Pyrrolic tripodal receptors for carbohydrates. Role of functional groups and binding geometry on carbohydrate recognition

Article abstract of DOI:10.1039/c0ob00651c

The contribution from several H-bonding groups and the impact of geometric requirements on the binding ability of benzene-based tripodal receptors toward carbohydrates have been investigated by measuring the affinity of a set of structures toward octyl β-

Full text of DOI:10.1039/c0ob00651c

View Article Online / Journal Homepage / Table of Contents for this issue
Organic &
Biomolecular
Chemistry
Dynamic Article Links
Cite this: Org. Biomol. Chem., 2011, 9, 1085
www.rsc.org/obc
PAPER
Pyrrolic tripodal receptors for carbohydrates. Role of functional groups and
binding geometry on carbohydrate recognition†
Martina Cacciarini,*a Cristina Nativi,a,b Martina Norcini,a Samuele Staderini,a Oscar Francesconia and
Stefano Roelens*c
Received 31st August 2010, Accepted 29th October 2010
DOI: 10.1039/c0ob00651c
The contribution from several H-bonding groups and the impact of geometric requirements on the
binding ability of benzene-based tripodal receptors toward carbohydrates have been investigated by
measuring the affinity of a set of structures toward octyl b-D-glucopyranoside, selected as a
representative monosaccharide. The results reported in the present study demonstrate that a judicious
choice of correct geometry and appropriate functional groups is critical to achieve the complementary
hydrogen bonding interactions required for an effective carbohydrate recognition.
Introduction
Molecular recognition of carbohydrates is essential in several bi-
ological processes, from carbohydrate metabolism and transport,
to cell to cell adhesion, cell infection by pathogens, the immune
response, and enzyme activity regulation.1 Since the principles
governing these recognition events are yet poorly understood,
considerable effort has been directed toward the investigation
of saccharide binding using artificial receptors.2 Over the last
two decades, several synthetic receptors have been designed
and investigated, showing various levels of recognition towards
carbohydrate substrates,3 some of which exhibited outstanding
recognition properties even in water.4 Some examples from this
group are the tripodal receptors 1a–b (Fig. 1), which bind
octyl-b-D-glucopyranoside in chloroform with high affinity and
remarkable selectivity.5 Measurable affinity was also observed
in acetonitrile, a significantly more polar solvent, while a mod-
ified receptor 2 featuring acetalic substituents was found to
possess increased affinity and marked selectivity toward octyl-
b-D-mannopyranoside.6 A common feature of compounds 1–2
Fig. 1 Structure of the tripodal receptors.
is the hexasubstituted benzene scaffold, bearing aminopyrrolic
(or iminopyrrolic) units that can interact with carbohydrates
through non-covalent forces, mainly hydrogen bonding and CH–p
interactions.
In an effort to expand on recognition properties, we explored
aDepartment of Chemistry, University of Firenze, via della Lastruccia 3-13,
I-50019, Sesto F.no (Firenze), Italy. E-mail: martina.cacciarini@unifi.it;
Fax: +39 055 4573531; Tel: +39 055 4573544
alternative binding groups incorporated in the same scaffold.
Herein we report on the synthesis of a series of symmetrically
substituted tripodal receptors and on their binding proper-
ties towards octyl-b-D-glucopyranoside (OctbGlc), selected as
a representative monosaccharide. The structural and functional
variations described in the present study aimed at the modulation
of hydrogen bonding ability, either touching the amine nitrogen
or replacing the pyrrolic heterocycle, with the goal of tuning the
strength, the directionality and the geometry of interaction within
the tripodal architecture.
bCentro Risonanze Magnetiche (CERM), Polo Scientifico e Tecnologico,
50019, Sesto F.no (Firenze), Italy
cIstituto di Metodologie Chimiche (IMC), Consiglio Nazionale delle
Ricerche (CNR), Department of Chemistry, University of Firenze, via
della Lastruccia 3-13, I-50019, Sesto F.no (Firenze), Italy. E-mail: stefano.
roelens@unifi.it; Fax: +39 055 4573570; Tel: +39 055 4573546
† Electronic supplementary information (ESI) available: 1H-NMR spectra
of 17a with OctbGlc in CDCl3 and CD3CN. 1H-NMR spectra of
compounds 6, 7, 9, 10, 15–18. See DOI: 10.1039/c0ob00651c
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 1085–1091 | 1085
©

View Article Online
Table 1 Cumulative binding constants (log bn) for 1 : 1, 2 : 1, 3 : 1 and 1 : 2
host-to-guest complexes of receptors with OctbGlc and corresponding in-
trinsic median binding concentration BC500 (mM) with standard deviationa
Entry log b11
log b21
log b31
log b12
BC500/mM
1ab
1bb
3b
4.61 0.03
7.79 0.06
9.04 0.09
24
4.8 0.5
3690 50
62 000 1000
1970 90
700 60
2
5.30 0.05
2.616 0.004
1.21 0.01
2.67 0.04
3.07 0.06
2.50 0.06
n.d.
2.61 0.09
1.154 0.007
n.d.
4
5b
4.88 0.06
5.86 0.04 7.96 0.17
6b,c
7
5.11 0.05 1490 80
n.d.
5.19 0.12 1300 100
70 000 1000
n.d.
9
10
15
16
17b
4.24 0.13
3.22 0.07
5.46 0.27 8.00 0.20
6.31 0.04
540
130
1170 20
7
4
18bb 3.87 0.01
18cb 2.978 0.003
Scheme 1 Synthesis of the sulfonamidic receptor 7.
a Measured by 1H-NMR (400 MHz) from titration experiments at T =
298 K in CDCl3 on 0.8–1.2 mM stock solutions of OctbGlc using
receptor concentration up to 25 mM. Binding constants were calculated
by simultaneous nonlinear least-square fit of all the available signals
to 1a. Results clearly show that increased acidity does not improve
the H-bonding ability of the NH function and that the slightly
larger affinity of sulfonamide 6 compared to 3 and 5 may rather be
ascribed to a more favorable geometry of binding achieved by the
sulfonamide NH moiety when located in the benzylic position.
0
shifts. BC50 values were calculated from log bn values using the “BC50
Calculator”, available for free upon request from one of the authors (S.
R.). b log bdim: 1a: 1.07 0.01; 1b: 0.92 0.02; 3: 1.83 0.02; 5: 1.732
0.009; 6: 2.114 0.006; 18b: 1.29 0.29; 18c: 1.74 0.05. c log btrim: 6: 3.69
0.08.
Nitronopyrrolic receptor
Nitrones are known to effectively coordinate to metallic Lewis
acids,9 and to interact via hydrogen bonding with ureas;10 more-
over, nitrones have been shown to be involved in intramolecular
hydrogen bonding, as in the case of N-(salicylidene)phenylamine
N-oxide reported by Brzezinski.11 Taking advantage of the trans-
formation of an imine into the N-oxide by the mild methyltriox-
orhenium/urea/hydrogen peroxide catalytic oxidation system,12
the nitrone derivative 9 has been prepared in 58% yield in one step
starting from the imine 1b5 (Scheme 2). A strong intramolecular
hydrogen bond (as indicated in Scheme 2) has been evidenced from
the1H-NMR spectrum by the downfield shift of the pyrrolic NH
to 11.8 ppm. Treatment at room temperature of receptor 9 with
increasing amounts of OctbGlc did not induce any shift of the
1H-NMR signals, neither from the sugar nor from the receptor,
showing no evidence of interaction with the selected carbohydrate.
Most likely, the carbohydrate hydroxyl groups cannot compete
with a strategically located pyrrole/nitrone arrangement forming
Results and discussion
Sulfonamidic receptors
Since compounds 3–5,5,7 bearing protons on nitrogen of different
acidity, have shown different binding abilities toward OctbGlc,
the effect of increased acidity of the NH function on hydrogen
bonding has been investigated by converting the aminic groups of
3 into sulfonamidic moieties.
The sulfonamidic receptor
6 (Fig. 1) was prepared in
81% yield from the parent amine 3 by treatment with 4-
methoxybenzenesulfonyl chloride in the presence of triethylamine.
The carbohydrate binding ability of 6 was tested in CDCl3 toward
OctbGlc by NMR titrations, following a previously established
protocol.7 Since 1 : 1, 2 : 1 and 3 : 1 host-to-guest adducts were
detected, in addition to dimerisation of the receptor, the affinity
was assessed through the BC50 parameter,5 a generalised affinity
descriptor univocally defining the intrinsic binding ability of
a receptor in chemical systems involving multiple equilibria.
0
0
Analogous to the IC50 parameter, the lower the BC50 value,
the higher the affinity. The BC50 value for 6, calculated from
0
cumulative binding constants, is reported in Table 1 together
with the values previously obtained for the parent amine 3,5 the
acetamide 45 and the ureidic derivative 5.7
0
Comparison of the BC50 values indicated a somewhat higher
affinity of 6 for OctbGlc with respect to receptors 3 and 5, and
much larger than that of 4. This evidence prompted us to combine
the sulfonamidic groups with other H-bonding groups in the tripo-
dal architecture. Homologous replacement of a sulfonamidic NH
for the pyrrolic NH of 1a to give 7 has been achieved in 49% yield
by amination of the trialdehyde 88 with N-tosylethylenediamine,
followed by reduction with NaBH4 (Scheme 1). Disappointingly,
this structural variation resulted in a 2-fold decrease in the affinity
for OctbGlc with respect to 6 and over a 60-fold drop with respect
Scheme 2 Synthesis of the nitronopyrrolic receptor 9.
1086 | Org. Biomol. Chem., 2011, 9, 1085–1091
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
Scheme 3 Synthesis of the oxypyrrolic receptor 10.
a 6-membered H-bonded ring, in order to establish hydrogen
bonding interactions.
decrease in affinity with respect to the amine 3 and over 50-fold
with respect to the oxypyrrolic receptor 10. It is evident that the
oxygen atom, which can behave as a hydrogen bonding acceptor
exclusively, cannot effectively replace the aminic nitrogen. The
results suggest that the amino group takes part in the binding
process, and that it may likely participate as a hydrogen bonding
donor.
Oxypyrrolic and pyrrolic receptors
The ether analogue 10 of the amino-pyrrolic receptor 1a was
prepared to ascertain the contribution from the heteroatom
located at the benzylic position on the recognition properties of the
receptor (Scheme 3). The pyrrolic nitrogen of the aldehyde 11 was
protected by reaction with 2-(trimethylsilyl)ethoxymethyl chloride
in the presence of sodium hydride in DMF to give 12,13 which was
reduced with sodium borohydride to the corresponding alcohol 13.
The Williamson etherification of 1,3,5-tris(bromomethyl)-2,4,6-
triethylbenzene with 13 and potassium tert-butoxide in DMF gave
14 in 58% yield over three steps. Deprotection of the pyrrolic
groups has been achieved with tetrabutylammonium fluoride and
1,2-diaminoethane13 to give the ether receptor 10. The binding
ability of 10 toward OctbGlc was evaluated in CDCl3, showing
a binding model which included 1 : 1, 2 : 1 and 1 : 2 host-to-guest
Furthermore, cross-comparison of the affinities of 1a, 3, 10,
and 15 shows that the contribution from the pyrrolic group is
substantially larger than that of the amine but, when located in
the tripodal architecture with the appropriate geometry, both con-
tribute synergetically to the overall binding ability of the receptor,
giving an affinity enhancement larger than that expected from
their independent contributions. Indeed, although the pyrrolic
H-bonding unit has been shown to be essential for the binding
properties of the tripodal receptors, its precise location is crucial.
This conclusion can be drawn from the results obtained with
the receptor 16, in which a 2-pyrrolyl substituent replaced the
amino groups or the oxy- of 3 and 15, respectively. Receptor
16 has been prepared by incorporating pyrrole rings into the
tripodal scaffold through a direct nucleophilic substitution on the
alkyl halide.14 Binding experiments did not show any evidence of
interaction of 16 with OctbGlc and the cause for this behaviour
may most likely reside in the length of the spacer between the
scaffold and the binding group, which prevents the receptor
from achieving the correct binding geometry. This result confirms
previous observations,5 showing that elongation of the spacer by
one methylene depleted the binding ability of derivatives of 3.
0
complexes. The corresponding BC50 value was calculated from
cumulative binding constants (see Table 1), and revealed a drop
in affinity of over 50-fold with respect to 1a. The trimethylether
derivative 15 of 1,3,5-tris(hydroxymethyl)-2,4,6-triethylbenzene
(Fig. 2) was prepared as a reference compound to evaluate the
contribution from the pyrrolic group to the binding ability of the
ether receptor and from the aminic group to the binding ability of
the plain triamine 3. The 1 : 1 binding constant measured in CDCl3
toward OctbGlc and the corresponding BC500 value calculated for
the methyl ether 15 are reported in Table 1, showing a 20-fold
Imidazolic and indolic receptors
Receptors 17 and 18 (Scheme 4) were designed to explore
functional groups alternative to pyrrolic H-bonding donors.15
Imidazole features both a pyrrole-like and a pyridine-like nitro-
gen, thus potentially behaving as a hydrogen bonding donor and
acceptor at the same time. This dual character is responsible for the
role of imidazole in biological processes, such as those occurring
in the active site of enzymes with histidine residues.16 Receptor 18a
may shed light on the preference of this group to act as a donor
or an acceptor of hydrogen bonds, since the two nitrogens occupy
structurally equivalent positions in the receptor architecture. 18a
has been synthesized (54%, two steps) by condensation of the
Fig. 2 Structures of methoxymethyl and pyrrolomethyl receptors 15 and
16.
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 1085–1091 | 1087
©

View Article Online
Scheme 4 Synthesis of imidazolic and indolic receptors.
parent amine 3 with 2-formylimidazole to give the imine derivative
17a (71%) which has been reduced to receptor 18a. The affinity
of 18a for OctbGlc was tentatively investigated by 1H-NMR
titration in CDCl3 and CD3CN but, unfortunately, precipitation of
insoluble material during measurements prevented the evaluation
of the binding affinity. In addition, the imine precursor 17a
was insoluble in most deuterated solvents (CD3OD, CD3CN and
CDCl3) whereas good solubility was observed in (CD3)2SO, in
which no evidence of interaction with OctbGlc could be detected.
However, when solid 17a was shaken with a millimolar solution of
OctbGlc in CDCl3, the solid partially dissolved and the resulting
spectrum showed that 20% of 17a was present in solution (see
Electronic Supplementary Information†). Bound 17a was still
detected (5%) when the experiment was performed in CD3CN,
thus proving that OctbGlc is indeed capable of bringing 17a into
solution by complexation, even though quantitative measurements
could not be obtained.
geometry,5 the loss of binding ability may be reasonably ascribed
to the steric hindrance of the indolic benzene moiety, hampering
the achievement of a correct binding conformation and affecting
the rigid chelate iminopyrrolic geometry to a larger extent than
the flexible aminopyrrolic arrangement.
Conclusions
In summary, a systematic analysis of the structural units consti-
tuting the tripodal receptors designed for molecular recognition
of carbohydrates has been performed. The results presented
highlight that (a) the acidity of sulfonamidic NH groups do not
improve H-bonding ability; (b) ethereal oxygen cannot effectively
replace amine as an H-bonding group, suggesting that the amine
contribution to the recognition process may most likely reside
in acting as an H-bonding donor rather than as an H-bonding
acceptor; (c) pyrrolic H-bonding units are essential for recognition
but a precise location in the architecture is crucial to achieve
the correct binding geometry; (d) when the correct geometry is
achieved, the aminic and the pyrrolic H-bonding groups exert
a synergistic effect, boosting the affinity of the receptor more
than their individual contributions; (e) connecting the tripodal
scaffold to the 2-position of the indole ring is mandatory for
effective recognition, whereas substitution at the 3-position causes
a marked drop in binding ability; (f) pyrrole is much more effective
than indole as an H-bonding donor when located in the tripodal
architecture with the correct geometry, likely because of steric and
conformational reasons.
The indolic receptors 17b and 18b were obtained by condensa-
tion of the amine 3 with indole-2-carboxaldehyde, followed by
reduction of the Schiff-base according to the same procedure
described for the imidazole derivatives (18b, 56% yield, Scheme
4). Likewise, indole-3-carboxaldehyde has been used to give
compound 18c by the same procedure (Scheme 4).17 The binding
abilities of 17b, 18b and 18c toward OctbGlc were tested in
CDCl3, where multiple association equilibria were detected in
0
most cases. BC50 values were thus calculated from cumulative
binding constants (see Table 1), showing that 18b is the receptor
of highest affinity. It can be appreciated that a drop in affinity of
an order of magnitude is observed between 18b and 18c, clearly
pointing out that connecting the spacer to the 2-position of the
pyrrole ring is crucial for achieving an effective binding geometry.
On the contrary 18c, which has the NH located one bond further
from the amine, most likely cannot achieve a convergent binding
arrangement.
Experimental section
All solvents were of reagent grade quality and purchased com-
mercially. All starting materials were purchased commercially and
used without further purification. NMR spectra used for charac-
terization of products and binding experiments were recorded on
a Varian Inova 400 instrument. The NMR spectra were referenced
to solvent. Mass spectra were recorded on an Agilent Technologies
6110 Quadrupole LC/MS. ESI-MS analysis was performed both
in positive or negative ion mode. HRMS were performed on a
LTQ-IT-Orbitrap with a spray voltage of 2.10 kV and a resolution
of 100 000. C, H and N elemental analysis was performed on a
Perkin–Elmer 2400 elemental analyser.
0
Comparison between the BC50 values of 18b and 1a shows
that indole is not as effective as pyrrole as a hydrogen bonding
donor, the affinity of the former receptor being 5-fold lower
than that of the latter. Whether this evidence can be ascribed
to electronic factors, steric hindrance, or restricted adaptivity
of indole cannot be ascertained from the present data, but the
observed loss of affinity is markedly larger when comparing the
corresponding iminic receptors 17b and 1b, which display an
affinity difference of over two orders of magnitude. Considering
that the binding ability of the iminopyrrolic receptor 1a has
been shown to rely on the achievement of a chelate H-bonding
Synthesis of sulfonamidic receptor 6. To a solution of 35
(101 mg, 0.405 mmol) and triethylamine (225 mL, 1.61 mmol) in
1088 | Org. Biomol. Chem., 2011, 9, 1085–1091
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
CH2Cl2 (4 mL), p-methoxysulfonyl chloride (253 mg, 1.22 mmol)
was added at 0 ◦C. The reaction mixture was allowed to warm to
r.t. and stirred for 1 h. Then it was diluted with CH2Cl2 (5 mL) and
washed with sat. sol. of NH4Cl (3 ¥ 5 mL), dried over Na2SO4,
filtered and concentrated. Purification of the crude product by
flash chromatography (CH2Cl2–CH3OH = 20/1, silica gel) gave 6
(130 mg, 0.171 mmol, 42%) as a white solid. M.p. 202–204 ◦C.
Found: C, 57.04; H, 5.93; N, 5.49. Calc. for C36H45N3O9S3: C,
56.90; H, 5.97; N, 5.53%; dH (400 MHz, CDCl3) 7.84–7.80 (m,
6H, Ph); 7.02–6.99 (m, 6H, Ph); 4.80 (t, J = 4.3 Hz, 3H, NHSO);
3.91 (s, 9H, OCH3); 3.85 (d, J = 4.3 Hz, 6H, CH2N); 2.22 (q, J =
7.5 Hz, 6H, CH2CH3); 0.80 (t, J = 7.5 Hz, 9H, CH3). dC (100 MHz,
CDCl3): 163.2; 144.5; 130.18; 130.14; 129.5; 114.3; 55.7; 40.8; 22.2;
16.1 ppm. MS(ESI): [M+Na]+ = 782.5; [M+K]+ = 798.4.
0
◦C, then poured into 550 mL of ice-cold NaHCO3 10% and
extracted with CH2Cl2 (3 ¥ 200 mL). The organic layers were
washed with water (3 ¥ 200 mL), dried over Na2SO4, filtered and
concentrated to give crude 12 (5.86 g, 26.0 mmol, 92%) as a pale
yellow oil. The product was used without further purification in
the next reaction. dH (200 MHz, CDCl3) 9.59 (s, 1H); 7.15–7.14
(m, 1H); 6.99–6.97 (m, 1H); 6.31–6.29 (m, 1H); 5.71 (s, 2H); 3.58–
3.50 (m, 2H); 0.94–0.86 (m, 2H); -0.02–(-0.05) (m, 9H). To a
solution of 12 (5.86 g, 26.0 mmol) in CH2Cl2 (260 mL), a freshly
prepared suspension of NaBH4 (1.97 g, 52.1 mmol) in MeOH
(75 mL) was added. The reaction was stirred for 1 h at r.t., poured
into water (500 mL) and extracted with CH2Cl2 (3 ¥ 200 mL).
The organic layers were washed with water (3 ¥ 200 mL), dried
over Na2SO4, filtered and concentrated. The crude product was
purified by flash chromatography (CH3OH–CH2Cl2 = 4/96, silica
gel) to give 13 (5.32 g, 23.4 mmol, 90%) as yellow solid. M.p.
36–38 ◦C. dH (200 MHz, CHCl3) 6.78–6.73 (m, 1H); 6.23–6.18
(m, 1H); 6.11–6.05 (m, 1H); 5.29 (s, 2H); 4.62 (d, J = 6 Hz, 2H);
3.55–3.44 (m, 2H); 2.49 (t, J = 6 Hz, 1H); 0.95–0.84 (m, 2H); 0.05–
(-0.05) (m, 9H). dC (50 MHz, CDCl3) 132.30; 123.00; 110.52;
107.42; 76.30; 65.89; 56.48; 17.98; 1.26 ppm. To a solution of 13
(594 mg, 2.62 mmol) in anhydrous DMF (5.2 mL), potassium tert-
butoxide (253 mg, 2.25 mmol) was slowly added. To the reaction
mixture, 1,3,5-triethyl-2,4,6-tris(bromomethyl)benzene (195 mg,
0.442 mmol) was added with constant stirring over 10 min. The
mixture was stirred at r.t. for 1 h, poured into water (70 mL),
neutralized with phosphate buffer and extracted with CH2Cl2 (3 ¥
25 mL). The organic layers were combined, washed with water (3 ¥
50 mL), dried over Na2SO4, filtered and concentrated. The crude
mixture was purified by flash chromatography (acetone/CH2Cl2 =
3/97, silica gel) to give 14 (272 mg, 0.309 mmol, 70%) as a pale
yellow glassy solid. dH (200 MHz, CHCl3) 6.77–6.70 (m, 3H); 6.24–
6.17 (m, 3H); 6.11–6.05 (m, 3H); 5.25 (s, 6H); 4.59 (s, 6H); 4.40 (s,
6H); 3.51–3.39 (m, 6H); 2.61 (q, J = 7.3 Hz, 6H); 1.04 (t, J = 7.3 Hz,
9H); 0.94–0.80 (m, 6H); 0.03–(-0.14) (m, 27H). dC (50 MHz,
CDCl3) 144.84; 131.75; 128.96; 123.05; 111.57; 107.29; 76.19;
65.50; 65.44; 63.96; 22.66; 17.89; 16.56; 1.18 ppm. To a solution
of 14 (676 mg, 0.768 mmol) in DMF (2.5 mL), ethylenediamine
(1.03 g, 17.1 mmol) and TBAF (2.18 g, 6.91 mmol) were added.
The solution was stirred for 60 h at 45 ◦C, then poured into
water (70 mL) and extracted with CH2Cl2 (3 ¥ 25 mL). The
combined organic layers were washed with water (3 ¥ 50 mL), dried
over Na2SO4, filtered and concentrated. The crude product was
purified by flash chromatography (acetone/CH2Cl2 = 10/90, then
acetone/CH2Cl2 = 20/80, silica gel) to give 10 (55 mg, 0.112 mmol,
Synthesis of sulfonamidic receptor 7. To a solution of 88
(218 mg, 0.885 mmol) in CH3OH (9 mL), n-tosylethylenediamine
(592 mg, 2.84 mmol) was added at r.t. The solution was stirred for
24 h, then solid NaBH4 (106 mg, 2.79 mmol) was slowly added and
evolution of hydrogen observed. After stirring for another 2 h, the
mixture was diluted with CHCl3 (50 mL), washed with brine (3 ¥
5 mL), dried over Na2SO4, filtered and concentrated. Purification
of the crude by flash chromatography (CHCl3/CH3OH/NH3
30% = 20/1/0.15, silica gel) gave 7 (365 mg, 0.434 mmol, 49%) as
a white solid. M.p. 58–60 ◦C. Found: C, 60.32; H, 6.92; N, 10.06.
Calc. for C42H60N6O6S3: C, 59.97; H, 7.19; N, 9.99%; dH (400 MHz,
CDCl3, 1.39 mM) 7.73 (d, J = 8.2 Hz, 6H, Ph); 7.30 (d, J = 8.0 Hz,
6H, Ph); 5.04 (br s, 3H, NHSO); 3.61 (s, 6H, NCH2Ph); 3.02 (m,
6H, CH2N); 2.81(m, 6H, CH2N); 2.69 (q, J = 7.4, 6H, CH2CH3);
2.41 (s, 9H, CH3); 1.14 (t, J = 7.4, 9H, CH2CH3). dC (50 MHz,
CDCl3): 143.2; 142.1; 136.9; 133.7; 129.6; 127.1; 48.9; 47.2; 42.7;
22.9; 21.7; 17.1 ppm. MS(ESI): [M+H]+ = 841.00; [M+Na]+
=
863.25.
Synthesis of nitrone receptor 9. To a yellow solution of
methyltrioxorhenium (5 mg, 0.020 mmol) and urea hydrogen
peroxide (280 mg, 3 mmol) in CH3OH (3 mL), the solid imine
1b5 (150 mg, 0.312 mmol) was added at r.t. Solubilization of the
suspension was noted after 10 min, and the reaction mixture was
stirred for 1 h more. After solvent removal under reduced pressure,
the reaction mixture was diluted with CH2Cl2 (10 mL) and the
undissolved urea filtered off. Concentration of the solute resulted
in a crude mixture, which was purified by flash chromatography
(CHCl3/CH3OH/NH3 30% = 20/1/0.15, silica gel) to give 9
(95 mg, 0.180 mmol, 58%) as a brown solid. M.p. 144–146 ◦C.
Found: C, 61.76; H, 7.38; N, 14.71. Calc. for C30H36N6O3·3H2O:
C, 61.84; H, 7.27; N, 14.42%; dH (400 MHz, CDCl3) 11.86 (br s,
3H, NH); 7.02 (s, 3H, CHN); 6.98–6.95 (m, 3H, Ar); 6.43–6.27 (m,
6H, Ar); 5.19 (s, 6H, CH2N); 2.86 (q, J = 7.6 Hz, 6H, CH2CH3);
1.21 (t, J = 7.5 Hz, 9H, CH3). dC (50 MHz, CDCl3): 147.9; 128.3;
126.3; 124.1; 120.7; 114.5; 110.5; 61.2, 23.8, 15.6 ppm. MS(ESI):
[M+H]+ = 529.17; [M+Na]+ = 551.33; [M+K]+ = 567.33.
◦
15%) as a yellow solid. M.p. 113–114 C. dH (200 MHz, CDCl3)
8.52 (s, 3H); 6.57–6.50 (m, 3H); 6.21–6.14 (m, 3H); 6.14–6.07
(m, 3H); 4.54 (s, 6H); 4.46 (s, 6H); 2.61 (q, J = 7.3, 6H); 1.03
(t, J = 7.3, 9H). dC (50 MHz, CDCl3) 144.95; 131.50; 127.75;
118.17; 107.90; 107.59; 64.99; 64.93; 22.22; 16.23 ppm. MS(ESI):
[M+Na]+ = 512.4.
Synthesis of receptor 10. To a suspension of sodium hydride
(1.13 g, 47.1 mmol) in anhydrous DMF (7 mL), pyrrole-2-
carboxaldehyde (2.68 g, 28.2 mmol) was added and evolution
of hydrogen was observed. The mixture was stirred at r.t. until
solubilization and 30 min further. The solution was cooled to 0 ◦C
and 2-(trimethylsilyl)ethoxymethyl chloride (4.71 g, 28.3 mmol)
was slowly added. The reaction mixture was stirred for 1 h at
Synthesis of receptor 15. To a suspension of 1,3,5-triethyl-
2,4,6-tris(bromomethyl)benzene (197 mg, 0.447 mmol) in anhy-
drous DMF (2.7 mL), sodium methoxide (82 mg, 8.52 mmol)
was added at r.t. and the reaction mixture was stirred for 2
h. The mixture was poured into water (30 mL) and extracted
with CH2Cl2 (3 ¥ 10 mL). The organic layers were washed with
water (3 ¥ 20 mL), dried over Na2SO4, filtered and concentrated.
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 1085–1091 | 1089
©

View Article Online
The crude mixture was purified by flash chromatography (ethyl
acetate/petroleum ether = 20/80, silica gel) to give 15 (90 mg,
0.306 mmol, 68%) as a white solid. M.p. 84–85 ◦C. Found C, 73.33;
H, 10.27. Calc. for C18H30O3: C, 73.43; H, 10.27; O, 16.30%; dH
(200 MHz, CDCl3) 4.45 (s, 6H); 3.42 (s, 9H); 2.83 (q, J = 1.75 Hz,
6H); 1.19 (t, J = 1.75 Hz, 9H). dC (50 MHz, CDCl3): 144.52; 131.57;
68.37; 57.96; 22.54; 16.26 ppm. MS(ESI): [M+Na]+ = 317.3.
for [C27H39N9 + H]+ = 490.34012; found 490.34032; delta (ppm):
+0.41.
18b: M.p. 152–154 ◦C. dH (400 MHz, CDCl3, 1.6 mM) 8.41 (br s,
3H, NH-Ind); 7.56–7.54 (m, 3H, Ar); 7.31–7.29 (m, 3H, Ar); 7.18–
7.07 (m, 6H, Ar); 6.38 (s, 3H, Ar); 4.07 (s, 6H, CH2N); 3.75 (s, 6H,
CH2N); 2.74 (q, J = 7.3 Hz, 6H, CH2CH3); 1.14 (t, J = 7.4 Hz, 9H,
CH3). dC (50 MHz, CDCl3, 40 mM): 142.4; 137.3; 136.1; 133.9;
128.4; 121.5; 120.1; 119.6; 110.7; 100.5; 47.6; 47.1; 22.7; 16.9 ppm.
MS(ESI): [M+H]+ = 637.08; [M+Na]+ = 659.25. HRMS (ESI):
calcd. for [C42H48N6 + H]+ = 637.40099; found 637.40132; delta
(ppm): -0.52.
Synthesis of receptor 16. To a suspension of 1,3,5-triethyl-
2,4,6-tris(bromomethyl)benzene (1 g, 2.27 mmol) and K2CO3
(0.941g, 6.81 mmol) inCH2Cl2 (5 mL) was added pyrrole (15.7 mL,
227 mmol). The whole mixture was allowed to stir for 4 h at room
temperature. The reaction was combined with water (50 mL), then
the mixture was extracted with CH2Cl2 (3 ¥ 30 mL). The organic
layer was dried over Na2SO4, filtered and concentrated. The
crude mixture was purified by flash column chromatography (ethyl
acetate/petroleum ether = 20/80, silica gel) to afford compound
16 (262 mg, 0.66 mmol, 29%) as glassy white solid. dH (200 MHz,
CDCl3) 7.69 (br s, 3H); 6.62–6.60 (m, 3H); 6.12–6.10 (m, 3H); 5.75–
5.73 (m, 3H); 4.04 (s, 6H); 2.62 (q, J = 7.5 Hz, 6H); 1.03 (t, J =
7.5 Hz, 9H). dC (50 MHz, CDCl3): 141.31; 133.31; 130.94; 116.18;
18c: M.p. 116–118 ◦C. Found: C, 78.95; H, 7.30; N, 13.00. Calc.
for C42H48N6: C, 79.21; H, 7.60; N, 13.20%; dH (400 MHz, CDCl3,
10.2 mM) 8.14 (br s, 3H, NH-Ind); 7.64–7.62 (m, 3H, Ar); 7.30–
7.28 (m, 3H, Ar); 7.18–7.14 (m, 3H, Ar); 7.10–7.06 (m, 3H, Ar);
7.05–7.04 (m, 3H, Ar); 4.04 (s, 6H, CH2N); 3.72 (s, 6H, CH2N);
2.61 (q, J = 7.3 Hz, 6H, CH2CH3); 1.00 (t, J = 7.3 Hz, 9H, CH3). dC
(100 MHz, CDCl3): 142.1; 136.4; 134.3; 127.2; 122.0; 119.4; 119.0;
111.0; 107.5; 47.3; 45.5; 22.4; 16.7 ppm. MS(ESI): [M+H]+ = 637.6;
[M+Na]+ = 659.6; [M+K]+ = 675.6.
108.71; 105.58; 27.93; 23.58; 15.38 ppm. MS(ESI): [M+H]+
400.17; [M+K]+ = 438.08.
=
Titrations and data analysis. Titrations were performed in
5 mm NMR tubes using Hamilton microsyringes, following a
previously described technique.7 To avoid interference of traces
of acid in solution, CDCl3 was additionally treated by eluting
through a short column of basic alumina right before use.
Mathematical analysis of data and graphics presentation of results
were done using the HypNMR 200618 computer program from
Protonic Software. The program performs simultaneous fit of
multiple signals to models involving multiple equilibria, giving
binding constants and chemical shifts of individual species. “BC50
Calculator”, the utility program for computing BC50 and BC500, is
available for free upon request from one of the authors (S.R.).
Synthesis of receptors 17a,b. To a solution of 35 (100 mg,
0.4 mmol) in CH3OH (3 mL), the corresponding aldehyde
(1.2 mmol) was added at r.t. The solution was stirred overnight at
r.t., during which a precipitate was formed. The suspension was
filtered and washed with fresh CH3OH, to yield pure imine (17a:
71%; 17b: 73%), as white solid.
17a: M.p. 158–161 ◦C. Found: C, 64.61; H, 6.98; N, 25.20. Calc.
for C27H33N9·H2O: C, 64.65; H, 7.03; N, 25.13%; dH (400 MHz,
DMSO-d6) 8.13 (s, 3H, CH); 7.15 (s, 3H, Ar); 7.02 (s, 3H, Ar);
4.84 (s, 6H, CH2N); 2.66 (q, J = 7.4 Hz, 6H, CH2CH3); 1.12 (t, J =
7.3 Hz, 9H, CH3). MS(ESI): [M+H]+ = 484.25; [M+Na]+ = 506.42.
17b: M.p. 125–128 ◦C. Found: C, 77.72; H, 6.87; N, 12.99. Calc.
for C42H42N6·H2O: C, 77.75; H, 6.84; N, 12.95%; dH (200 MHz,
CDCl3) 9.15 (br s, 3H, NH-Ind); 8.27 (s, 3H); 7.63–7.59 (m, 3H,
Ar); 7.35–7.19 (m, 6H, Ar); 7.12–7.04 (m, 3H, Ar); 6.74 (s, 3H);
4.95 (s, 6H, CH2N); 2.81 (q, J = 7.3 Hz, 6H, CH2CH3); 1.26 (t,
J = 7.5 Hz, 9H, CH3). dC (50 MHz, CDCl3): 151.6; 143.1; 136.5;
134.9; 132.5; 127.7; 124.1; 121.4; 119.8; 111.1, 107.5; 56.8; 22.8;
15.6 ppm. MS(ESI): [M+H]+ = 631.6; [M+Na]+ = 653.6; [M+K]+ =
669.6.
Acknowledgements
A C.I.N.M.P.I.S. fellowship to Oscar Francesconi is gratefully
acknowledged.
Notes and references
1 (a) The sugar code: Fundamentals of Glycosciences, ed. H.-J. Gabius,
Wiley-VCH, Weinheim, 2009; (b) B. Ernst, W. Hart and P. Sinay,
Carbohydrates in Chemistry and Biology, Wiley-VCH, Weinheim, 2000,
Part I, Vol. 2 and Part II, Vol. 4; (c) T. K. Lindhorst, Essentials
of Carbohydrate Chemistry and Biochemistry, Wiley-VCH, Weinheim,
2000.
Synthesis of receptors 18. To a solution of 35 (100 mg,
0.4 mmol) in CH3OH (5 mL), the corresponding aldehyde
(1.2 mmol) was added at r.t. The solution was stirred overnight at
r.t., during which the Schiff base was formed. The reaction mixture
was diluted with CHCl3 (20 mL), solid NaBH4 was slowly added
and evolution of hydrogen observed. After stirring for another
2 h, the mixture was diluted with CHCl3 (20 mL), washed with
brine (3 ¥ 20 mL), dried over Na2SO4, filtered and concentrated.
Purification of the crude products by flash chromatography (silica
gel, CH2Cl2/CH3OH/NH3 30% = 4/1/0.1 (18a), 12/1/0.15 (18b)
and 5 : 1 : 0.1 (18c)) gave 18 (18a: 54%, 18b: 56%, 18c: 38%) as
white solids.
2 (a) S. Jin, Y. Cheng, S. Reid, M. Li and B. Wang, Medicinal Research
Reviews, 2009, 1–87; (b) D. B. Walker, G. Joshi and A. P. Davis, Cell.
Mol. Life Sci., 2009, 66, 3177–3191; (c) M. Mazik, Chem. Soc. Rev.,
2009, 38, 935–956; (d) A. P. Davis and T. D. James, in Functional
Synthetic Receptors, T. Schrader and A. D. Hamilton, ed., Wiley-
VCH, Weinheim, Germany, 2005, 45–109; (e) Host–Guest Chemistry.
Mimetic Approaches to Study Carbohydrate Recognition, in Topics
in Current Chemistry, S. Penades, Ed., Springer-Verlag, Heidelberg,
Germany, 2002, 218; (f) A. P. Davis and R. S. Wareham, Angew. Chem.,
Int. Ed., 1999, 38, 2979–2996; (g) T. D. James, K. R. A. S. Sandanayake
and S. Shinkai, Angew. Chem., Int. Ed. Engl., 1996, 35, 1910–1922;
(h) Y. C. Lee and R. T. Lee, Acc. Chem. Res., 1995, 28, 321–327.
3 (a) N. Y. Edwards, T. W. Sager, J. T. McDevitt and E. V. Anslyn, J.
Am. Chem. Soc., 2007, 129, 13575–13583; (b) Y. Kikuchi, Y. Tanaka, S.
Sutarto, K. Kobayashi, H. Toi and Y. Aoyama, J. Am. Chem. Soc., 1992,
114, 10302–10306; (c) N. P. Barwell, M. P. Crump and A. Davis, Angew.
Chem., Int. Ed., 2009, 48, 7673–7676; (d) S. Anderson, U. Neidlein, V.
Gramlich and F. Diederich, Angew. Chem., Int. Ed. Engl., 1995, 34,
◦
18a: M.p. 116–119 C. dH (200 MHz, CDCl3) 7.09 (s, 6H, CH
Ar); 3.83 (s, 6H, CH2N); 3.29 (s, 6H, CH2N); 1.87 (q, J = 7.3 Hz,
6H, CH2CH3); 0.85 (t, J = 7.3 Hz, 9H, CH3). HRMS (ESI): calcd.
1090 | Org. Biomol. Chem., 2011, 9, 1085–1091
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
1596–1600; (e) J.-M. Fang, S. Selvi, J.-H. Liao, Z. Slanina, C.-T. Chen
and P.-T. Chou, J. Am. Chem. Soc., 2004, 126, 3559–3566; (f) G. Das
and A. D. Hamilton, J. Am. Chem. Soc., 1994, 116, 11139–11140; (g) Y.-
H. Kim and J.-I. Hong, Angew. Chem., Int. Ed., 2002, 41, 2947–2950;
(h) O. Rusin, K. Lang and V. Kral, Chem.–Eur. J., 2002, 8, 655–663;
(i) H. Abe, A. Horii, S. Matsumoto, M. Shiro and M. Inouye, Org.
Lett., 2008, 10, 2685–2688; (j) M. Mazik and M. Kuschel, Eur. J. Org.
Chem., 2008, 1517–1526; (k) C. Schmuck and M. Schwegmann, Org.
Lett., 2005, 7, 3517–3520; (l) T. Schrader, J. Am. Chem. Soc., 1998, 120,
11816–11817; (m) T. Ishi-I, M. A. Mateos-Timoneda, P. Timmerman,
M. Crego-Calama, D. N. Reinhoudt and S. Shinkai, Angew. Chem.,
Int. Ed., 2003, 42, 2300–2305; (n) R. Liu and W. C. Still, Tetrahedron
Lett., 1993, 34, 2573–2576; (o) S. Striegel and M. Dittel, J. Am. Chem.
Soc., 2003, 125, 11518–11524.
4 (a) N. P. Barwell, M. P. Crump and A. P. Davis, Angew. Chem., Int.
Ed., 2009, 48, 7673–7676; (b) Y. Ferrand, M. P. Crump and A. P. Davis,
Science, 2007, 318, 619–622.
5 C. Nativi, M. Cacciarini, O. Francesconi, A. Vacca, G. Moneti, A.
Ienco and S. Roelens, J. Am. Chem. Soc., 2007, 129, 4377–4385.
6 C. Nativi, M. Cacciarini, O. Francesconi, G. Moneti and S. Roelens,
Org. Lett., 2007, 9, 4685–4688.
8 A. J. Lampkins, O. Abdul-Rahim and R. K. Castellano, J. Org. Chem.,
2006, 71, 5815–5818.
9 S. Murahashi, Y. Imada, T. Kawakami, K Harada, Y. Yonemushi and
N. Tomita, J. Am. Chem. Soc., 2002, 124, 2888–2889.
10 T. Okino, Y. Hoashi and Y. Takemoto, Tetrahedron Lett., 2003, 44,
2817–2821.
11 T. Dziembowska, E. Majewski, Z. Rozwadowski and B. Brzezinski, J.
Mol. Struct., 1997, 403, 183–187.
12 G. Soldaini, F. Cardona and A. Goti, Org. Lett., 2007, 9, 473–476.
13 J. M. Muchowski and D. R. Solas, J. Org. Chem., 1984, 49, 203–205.
14 S.-J. Hong, J. Yoo, S.-D. Jeong and C.-H. Lee, J. Inclusion Phenom.
Macrocyclic Chem., 2010, 66, 209–212.
15 (a) For tripodal, nonsymmetrically substituted receptors bearing
imidazole and indole groups, see also: M. Mazik and A. Hartmann,
Beilstein J. Org. Chem., 2010, 6(9); (b) M. Mazik and M. Kuschel,
Chem.–Eur. J., 2008, 14, 2405–2419.
16 S. A. Kuby, A Study of Enzymes - vol I - Enzyme Catalysis, Kinetics,
and Substrate Binding, CRC Press, 1991.
17 Because the 3-indolyl Schiff base does not precipitate from solution,
compound 18c was obtained by in situ reduction of 17c without
isolation of the imine.
7 A. Vacca, C. Nativi, M. Cacciarini, R. Pergoli and S. Roelens, J. Am.
Chem. Soc., 2004, 126, 16456–16465.
18 G. Frassineti, S. Ghelli, P. Gans, A. Sabatini, M. S. Moruzzi and A.
Vacca, Anal. Biochem., 1995, 231, 374–382.
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 1085–1091 | 1091
©