl3): δ 15.6, 16.3, 20.0, 21.6, 23.1, 39.5, 46.0,
54.7, 102.8, 112.4, 120.0, 131.8, 133.3, 135.7, 141.0, 141.8, 157.2, 158.0.
HR-MS (ESI): calcd for C58H79N10 915.64836 [M + H]+; found
915.64839. Rf = 0.41 [CHCl3/MeOH (incl 1% NH3), 7:1 v/v].
Compound 11. Yield: 52% (200 mg). Mp: 156−157 °C. 1H NMR
(400 MHz, CDCl3): δ 1.21 (m, 18H), 2.28 (s, 12H), 2.84 (q, J = 7.5 Hz,
8H), 2.90 (q, J = 7.4 Hz, 4H), 3.74 (s, 8H), 4.01 (s, 8H), 4.55 (d, J = 4.05
Hz, 4H), 4.78 (s, 2H), 6.29 (s, 2H), 7.14 (d, J = 7.6 Hz, 4H), 7.59 (t, J =
7.6 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 16.5, 22.7, 23.9, 39.9, 47.0,
55.7, 109.5, 121.0, 132.7, 134.3, 136.7, 142.0, 142.9, 159.0, 161.8, 167.3.
HR-MS (ESI): calcd for C56H77N12 917.63886 [M + H]+; found
917.63910. Rf = 0.10 (CHCl3/MeOH, 7:1 v/v).
Macrocycle 9 and the acyclic compound 14, containing
aminopyrimidine groups, were shown to be more effective
carbohydrate receptors than the aminopyridine-based analogues
8 and 13, respectively.
The binding properties of compounds 8−16 will be analyzed
in more detail by 1H NMR and fluorescence spectroscopy as well
as isothermal titration calorimetry (ITC) in competitive and
noncompetitive organic media; it is hoped that suitable
derivatives will allow also complexation in aqueous media. X-
ray crystallographic investigations are also carried out in our
laboratory. Efforts to examine the three-dimensional structures
of the receptor−sugar complexes and toward the development of
a more complete structural understanding of the factors
influencing the complex stability are currently underway.
The binding efficiency of the macrocyclic and acyclic receptors
can be further influenced by introducing of other groups, such as
imidazole, indole, pyrrole, pyridinium, quinolinium, and
imidazolium units as well as other groups, which are shown in
Figure 2. The properties of the triethylbenzene-based receptors
will be compared with those of the trimethylbenzene- and
trimethoxybenzene-based systems. The syntheses of these
compounds, including water-soluble analogues, which are
expected to perform effective carbohydrate recognition in
aqueous solutions, are the subject of current work.
Compound 12. Yield: 43% (130 mg). Mp: 151−152 °C; 1H NMR
(500 MHz, CDCl3): δ 1.12 (t, J = 7.2 Hz, 6H), 1.22 (t, J = 7.4 Hz, 12H),
2.17 (s, 6H), 2.34 (s, 6H), 2.63 (q, J = 7.3 Hz, 8H), 2.74 (q, J = 7.3 Hz,
4H), 3.75 (s, 8H), 3.97 (s, 8H), 4.06 (s, 2H), 4.35 (d, J = 3.7 Hz, 4H),
5.97 (s, 2H), 6.29 (s, 2H), 6.76 (t, J = 7.5 Hz, 2H), 7.06 (d, J = 7.5 Hz,
4H). 13C NMR (100 MHz, CDCl3): δ 16.8, 21.0, 22.5, 22.8, 24.2, 40.0,
47.3, 52.2, 104.0, 113.6, 118.7, 124.5, 128.8, 133.0, 133.6, 142.4, 142.9,
148.4, 156.6, 158.2. HR-MS (ESI): calcd for C60H81N8O2 945.64770 [M
+ H]+; found 945.64795. Rf = 0.35 [CHCl3/MeOH (incl. 1% NH3) 10:1
v/v].
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General Procedure for the Synthesis of the Acyclic
Compounds 13−16. In a 25 mL flask were dissolved the
corresponding amine (compound 29a or 29b, 2 equiv) and the
aldehyde (compound 22a or 22b, 1 equiv) in dry EtOH. One drop of
acetic acid and molecular sieves (3 Å) were added, and the mixture was
stirred at 70 °C for 24 h. Afterward the mixture was cooled to room
temperature, the resulting precipitate was filtered off and washed with
small amounts of EtOH. The solid imine was dissolved in dry MeOH/
CHCl3 (20:1 v/v), NaBH4 (12 equiv) was slowly added, and the mixture
was stirred for 3 h at room temperature. After addition of H2O, the
resulting suspension was extracted with CHCl3, all combined organic
layers (50 mL) were washed with H2O (50 mL) and dried over MgSO4,
and the solvent was removed under reduced pressure. Pure product was
obtained as light yellowish or white solid.
143.7, 144.9, 161.6, 167.5. HR-MS (ESI): calcd for C27H38N6Br
527.23192 [M + H]+; found 527.23169. Rf = 0.38 (EtOAc/toluene, 1:3
v/v).
1,3-Bis(phthalimidomethyl)-5-[(4,6-dimethylpyrimidin-2-yl)-
aminomethyl]-2,4,6-triethyl-benzene (20b). A mixture of com-
pound 19b (2.00 g, 4.15 mmol) and potassium phthalimide (2.30 g,
12.44 mmol) in dry DMSO (50 mL) was heated to 95 °C for 8 h. After
the mixture was cooled to room temperature, H2O (150 mL) was added,
and the formed precipitate was filtered and washed with H2O (200 mL).
Then the precipitate was suspended in H2O (100 mL), and the
suspension was extracted with CH2Cl2 (3 × 50 mL). The combined
organic layers were washed with H2O (50 mL), dried over MgSO4, and
evaporated, and the residue was purified by column chromatography
(toluene/EtOAc, 3:1 v/v). Yield: 63% (1.60 g). Mp: 120−121 °C. 1H
NMR (400 MHz, CDCl3): δ 1.00 (t, J = 7.5 Hz, 3H), 1.09 (t, J = 7.5 Hz,
6H), 2.27 (s, 6H), 2.87 (q, J = 7.5 Hz, 4H), 3.18 (q, J = 7.5 Hz, 2H), 4.58
(d, J = 4.0 Hz, 2H), 4.80 (t, J = 4.0 Hz, 1H), 4.95 (s, 4H), 6.30 (s, 1H),
7.69 (m, 4H), 7.80 (m, 4H). 13C NMR (100 MHz, CDCl3): δ 168.2,
167.4, 161.8, 145.4, 144.6, 133.9, 132.8, 132.0, 129.5, 123.2, 109.6, 39.8,
37.4, 23.9, 23.3, 23.4, 16.1, 15.7. MS (EI; 70 eV): m/z (%): 615 (15)
[M]+, 586 (100), 455 (20), 332 (39), 160 (62). Anal. Calcd for
C37H37N5O4: C, 72.17; H, 6.06; N, 11.37. Found: C, 72.10; H, 6.08; N,
11.41. Rf = 0.23 (toluene/EtOAc, 3:1 v/v).
1
Compound 13. Yield: 56% (40 mg). Mp: 100−101 °C. H NMR
(400 MHz, CDCl3): δ 1.18 (m, 18H), 2.22 (s, 12H), 2.34 (s, 12H), 3.70
(s, 4H), 3.92 (s, 4H), 4.14 (s, 4H), 4.33 (d, J = 4.0 Hz, 8H), 6.06 (s,
12H), 6.33 (s, 12H), 7.29 (m, 3H), 7.40 (s, 1H). 13C NMR (100 MHz,
CDCl3): δ 16.8, 21.0, 22.7, 22.8, 24.2, 40.5, 46.9, 54.7, 103.4, 113.7,
126.8, 128.1, 128.3, 132.7, 134.7, 140.3, 142.8, 143.1, 148.6, 156.6, 158.2.
HR-MS (ESI): calcd for C66H89N10 1021.72662 [M + H]+; found
1021.72678.
1
Compound 14. Yield: 17% (53 mg). Mp: 111−112 °C. H NMR
1,3-Bis(aminomethyl)-5-[(4,6-dimethylpyrimidin-2-yl)-ami-
nomethyl]-2,4,6-triethyl-benzene (21b). Compound 20b (1.45 g,
2.36 mmol) was dissolved in a mixture of dry EtOH/toluene (2:1 (v/v))
and refluxed with hydrazine hydrate (0.31 mL, 10.00 mmol) for 20 h.
Afterward the solvent was evaporated, the precipitate was suspended in a
solution of 40% aq KOH (100 mL), and the suspension was extracted
with CHCl3 (100 mL). The extraction was repeated three times, and the
combined organic extracts were washed with H2O, dried over MgSO4
and filtered. Yield: 60% (503 mg). Mp: 59−60 °C. 1H NMR (400 MHz,
CDCl3): δ 1.25 (m, 9H), 2.30 (s, 6H), 2.77 (q, J = 7.5 Hz, 4H), 2.84 (q, J
= 7.4 Hz, 2H), 3.88 (s, 4H), 4.56 (d, J = 4.4 Hz, 2H), 4.76 (s, 1H), 6.34
(s, 1H). 13C NMR (100 MHz, CDCl3): δ 167.5, 161.8, 141.5, 141.1,
137.5, 132.9, 109.8, 39.9, 39.7, 23.9, 22.7, 22.6, 16.8. MS (EI; 70 eV): m/
z (%): 355 (15) [M]+, 338 (75), 309 (100), 295 (36), 232 (18), 187
(19), 124 (92). Anal. Calcd for C21H33N5 C, 70.95; H, 9.36; N, 19.70.
Found: C, 71.00; H, 9.37; N, 19.63. Rf = 0.11 (CHCl3/MeOH, 5:1 v/v).
1-Aminomethyl-3,5-bis[(4,6-dimethylpyrimidin-2-yl)-
aminomethyl]-2,4,6-triethyl-benzene (29b). In a 50-mL three-
necked flask with a dropping funnel, a solution of 1-(brommethyl)-3,5-
bis-[(4,6-dimethylpyrimidin-2-yl)aminomehtyl]-2,4,6-triethylbenzene
(28b) (211 mg, 0.46 mmol) in methanol was slowly dropped into a
solution of 2 N ammonia in methanol. The mixture was stirred at room
temperature for 48 h, the solvent was removed under reduced pressure,
and the residue was purified by column chromatography (CHCl3/
(400 MHz, CDCl3): δ 1.18 (m, 18H), 2.29 (s, 24H), 2.73 (m, 12H),
3.70 (s, 4H), 3.92 (s, 4H), 4.54 (d, J = 4.3 Hz, 8H), 4.73 (t, J = 4.2 Hz,
4H), 6.32 (s, 4H), 7.30 (m, 3H), 7.38 (s, 1H). 13C NMR (100 MHz,
CDCl3): δ 16.6, 16.7, 22.7, 22.8, 23.9, 39.9, 47.0, 54.7, 109.7, 126.7,
128.1, 128.3, 132.7, 134.7, 140.3, 142.9, 143.2, 161.8, 167.4; HR-MS
(ESI): calcd for C62H85N14 1025.70761 [M + H]+; found 1025.70690.
1
Compound 15. Yield: 36% (83 mg). Mp: 118−19 °C. H NMR
(400 MHz, CDCl3): δ 1.17 (t, J = 7.4 Hz, 12H), 1.20 (t, J = 7.4 Hz, 6H),
2.22 (s, 12H), 2.34 (s, 12H), 2.70 (q, J = 7.4 Hz, 4H), 2.78 (q, J = 7.4 Hz,
8H), 3.74 (s, 4H), 4.04 (s, 4H), 4.34 (d, J = 4.1 Hz, 4H), 6.06 (s, 4H),
6.32 (s, 4H), 7.29 (d, J = 7.7 Hz, 1H), 7.64 (t, J = 7.7 Hz, 2H). 13C NMR
(100 MHz, CDCl3): δ 16.8, 21.0, 22.7, 22.8, 24.2, 40.6, 47.2, 56.0, 103.4,
113.7, 120.5, 132.8, 134.7, 136.8, 142.9, 143.2, 148.6, 156.6, 158.2, 159.5.
HR-MS (ESI): calcd for C65H88N11 1022.72187 [M + H]+; found
1022.72230.
1
Compound 16. Yield: 27% (40 mg). Mp: 110−111 °C. H NMR
(600 MHz, CDCl3): δ 1.17 (t, J = 7.5 Hz, 12H), 1.20 (t, J = 7.5 Hz, 6H),
2.29 (s, 24H), 2.72 (q, J = 7.5 Hz, 4H), 2.78 (q, J = 7.5 Hz, 8H), 3,74 (s,
4H), 4.05 (s, 4H), 4.55 (d, J = 4.3 Hz, 8H), 4.75 (t, J = 4.3 Hz, 4H), 6.33
(s, 4H), 7.30 (d, J = 7.7 Hz, 2H), 7.65 (t, J = 7.6 Hz, 1H). 13C NMR (150
MHz, CDCl3): δ 16.6, 16.7, 22.7, 22.9, 23.9, 39.9, 47.1, 56.0, 109.7,
120.4, 132.7, 134.6, 136.8, 142.9, 143.2, 159.5, 161.8. HR-MS (ESI):
calcd for C61H84N15 1026.70286 [M + H]+; found 1026.70360.
1,3-Bis(bromomethyl)-5[(4,6-dimethylpyrimidin-2-yl)-
aminomehtyl]-2,4,6-triethyl-benzene (19b) and 1-(Bromo-
methyl)-3,5-bis[(4,6-dimethylpyrimidin-2-yl)aminomethyl]-
2,4,6-triethylbenzene (28b). A suspension of 2-amino-4,6-dimethyl-
pyrimidine (5.00 g, 40.68 mmol), 1,3,5-tris(bromomethyl)-2,4,6-
trimethyl-benzene (6.00 g, 13.56 mmol), and K2CO3 (5.76 g, 40.68
mmol) in CH3CN/THF (1:2 v/v; 150 mL) was stirred at 50 °C for 72 h.
After cooling to room temperature, filtration, and evaporation of
solvents, the crude product was purified by column chromatography
(EtOAc/toluene, 1:3 v/v).
1
MeOH, 5:1 v/v). Yield: 87% (184 mg). Mp: 206−207 °C. H NMR
(500 MHz, CDCl3): δ 1.17 (t, J = 7.5 Hz, 6H), 1.22 (t, J = 7.5 Hz, 3H),
2.27 (s, 12H), 2.71 (q, J = 7.5 Hz, 2H), 2.77 (q, J = 7.5 Hz, 4H), 4.24 (s,
2H), 4.52 (s, 4H), 5.18 (s, 2H), 6.33 (s, 2H). 13C NMR (100 MHz,
CDCl3): δ 16.4, 16.5, 23.5, 23.7, 37.1, 39.8, 109.9, 127.9, 133.0, 144.1,
145.6, 161.4, 167.6. HR-MS (ESI): calcd for C27H40N7 462.33397 [M +
H]+; found 462.33410. Rf = 0.33 (CHCl3/MeOH, 5:1 v/v).
ASSOCIATED CONTENT
■
Compound 19b. Yield 20% (1.29 g). Mp: 70−71 °C. 1H NMR (400
MHz, CDCl3): δ 1.27 (t, J = 7.6 Hz, 6H), 1.35 (t, J = 7.6, 3H), 2.31 (s,
6H), 2.85 (q, J = 7.6 Hz, 4H), 2.95 (q, J = 7.6 Hz, 2H), 4.57 (d, J = 3.7
Hz, 2H), 4.59 (s, 4H), 4.77 (s, 1H), 6.36 (s, 1H). 13C NMR (100 MHz,
CDCl3): δ 167.5, 161.7, 145.0, 143.9, 133.8, 132.3, 109.9, 39.7, 29.1,
24.0, 23.9, 16.2. MS (EI; 70 eV): m/z (%): 483 (2) [M+], 402 (15), 322
(100), 296 (55), 187 (35), 171 (46), 136 (37), 124 (45), 81 (25). Anal.
Calcd for C21H29Br2N3: C, 52.19; H, 6.05; N, 8.69. Found: C, 52.09; H,
6.07; N, 8.72. Rf = 0.60 (EtOAc/toluene, 1:3 v/v).
S
* Supporting Information
Description of the binding studies; examples of 1H NMR
titrations; representative mole ratio plots; representative
1
EQNMR plots; change in chemical shift observed during H
NMR titrations; copies of the 1H and 13C NMR spectra of 8−16,
19b, 20b, 21b, 28b, and 29b. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
Compound 28b. Yield: 7% (480 mg). Mp: 79−80 °C. H NMR
(400 MHz, CDCl3): δ 1.21 (t, J = 7.5 Hz, 3H), 1.28 (t, J = 7.5 Hz, 6H),
2.30 (s, 12H), 2.74 (q, J = 7.5 Hz, 2H), 2.68 (q, J = 7.5 Hz, 4H), 4.57 (d, J
= 4.3 Hz, 4H), 4.61 (s, 2H), 4.82 (br.s, 2H), 6.35 (s, 2H). 13C NMR (100
MHz, CDCl3): δ 16.2, 16.5, 22.7, 23.9, 29.6, 39.7, 109.8, 131.8, 133.3,
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: monika.mazik@chemie.tu-freiberg.de.
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Article
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the Deutsche Forschungsgemein-
schaft.
■
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