Carbohydrate-based receptors with multiple thiourea binding sites. Multipoint hydrogen bond recognition of dicarboxylates and monosaccharides

Article abstract of DOI:10.1021/jo001508n

The binding properties of multitopic sugar thiourea receptors toward dicarboxylate and monosaccharide guests have been examined taking glutarate and octyl β-D-glucopyranoside as model ligands. For the anionic hydrogen bond acceptor, both the complex stoichiometry and the association constants (K_as) were found to be strongly dependent on the relative disposition of recognition elements in the host. In contrast, for the glucoside guest a 1:1 stoichiometry was observed in all cases, the K_as values being largely independent of the unbound state provided that geometrically equivalent supramolecular topologies can be achieved.

Full text of DOI:10.1021/jo001508n

1366
J . Org. Chem. 2001, 66, 1366-1372
Ca r boh yd r a te-Ba sed Recep tor s w ith Mu ltip le Th iou r ea Bin d in g
Sites. Mu ltip oin t Hyd r ogen Bon d Recogn ition of Dica r boxyla tes
a n d Mon osa cch a r id es†
J uan M. Benito,‡ Marta Go´mez-Garc´ıa,‡ J ose´ L. J ime´nez Blanco,‡ Carmen Ortiz Mellet,*,‡ and
J ose´ M. Garc´ıa Ferna´ndez*,§
Departamento de Quı´mica Orga´nica, Facultad de Qu´ımica, Universidad de Sevilla, Aptdo. 553,
E-41071 Sevilla, Spain, and Instituto de Investigaciones Quı´micas, CSIC, Ame´rico Vespucio s/ n,
Isla de la Cartuja, E-41092 Sevilla, Spain
jogarcia@cica.es
Received October 23, 2000
The binding properties of multitopic sugar thiourea receptors toward dicarboxylate and monosac-
charide guests have been examined taking glutarate and octyl â-D-glucopyranoside as model ligands.
For the anionic hydrogen bond acceptor, both the complex stoichiometry and the association
constants (Kas) were found to be strongly dependent on the relative disposition of recognition
elements in the host. In contrast, for the glucoside guest a 1:1 stoichiometry was observed in all
cases, the Kas values being largely independent of the unbound state provided that geometrically
equivalent supramolecular topologies can be achieved.
In tr od u ction
nating force in the association process if several act
concertedly.5 On the other hand, the recognition and
subsequent complementary binding between an oligosac-
charide epitope and its cognate receptor is the first step
in many vital supramolecular processes in living systems,
such as cell-cell recognition and adhesion.6 A common
feature of the protein-sugar complexes is the existence
of extensive hydrogen bonding between host and guest.7
Mimicking these strategies by designing hydrogen bond-
ing abiotic receptors should help to unravel the principles
of saccharide recognition by biomolecules.
The design and synthesis of model receptors to recog-
nize polar molecules of biochemical significance is receiv-
ing much increased attention.1 The intermolecular inter-
actions involved in these processes are of direct relevance
to many biological events and may also lead to new bio-
sensors, therapeutics, and transport systems.2 Consider-
able effort has recently been directed toward the develop-
ment of synthetic receptors that rely solely on hydrogen
bond arrays for molecular recognition of carboxylic acid3
and carbohydrate derivatives.4 The former are simpler
model compounds for amino acids and peptides. Although
hydrogen bonds are in general significantly weaker than
electrostatic interactions when the corresponding car-
boxylate anions are involved, they may become a domi-
Thiourea derivatives have proven particularly useful
in the construction of neutral hydrogen bonding recep-
tors.3a,b,8 The relatively acidic thiourea NH protons,9 with
a strong hydrogen-bond donor capability, can establish
(4) For reviews on artificial hosts for sugars, see: (a) Davis, A. P.;
Wareham, R. S. Angew. Chem., Int. Ed. Engl. 1999, 38, 2978-2996.
(b) Haseltine, J .; Doyle, T. J . In Organic Synthesis: Theory and
Applications; Hudlicky, T., Ed.; J AI Press: Greenwich, 1996; Vol. 3,
85-107. For recent reports on hydrogen-bond receptors for carbohy-
drates, see: (c) Budka, J .; Tkadlekova´, M.; Lhota´h, P.; Stibor, I.
Tetrahedron 2000, 56, 1883-1887. (d) Inouye, M.; Takahashi, K.;
Nakazumi, H. J . Am. Chem. Soc. 1999, 121, 341-345. (e) Inouye, M.;
Chiba, J .; Nakazumi, H. J . Org. Chem. 1999, 64, 8170-8176. (f) Rusin,
O.; Kra´l, V. Chem. Commun. 1999, 2367-2368. (g) Davis, A. P.;
Wareham, R. S. Angew. Chem., Int. Ed. Engl. 1998, 37, 2270-2273.
(5) A paradigmatic example from nature, that has inspired much
work on artificial neutral receptors for carboxylate anions, is the
carboxylate binding pocket of the vancomycin group antibiotics. See:
Loll, P. J .; Bevivino, B. D.; Korty, B. D.; Axelsen, P. H. J . Am. Chem.
Soc. 1997, 119, 1516-1522. (b) Sharman, G. J .; Williams, D. H. Chem.
Commun. 1997, 723-724. (c) Bardsley, B.; Williams, D. H. Chem.
Commun. 1997, 1049-1050. (d) Albert, J . S.; Hamilton, A. D. Tetra-
hedron Lett. 1993, 34, 7363-7366, and references therein.
(6) For leading references see: (a) Siebert, H.-C.; von der Lieth, C.-
W.; Renter, G.; Wittmann, J .; Vliegenthart, J . F. G.; Gabius, H.-J . In
Glycosciences, Status and Perspectives; Gabius, H.-J ., Gabius, S., Eds.;
Chapman & Hall: Weinheim, 1997; pp 291-310. (b) Dwek, R. Chem.
Rev. 1996, 96, 683-720. (c) Fukuda, M. Bioorg. Med. Chem. 1995, 3,
207-215. (d) Varki, A. Glycobiology 1993, 3, 97-130.
(7) (a) Sears, P.; Wong, C.-H. Angew. Chem., Int. Ed. Engl. 1999,
38, 2300-2334. (b) Poveda, A.; J ime´nez-Barbero, J . Chem. Soc. Rev.
1998, 27, 133-143. (c) Weiss, W. I.; Drickamer, K. Annu. Rev. Biochem.
1996, 65, 441-473. (d) Toone, E. J . Curr. Opin. Struct. Biol. 1994, 4,
719-728. (e) J effrey, G. A. Adv. Enzymol. 1992, 65, 217-254. (f) Vyas,
N. K. Curr. Opin. Struct. Biol. 1991, 1, 732-740.
† Dedicated to Professor Ge´rard Descotes on the occasion of his
retirement.
* To whom correspondence should be addressed. Tel: + 34 954489559.
Fax: + 34 954460565. E-mail: jogarcia@cica.es.
‡ Universidad de Sevilla.
§ Instituto de Investigaciones Qu´ımicas, CSIC.
(1) For leading references, see: (a) Lehn, J .-M. Supramolecular
Chemistry. Concepts and Perspectives; VCH: Weinheim: 1995. (b)
Dugas, H. Bioorganic Chemistry; Springer-Verlag: New York, 1996.
(c) Aoyama, Y. In Comprehensive Supramolecular Chemistry; Atwood,
J . L., Davies, J . E. D., MacNicol, D. D., Vo¨gtle, F., Eds.; Pergamon:
Oxford, 1996; Vol. 2, pp 279-307. (d) Seel, C.; de Mendoza, J . In
Comprehensive Supramolecular Chemistry; Atwood, J . L., Davies, J .
E. D., MacNicol, D. D., Vo¨gtle, F., Eds.; Pergamon: Oxford, 1996; Vol.
2, pp 519-552. (e) Chen, H.; Weiner, W. S.; Hamilton, A. D. Curr. Opin.
Chem. Biol. 1997, 1, 458-466. (f) Beer, P. D.; Schmitt, P. Curr. Opin.
Chem. Biol. 1997, 1, 475-482.
(2) (a) Bell, T. W. Curr. Opin. Chem. Biol. 1998, 2, 711-716. (b)
Snowden, T. S.; Anslyn, E. V. Curr. Opin. Chem. Biol. 1999, 3, 740-
746.
(3) For a complete account on carboxylate receptors, see ref 1d,f.
For recent reports on hydrogen-bond receptors for carboxylates see:
(a) J ullian, V.; Shephard, E.; Hursthouse, M. B.; Kilburn, J . D.
Tetrahedron Lett. 2000, 41, 3963-3966. (b) Sasaki, S,; Mizuno, M.;
Naemura, K.; Tobe, Y. J . Am. Chem. Soc. 2000, 65, 275-283. (c) Kubo,
Y.; Tsukahara, M.; Ishihara, S.; Tokita, S. Chem. Commun. 2000, 653-
654. (d) Snellink-Rue¨l, B. H. M.; Antonisse, M. M. G.; Engbersen, J .
F. J .; Reinhoudt, D. N. Eur. J . Org. Chem. 2000, 165-170. (e) Al-Sayah,
M. H.; Branda, N. R. Angew. Chem., Int. Ed. Engl. 2000, 39, 945-
947.
10.1021/jo001508n CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/01/2001

Carbohydrate-Based Receptors
J . Org. Chem., Vol. 66, No. 4, 2001 1367
multipoint hydrogen bonded patterns with complemen-
tary acceptor groups in a specific and predictable manner.
Moreover, the diffusiveness of the electronic charge in
the lone pairs of sulfur leads to the thiocarbonyl group
being a weak hydrogen-bonding acceptor, unlikely to
interfere in conformational or complexing studies involv-
ing other stronger acceptor centers.10 Our own results
have shown that thiourea functionalities embedded into
an oligosaccharide structure provide efficient anchoring
points for hydrogen bonding of carboxylates.11 The sugar
moieties allow conformational control at the pseudoamide
bonds and act as intramolecular probes during NMR
titration experiments. In the frame of a program aimed
at the understanding of molecular recognition processes
involving carbohydrates, we now report on the binding
properties of the multitopic sugar-thiourea conjugates
1-3. The complex stoichiometry and association con-
stants (Kas) for glutarate (4) and octyl â-D-glucopyrano-
1
side (5) have been determined by H NMR spectroscopy
in order to evaluate the effect of the number of hydrogen
bond donating sites and their relative disposition on the
binding efficiency for each type of ligand.
Resu lts a n d Discu ssion
Design Cr iter ia . In the present work, we have
purposely chosen systems having C2v symmetry to pro-
mote cooperative interactions on complexation while
facilitating NMR analysis. The p-xylylene bis(thiourea)
framework was selected as an appropriate motif for
receptor design because it has been shown to be comple-
mentary of dicarboxylate 4.8a,12 Since 4 has a chain length
close to that of a hexose, it was also expected to expand
properly through the pyranoid ring of the glucopyrano-
side 5.13 By incorporating additional thiourea function-
alities and m-xylylene linkers we intended to create a
deeper cavity, favoring ligand desolvation and maximiz-
ing hydrogen bond donating interactions.14 The relative
disposition of the thiourea groups was chose to allow an
optimal contact between the corresponding binding sites
without building up of substantial strain. In contrast to
the tetradentated receptor 1, the hydrogen-bonding
centers of 2 and 3 can converge to complement all
acceptor centers of ligand 4, in its most stable transoid
conformation, by rotation about a limited number of
single bonds; this was checked by computer-aid molecular
modeling. The goal of considering both alternating com-
binations of p- and m-xylylene spacers was to examine
binding characteristics as the arrangement of recognition
elements in the receptor changed with the overall archi-
tecture of the 1:1 complex remaining the same. The
terminal glucopyranosyl subunits were expected to fix the
Z-configuration at the contiguous NH;C(dS) bonds, as
previously observed for other sugar thioureas, promoting
the “active” 1,3-parallel disposition of the thiourea NH
protons.15
(8) (a) Fan, E.; Van Arman, S. A.; Kincaid, S.; Hamilton, A. D. J .
Am. Chem. Soc. 1993, 115, 369-370. (b) Scheerder, J .; Fochi, M.;
Engbersen, J . F. J .; Reinhoudt, D. N. J . Org. Chem. 1994, 59, 7815-
7820. (c) Pern´ıa, G. J .; Kilburn, J . D.; Rowley, M. J . Chem. Soc., Chem.
Commun. 1995, 305-306. (d) Goodman, M. S.; J ubian, V.; Linton, B.;
Hamilton, A. D. J . Am. Chem. Soc. 1995, 117, 11610-11611. (e)
Nishizawa, S.; Bu¨hlmann, P.; Iwao, M.; Umezawa, Y. Tetrahedron Lett.
1995, 36, 6483-6486. (f) Scheerder, J .; Engbersen, J . F. J .; Reinhoudt,
D. N. Recl. Trav. Chim. Pays-Bas 1996, 115, 307-320. (g) Scheerder,
J .; Engbersen, J . F. J .; Verboom, W.; van Duynhoven, J . P. M.;
Reinhoudt, D. N. J . Org. Chem. 1996, 61, 3476-3481. (h) Pern´ıa, G.
J .; Kilburn, J . D.; Essex, J . W.; Mortishire-Smith, R. J .; Rowley, M. J .
Am. Chem. Soc. 1996, 118, 10220-10227. (i) Bu¨hlmann, P.; Nishizawa,
S.; Xiao, K. P.; Umezawa, Y. Tetrahedron 1997, 53, 1647-1654. (j)
Tobe, Y.; Sasaki, S.; Mizuno, M.; Naemura, K Chem. Lett. 1998, 835-
836. (k) Linton, B. R.; Goodman, M. S.; Hamilton, A. D. Chem. Eur. J .
2000, 6, 2449-2455.
(9) Bordwell, F. G.; Algrim, D. J .; Harrelson, J . A., J r. J . Am. Chem.
Soc. 1988, 110, 5903-5904.
(10) (a) Molina, M. T.; Ya´n˜ez, M.; Mo´, O.; Notario, R.; Abboud, J .-L.
M. In Supplement A3, The Chemistry of Double-Bonded Functional
Groups, Part 2; Patai, S., Ed.; J ohn Wiley & Sons: Chichester, 1997;
pp 1355-1496. (b) Riena¨cker, C. M.; Klapo¨tke, T. In Supplement A3,
The Chemistry of Double-Bonded Functional Groups, Part 1; Patai, S.,
Ed.; J ohn Wiley & Sons: Chichester, 1997; pp 341-365.
Syn th esis a n d Str u ctu r e of Recep tor s 1-3. The
tetradentated host 1 was efficiently prepared by direct
(11) J ime´nez Blanco, J . L.; Benito, J . M.; Ortiz Mellet, C.; Garc´ıa
Ferna´ndez, J . M. Org. Lett. 1999, 1, 1217-1220.
(12) For leading references on artificial receptors for dicarboxylic
acids and their salts, see: (a) Goswami, S.; Ghosh, K.; Dasgupta, S.;
J . Org. Chem. 2000, 65, 1907-1914. (b) Lavigne, J . J .; Anslyn, E. V.
Angew. Chem., Int. Ed. Engl. 1999, 38, 3666-3669. (c) Prohens, R.;
Toma`s, S.; Deya`, P. M.; Ballester, P.; Costa, A. Tetrahedron Lett. 1998,
39, 1063-1066. (d) Takeuchi, M.; Imada, T.; Shinkai, S. Angew. Chem.,
Int. Ed. Engl. 1998, 37, 2096-2099. (e) Eblinger, F.; Schneider, H.-J .
Angew. Chem., Int. Ed. Engl. 1998, 37, 826-829.
(13) Rigid receptors for five-carbon chain dicarboxylic acids have
been previously shown to be efficient for hexopyranoside binding:
Cuntze, J .; Owens, L.; Alca´zar, V.; Seiler, P.; Diederich, F. Helv. Chim.
Acta 1995, 78, 367-390.
(14) Most reported artificial receptors for carbohydrates relying
solely on hydrogen bonds for binding encompass simultaneously donor
and acceptor centers (cf. ref 4). A few examples of hosts having
exclusively hydrogen bond acceptor centers have been reported: (a)
Das, G.; Hamilton, A. D. J . Am. Chem. Soc. 1994, 116, 11139-11140.
(b) Neidlein, U.; Diederich, F. Chem. Commun. 1996, 1493-1494. To
the best of our knwoledge, there are no reports on sugar binding by
receptors involving exclusively hydrogen bond donor centres.
(15) For recent reviews on the synthesis and conformational proper-
ties of sugar thioureas, see: (a) Garc´ıa Ferna´ndez, J . M.; Ortiz Mellet,
C. Sulfur Reports 1996, 19, 61-169. (b) Garc´ıa Ferna´ndez, J . M.; Ortiz
Mellet, C. Adv. Carbohydr. Chem. Biochem. 2000, 55, 35-135.

1368 J . Org. Chem., Vol. 66, No. 4, 2001
Benito et al.
Sch em e 1
F igu r e 1. Relative disposition of the H-2, H-1, and NH
protons in the anti-Z,Z conformation of glucosylthioureido
segments. The diagnostic ROE contacts are shown.
Sch em e 2
the expected C2v symmetry. In all cases, ROESY experi-
ments exhibited intense cross-peaks between the reso-
nances of the sugar-NH proton and H-2 in the pyranose
ring, supporting the anti conformation at the anomeric
bond (Figure 1). The low-field chemical shift for the H-1
resonance is in agreement with a relative 1,3-parallel
disposition with respect to the thiocarbonyl sulfur atom,
implying the Z-configuration at the corresponding pseu-
doamide bond.21 The presence of intense ROE contacts
between NH protons located at the same thiourea group
indicated a significant proportion of Z,Z conformers in
the rotameric equilibria. Nevertheless, the low-temper-
1
ature range H NMR spectra of 1-3 in CDCl3 evidenced
Sch em e 3
the presence of several rotamers in the solution. The
relatively low coalescence temperatures (255-260 K), the
chemical shifts of the NH resonances (7.2-6.5 ppm) and
the complexity of the spectra discarded the presence of
intramolecular hydrogen-bonded stabilized folding pat-
terns that should be disrupt upon complexation.22
Bin d in g Stu d ies. Evaluation of complex stoichiome-
try and binding strength was effected by continuous
1
variation plots (J ob plots) and H NMR titration experi-
ments. Addition of aliquots of tetrabutylammonium
(TBA) glutarate 4 (in CDCl3 or DMSO-d6) or octyl â-D-
glucopyranoside 5 (in CDCl3) to 5 mM solutions of
receptors 1-3 at 303 K caused all NH proton signals to
shift downfield, presumably due to their involvement on
hydrogen bonding. In all cases, the 1H chemical shifts
were found to be independent of the concentration of host
molecule in the range 1.0-10.0 mM at this temperature,
discarding significant self-association under the condi-
tions used for association constant (Kas) measurements.
Dilution experiments in CDCl3 showed weak self-associa-
tion of the glucopyranoside 5 above 5 mM. To confirm
that the effect of self-association on the host-guest
binding is negligible, additional inverse titrations were
carried out in which the concentration of 5 was held
constant and that of receptors 1-3 varied. The Kas values
obtained by this procedure were virtually identical to
those obtained by the direct protocole.
coupling of commercial p-xylylene diamine (6) and 2,3,4,6-
tetra-O-acetyl-â-D-glucopyranosyl isothiocyanate16 (7)
(Scheme 1). The tetrathiourea derivatives 2 and 3 were
obtained in three steps by reaction of 1,4- (8)17 or 1,3-
bis(isothiocyanatomethyl)benzene18 (12), respectively, with
the corresponding mono-Boc-protected diamine 918,19 or
13,19 subsequent TFA-catalyzed hydrolysis of the car-
bamate groups in the adducts (10 or 14), and final
coupling of the resulting diamine (11 or 15) with the
glucosyl isothiocyanate 7 (Schemes 2 and 3).
As anticipated, receptor 1 displayed a binding isotherm
against 4 in DMSO-d6 compatible with structure 16
The 1H and 13C NMR spectra of receptors 1-3 recorded
at room temperature in CDCl3 and in DMSO-d6 showed
the typical line broadening associated with restricted
rotation at the pseudoamide NH-C(dS) bonds.20 At 313
K, a single set of signals was observed, thus confirming
(20) Duus, F. In Comprehensive Organic Chemistry; Barton, D.,
Ollis, W. D., Eds.; Pergamon Press: London, 1979; Vol. 3, pp 373-
487.
(21) D´ıaz Pe´rez, V. M.; Ortiz Mellet, C.; Fuentes, J .; Garcı´a Ferna´n-
dez, J . M. Carbohydr. Res. 2000, 326, 161-175.
(22) Typically, the presence of intramolecular hydrogen-bonding
stabilized folding patterns in sugar thioureas results in increased
caolescence temperatures (285-295 K), lowfield shifted NH resonances
(11-7.3 ppm) and a simplification of the rotameric equilibrium. See
cf. ref 11 and: (a) Garc´ıa-Moreno, M. I.; Benito, J . M.; Ortiz Mellet,
C.; Garc´ıa Ferna´ndez, J . M. Tetrahedron: Asymmetry 2000, 11, 1331-
1341. (b) Garc´ıa Ferna´ndez, J . M.; Ortiz Mellet, C.; J ime´nez Blanco,
J . L.; Fuentes, J .; Dia´nez, M. J .; Estrada, M. D.; Lo´pez-Castro, A.;
Pe´rez-Garrido, S. Carbohydr. Res. 1996, 286, 55-65. (c) Garc´ıa
Ferna´ndez, J . M.; J ime´nez Blanco, J . L.; Ortiz Mellet, C.; Fuentes, J .
J . Chem. Soc., Chem. Commun. 1995, 57-58. (d) Ortiz Mellet, C.;
Moreno Mar´ın, A.; J ime´nez Blanco, J . L.; Garc´ıa Ferna´ndez, J . M.;
Fuentes, J . Tetrahedron: Asymmetry 1994, 5, 2325-2334.
(16) (a) Camarasa, M. J .; Ferna´ndez-Resa, P.; Garc´ıa-Lo´pez, M. T.;
de las Heras, F. G., Me´ndez-Castrillo´n, P. P.; San Fe´lix, A. Synthesis
1984, 509-510. (b) Lindhorst, T. K.; Kieburg, C. Synthesis 1995, 1228-
1230.
(17) (a) Shibanuma, T.; Shiono, M.; Mukaiyama, T. Chem. Lett.
1977, 573-574. (b) Kritia´n, P.; Sprinzl, M., Antos, K. Collect. Czech.
Chem. Commun. 1965, 3658-3663.
(18) Gross, R.; Du¨rner, G.; Go¨bel, M. W. Liebigs Ann. Chem. 1994,
49-58.
(19) Smith, J .; Lira, J . L.; Schneider, S. E.; Anslyn, E. V. J . Org.
Chem. 1996, 61, 8811-8818.

Carbohydrate-Based Receptors
J . Org. Chem., Vol. 66, No. 4, 2001 1369
F igu r e 2. Probable mode of complexation of receptor 1 with
ligand 4 in DMSO-d6.
F igu r e 4. Probable struture of the 2:1 (host/guest) complex
of receptor 1 with ligand 4 in CDCl3.
F igu r e 3. (a) J ob plot of 1 + 4 in CDCl3 at a total concentra-
tion of 5 mM (H-1). (b) Plot of the chemical shifts of the H-1
resonance of receptor 1 (c ) 5 mM) vs concentration of ligand
4 in CDCl3 at 303 K.
(Figure 2). Furthermore, the resolution of the spectra
significantly improved in the presence of ligand, which
is consistent with the postulated formation of trans-
bidentated hydrogen bonds that anchors the Z,Z config-
uration at the thiourea segments upon complex forma-
tion.23 Nonlinear regression analysis24 of the resulting
curve indicated relatively weak binding (Kas 391 M-1),25
which is in contrast with the much higher association
constant reported for a similar bis(thiourea) structure
bearing butyl substituents at the N-atoms.8a Possibly, the
higher lipophilicity of the alkyl chains at the vicinity of
the hydrogen bond donor centers in the latter facilitate
the dessolvation processes of the incoming carboxylate
guest. In CDCl3, however, where interactions other that
hydrogen bonds are suppressed, the titration data and
continuous variation plots (Figure 3) were consistent with
an equilibrium between 16 and a 2:1 (host/guest) com-
plex, probably having structure 17 (Figure 4). Since the
isotherm begins with host 1 in excess, the 2:1 complex
17 is first formed, which exists in highest concentration
near 0.5 equiv of ligand. As the concentration of 4
increases, the equilibrium shifts to predominantly the 1:1
complex 16. Fitting the curve to a two-equilibria expres-
sion26 gave microscopic binding constants Kas1 and Kas2
of 235 and 247 M-1, respectively.25
F igu r e 5. Probable mode of complexation of receptor 2 with
ligand 4 in both DMSO-d6 and CDCl3.
F igu r e 6. (a) J ob plot of 3 + 4 in CDCl3 at a total concentra-
tion of 5 mM (H-3). (b) J ob plot of 3 + 4 in DMSO-d6 at a total
concentration of 5 mM (H-1).
tated complexes with 4. Notwithstanding, the corre-
sponding binding isotherms and J ob’s plots evidenced
significant differences in their binding behavior. Receptor
2 formed the expected 1:1 complex 18 either in CDCl3 or
in DMSO-d6 solution (Figure 5), with Kas values that
exceeded those that can be measured by NMR titration
experiments (>106 M-1). Binding was still clearly observ-
able at 10% D2O/DMSO-d6 (Kas 103 M-1). In the case of
3, the NMR data did not fit for the 1:1 host:guest binding
model (Figure 6). In CDCl3 the binding data suggested
an equilibrium between the 1:1 (19) and a 2:1 (host:guest)
complex (Kas1 104 M-1, Kas2 103 M-1),25 probably having
structure 20. This is in agreement with the high tendency
of m-xylylene bis(thiourea) fragments to form tetraden-
tated hydrogen bond patterns with carboxylate groups.8e
In DMSO-d6 the titration curves and J ob’s plots (Figure
6) were consistent with a 1:2 stoichiometry (21) at high
The podant-like tetrathiourea receptors 2 and 3 both
incorporate the potential to form bimolecular octaden-
(23) Although a fast equilibrium between monodentate complexes
is also possible, strong binding necessarily involves the Z,Z-rotameric
form of the thiourea group. See: Haushalter, K. A.; Lan, J .; Roberts,
J . D. J . Am. Chem. Soc. 1996, 118, 8891-8896.
(24) Wilcox, C. S. In Frontiers in Supramolecular Chemistry and
Photochemistry; Schneider, H. J ., Durr, H., Eds.; VCH: Weinheim,
1991; p 123.
(25) Binding constants were obtained in triplicate at 303 K. Esti-
mated errors are (10% for the host:guest fitting and (20% for the
host-host-guest and host-guest-guest fittings. We thank Dr. C. A.
Hunter for kindly providing the fitting program.
(26) Kneeland D. M.; Ariga, K.; Lynch, V. M.; Huang, C.-Y.; Anslyn,
E. V. J . Am. Chem. Soc. 1993, 115, 10042-10055.

1370 J . Org. Chem., Vol. 66, No. 4, 2001
Benito et al.
F igu r e 7. Probable struture of the 1:1 (19), 2:1 (20), and 1:2
(host/guest) complex (21) of receptor 3 with ligand 4 in CDCl3
or DMSO-d6.
F igu r e 8. Comparative 1H NMR spectra (sugar proton region,
500 MHz, CDCl3) of octyl â-D-glucopyranoside 5 (a), an
equimolecular mixture of 1 and 5 (b), and receptor 1 (c).
25
ligand concentrations (Kas1 105 M-1, Kas2 102 M-1
)
(Figure 7).
The above ensemble of thermodynamic data illustrates
the dramatic influence that subtle changes in the relative
disposition of recognition elements may have in the
binding properties of flexible hydrogen-bonding receptors.
Structures 17, 18, and 19 represent geometrically equiva-
lent supramolecular arrangements, with the glutarate
ligand, in the most stable transoid conformation, fully
hydrogen-bonded solvated by the four surrounding thio-
urea groups. The marked differences in their relative
stability (18 > 19 . 17) underline the advantages of
positioning hydrogen-bonding sites close together in the
host, where they are less effectively solvated than when
widely spaced. Moreover, the cooperativity of multipoint
interactions at both ends of the receptor facilitates an
optimal fitting between the host and the dicarboxylate
ligand.
The binding behavior of 1-3 toward glucoside 5 in
CDCl3 revealed a totally different scenario. First, the
titration experiments and J ob plots were indicative of the
1:1 stoichiometry in all cases. Second, the Kas value for
the 1:5 complex (900 M-1) was almost three times higher
than Kas values for the 2:5 (Kas 314 M-1) and 3:5 (Kas 304
M-1) complexes.25 No significant difference in binding
efficiency was found between the two latter. Most prob-
ably, compounds 2 and 3 act solely as hydrogen bond
donating receptors, whereas additional hydrogen-bonding
interactions between the OH groups of 5 and the ester
functionalities of 1 operate in the case of the 1:5 complex.
The observation of weak intermolecular ROE contacts
between the sugar units of the host and the ligand in
the latter, which were absent in the case of the 2:5 and
3:5 complexes, supports this hypothesis.
The dramatic improvement of spectral resolution in the
1:5 complex as compared with the unbound host 1 (Figure
8) is consistent with anchoring of the receptor conforma-
tion and formation of a relatively rigid supramolecular
architecture. The highfield complexation-induced chemi-
cal shifts of the ligand protons clearly shows that the
pyranose ring lies over the ring current of the benzene
π-system in the binding site. This effect is much stronger
for the H-2 and H-4 protons, suggesting that the â-face
of the glycoside is closer to the center of the aromatic
ring. This relative disposition allows the formation of
four NH‚‚‚O hydrogen bonds involving the ligand O-2,
O-3, O-4, and O-6 atoms, with both thiourea segments
in the Z,Z configuration as shown in Figure 9 (22).
Additionally, at least two acetyl groups of the host
(located at C-2 and C-6) can act as acceptor centers in
intermolecular OH‚‚‚O hydrogen-bonding interactions,
which probably accounts for the unexpectedly high as-
sociation constant. Interestingly, this binding mode do
not compete with the intramolecular hydrogen-bonding
network around the adjacent OH-2/OH-3/OH-4 triol
system of the glucoside.27 The presence of H-4/benzylic
NH, H-4/Ar and H-2/Ar intermolecular cross-peaks in the
ROESY spectra further supports this possible mode of
complexation.

Carbohydrate-Based Receptors
J . Org. Chem., Vol. 66, No. 4, 2001 1371
F igu r e 9. Probable mode of complexation of receptor 1 with
ligand 5 in CDCl3.
F igu r e 10. Schematic representation of the complexes of the
tetrathiourea receptors 2 and 3 with ligand 5 in CDCl3.
Significant complexation-induced highfield chemical
shifts were also observed for the ligand protons in the
complexes of the tetrathiourea hosts 2 and 3 with octyl
â-D-glucopyranoside 5 in CDCl3, suggesting its location
in an aromatic ring surrounded cavity. In contrast,
changes on line shape and chemical shifts in the 1H NMR
spectra of the tetrathiourea hosts 2 and 3 upon addition
of ligand 5 were much less pronounced. Line broadening
was still evident in the presence of high proportions of
ligand, indicating that slow chemical exchange processes,
probably associated to restricted rotations about NH-
C(dS) bonds, occur in the complexes. These observations
can be rationalized assuming that the recognition pattern
above-discussed, involving two xylylene-bridged thiourea
groups and four O-atoms of the pyranose ring, is the main
binding mode that operates in these systems.28 Although
supramolecular arrangements with a higher degree of
hydrogen-bonding solvation of the sugar ligand are
accessible, the additional intermolecular hydrogen bonds
would be formed either at the expenses of the intramo-
lecular OH‚‚‚OH interactions29 or would imply single two
centers NH‚‚‚O interactions. Such situations will con-
tribute only marginally to the complexing enthalpy, while
the entropy penalty will be presumably high. Only one
of the three xylylene bis(thiourea) fragments present in
the structutre of hosts 2 and 3 is efficiently participating
in binding at a time, the rest of the bound receptor
molecule keeping a high mobility degree. The ligand
molecule is probably in fast exchange between the cluster
of binding sites (Figure 10), the average result being
equivalent to an aromatic-surrounded cavity (23).
actions are strong (e.g., thiourea-carboxylate associa-
tions). In contrast, the need for a maximum effective
surface contact between host and guest in the bound state
for recognition processes characterized by weak affinities
(e.g., thiourea-hydroxyl associations) largely overcomes
the contributions to the binding energy arising from
differences in the arrangement of recognition elements
in the unbound state, a situation already encountered
in natural systems.30 Identification of recognition ele-
ments and quantification of individual interactions should
be, then, of particular relevance in the design of artificial
sugar-based receptors for hydrogen bond recognition of
molecules possessing simultaneously strong and weak
hydrogen-bonding domains, such as carboxylate (uronic
acids, ulosonic acids) and phosphate-containing carbo-
hydrate derivatives (nucleotides, sugar phosphates).
Work in this sense is currently under progress in our
group.
Exp er im en ta l Section
Gen er a l Meth od s. Optical rotations were measured at
1
room temperature in 1-dm tubes. H (13C) NMR spectra were
recorded at 500 (125.7) or 300 (75.5) MHz. 2D ROESY
experiments were obtained at 500 MHz in the phase-sensitive
mode with time proportional phase incrementation (TPPI) and
at two different mixing times (200 and 400 ms). In the FABMS
spectra, the primary beam consisted of Xe atoms with a
maximum energy of 8 keV. The samples were dissolved in
m-nitrobezyl alcohol or thioglycerol and the positive ions were
separated and accelerated over a potential of 7 keV. NaI was
added as cationizing agent. TLC was performed with precoated
TLC plates, silica gel 30F-245, with visualization by UV and
by charring with 10% sulfuric acid. Microanalyses were
performed by the Instituto de Investigaciones Quı´micas (Sevi-
lla, Spain).
Ma ter ia ls. Tetra-n-butylammonium (TBA) glutarate (4)
was prepared from glutaric acid by addition of 2 equiv of TBA
hydroxide and freeze-drying of the resulting dicarboxylate salt.
Octyl â-D-glucopyranoside 5 was a commercial compound.
2,3,4,6-Tetra-O-acetyl-â-D-glucopyranosyl isothiocyanate 7 was
synthesized from acetobromoglucose following the methodology
of Camarasa et al.16 1,4- and 1,3-bis(isothiocyanatomethyl)-
benzene 8 and 12 were obtained by isothiocyanation of the
corresponding commercially available diamines with thiophos-
gene as reported.17,18 The N-tert-butyloxycarbonyl monopro-
tected diamine derivatives 9 and 13 were prepared by the
reaction of bis(tert-butyloxycarbonyl) carbonate with an excess
of the corresponding diamine.18,19
It is noteworthy that the much weaker hydrogen bond
acceptor ligand 5, as compared with 4, exhibits a higher
efficiency in inducing an optimal 1:1 host-guest fitting.
This suggests that the requirements for multipoint
recognition and binding through hydrogen bonds depend
strongly on the nature of the individual interactions
between complementary recognition elements in the host
and guest. Preorganization of hydrogen bond donor
centers in a flexible host is crucial to achieve a well
defined supramolecular organization if individual inter-
(27) Docking studies were performed by using the MACROMODEL
v6.0 package and the GB/SA continuum solvent model for chloroform.
Initially, the host molecule was extensively minimized using the MM2*
program, with the thiourea groups in the Z,Z configuration and the
anti conformation at the glycosidic bonds. The global minimum was
then used to dock the ligand, with the octyl chain in the transoid
conformation and oriented to satisfy the exo-anomeric effect (Φ angle
H-1-C-1-O-1-octyl of ca. 60°). The ligand was manually docked into
the binding site and extensively minimized. The three-dimensional
geometries obtained were in agreement with the discussed NMR data.
(28) An analogous binding mode has been proposed for a m-xylylene-
derived receptor incorporating phosphonate groups as bidentated
hydrogen bond acceptor centres. See cf. ref 18a.
Bin d in g Titr a tion s. CDCl3 used for binding experiments
was deacidified and partially dried by storage over basic
alumina. DMSO-d6 was stored over 4 Å molecular sieve and
(30) (a) Navarre, N.; Amiot, N.; van Oijen, A.; Imberty, A.; Poveda,
A.; J ime´nez-Barbero, J .; Cooper, A.; Nutley, M. A.; Boons, G.-J . Chem.
Eur. J . 1999, 5, 2281-2294. (b) Wacowich-Sgarbi, Bundle, D. R. J .
Org. Chem. 1999, 64, 9080-9089. (c) Alibe´s, R.; Bundle, D. R. J . Org.
Chem. 1998, 63, 6288-6301.
(29) Huang C.-Y. Cabell, L. A., Anslyn, E. V. J . Am. Chem. Soc.
1994, 116, 2778-2792.

1372 J . Org. Chem., Vol. 66, No. 4, 2001
Benito et al.
contained 0.2% water as detected by 1H NMR. The association
constants at 303 K were experimentally determined by mea-
suring the proton chemical shift changes of 5 mM solutions of
the thiourea receptor 1-3 upon increased amounts of the
corresponding guest 4 or 5. In a typical titration experiment,
a 5 mM solution of host in CDCl3 or DMSO-d6 was prepared,
a 500 µL aliquot was transferred to a 5 mm NMR tube, and
the initial NMR spectrum was recorded. A solution (20-75
mM) of the guest in the previous host solution was prepared
and then added via microsyringe initially in 10 µL portions.
These amounts were increased until complete complexation
of the host. The 1H NMR spectrum of each solution was
recorded and the chemical shift of the diagnostic sugar protons
obtained at 12-15 different host-guest concentration ratios
were used in an iterative least-squares fitting procedure.
Inverse titration experiments were also carried out for the
complexes between receptors 1-3 and ligand 5.
used in the next step without further purification: FABMS
m/z 515 (10%, [M + Na]+); 1H NMR (500 MHz, CD3OD) δ 4.08
(s, 4 H, CH2NH2), 4.70, 4.77 (2 s, each 4 H, CH2NHCS), 7.20-
7.40 (m, 12 H, Ph); 13C NMR (125.7 MHz, CD3OD) δ 43.8, 48.5,
126.9-135.5, 183.1.
1,4-Bis[3-(2,3,4,6-t et r a -O-a cet yl-â-D-glu cop yr a n osyl)-
th iou r eid om eth yl]ben zen e (1). To a solution of 1,4-bis-
(aminomethyl)benzene (6, 68 mg, 0.5 mmol) in CH2Cl2 (5 mL)
was added 2,3,4,6-tetra-O-acetyl-â-D-glucopyranosyl isothio-
cyanate (7, 389 mg, 1 mmol). The reaction mixture was stirred
for 30 min at room temperature and concentrated. Further
purification of the residue by column chromatography (20:1
CH2Cl2-MeOH) yielded 1 (355 mg, 78%) as an amorphous
solid: Rf 0.56 (9:1 CH2Cl2-MeOH); [R]D +1.0 (c 1.0, CH2Cl2);
FABMS m/z 937 (40, [M + Na]+); 1H NMR (500 MHz, CDCl3)
δ 2.09, 2.06, 2.03, 2.01 (4 s, each 6 H, Ac), 3.82 (ddd, 2 H, J )
2.2, 4.7, 9.7 Hz, H-5), 4.00 (bd, 2 H, J ) 4.7, 12.6 Hz, H-6b),
4.27 (dd, 2 H, J ) 2.2, 12.6 Hz, H-6a), 4.67 (bs, 4 H, CH2),
4.94 (t, 2 H, J ) 9.7 Hz, H-2), 5.06 (t, 2 H, J ) 9.7 Hz, H-4),
5.34 (t, 2 H, J ) 9.7 Hz, H-3), 5.68 (bt, 2 H, J ) 9.7 Hz, H-1),
6.56, 6.62 (2 bs, each 2 H, NH), 7.26 (s, 4 H, Ph); 13C NMR
(125.7 MHz, CDCl3, 313 K) δ 20.7, 20.6, 20.5, 48.6, 61.7, 68.3,
69.5, 72.8, 73.4, 82.7, 128.4, 136.9, 169.1-171.2, 183.6. Anal.
Calcd for C38H50N4O18S2: C, 49.88; H, 5.51; N, 6.12. Found:
C, 49.88; H, 5.55; N, 6.13.
J ob P lots. Stock solutions of host and guest were prepared
(5 mM each) and separated into 5-mm NMR tubes to give the
following host:guest volume ratio: 5:0, 4:1, 3:3, 2:3, 1:4. 1H
NMR spectra of all samples were obtained and the concentra-
tion of complex ([C]) for each solution was determined from
the equation
[C] ) [H]0(δobs - δ0)/( δmax - δ0)
1,4-B is [3-[3-[3-(2,3,4,6-t e t r a -O -a c e t y l-â-D -g lu c o p y -
r a n osyl)t h iou r eid om et h yl]b en zyl]t h iou r eid om et h yl]-
ben zen e (2). A solution of the diamine 11 (73 mg, 0.15 mmol)
in acetone-water (1:1, 3 mL) was adjusted to pH 8 by addition
of saturated aqueous NaHCO3. 2,3,4,6-Tetra-O-acetyl-â-D-
glucopyranosyl isothiocyanate (7, 129 mg, 0.33 mmol) in
acetone (1.5 mL) was then added, and the reaction mixture
was stirred for 2 h at room temperature. Acetone was removed
by distillation and the aqueous residue was extracted with
CH2Cl2 (3 × 5 mL). The organic solution was concentrated,
and the resulting residue was purified by column chromatog-
raphy (5:1 EtOAc-petroleum ether) to afford 2 (134 mg, 70%)
as an amorphous solid: Rf 0.52 (5:1 EtOAc-petroleum ether);
[R]D 0.0 (c 1.0, CH2Cl2); FABMS m/z 1293 (20, [M + Na]+); 1H
NMR (500 MHz, CDCl3) δ 2.02, 2.01, 2.00, 1.99 (4 s, each 6 H,
Ac), 3.79 (m, 1 H, H-5), 3.89 (bd, 2 H, J ) 12.2 Hz, H-6b), 4.27
(dd, 2 H, J ) 4.0, 12.2 Hz, H-6a), 4.62 (bs, 12 H, CH2), 4.95 (t,
2 H, J ) 9.1 Hz, H-2), 5.01 (t, 2 H, J ) 9.1 Hz, H-4), 5.29 (t,
2 H, J ) 9.1 Hz, H-3), 5.70 (bt, 2 H, J ) 9.1 Hz, H-1), 6.77,
7.01 (2 bs, each 4 H, NH), 7.16-7.18 (m, 12 H, Ph); 13C NMR
(75.5 MHz, CDCl3, 313 K) δ 21.2, 20.8, 20.5, 20.3, 47.7, 47.9,
60.1, 68.3, 70.6, 72.9, 73.3, 82.6, 125.1-138.0, 169.5-170.8,
181.9, 183.5. Anal. Calcd for C56H70N8O18S4: C, 52.90; H, 5.55;
N, 8.81. Found: C, 52.89; H, 5.53; N, 8.80.
where [H]0 is the preequilibrium host concentration, δobs is the
observed chemical shift, δ0 is the chemical shift of the free host,
and δmax is the chemical shift of the complex. The conventional
J ob plot ([C]eq vs [H]0/[H]0 + [G]0) was then determined.
1,4-Bis[3-[3-(ter t-bu toxyca r bon yla m in om eth yl)ben zyl]-
t h iou r eid om et h yl]b en zen e (10). A solution of 1,4-bis-
(isothiocyanatomethyl)benzene 8 (125 mg, 0.3 mmol) in CH2Cl2
(5 mL) was added to a solution of the mono-Boc-protected
diamine 9 (280 mg, 1.2 mmol) in CH2Cl2 (5 mL). The reaction
mixture was stirred for 3 h, evaporated and purified by column
chromatography eluting first with 1:1 EtOAc-petroleum ether
and then with MeOH, to give 10 (181 mg, 47%) as an
amorphous solid: Rf 0.15 (1:1 EtOAc-petroleum ether);
FABMS m/z 715 (100, [M + Na]+); 1H NMR (300 MHz, DMSO-
d6) δ 1.38 (s, 18 H, CMe3), 4.09 (d, 4 H, J ) 6.2 Hz, CH2NHCO),
4.64 (bs, 8 H, CH2NHCS), 7.09-7.28 (m, 12 H, Ph), 7.36 (t, 2
H, J ) 6.2 Hz, NHCO), 7.91 (bs, 4 H, NHCS); 13C NMR (75.5
MHz, DMSO-d6) δ 28.3, 43.4, 46.8, 77.8, 125.4-140.2, 155.1,
182.9. Anal. Calcd for C36H48N6O4S2: C, 62.40; H, 6.98; N,
12.13. Found: C, 62.40; H, 6.90; N, 12.12.
1,4-Bis[3-[3-(a m in om eth yl)ben zyl]th iou r eid om eth yl]-
ben zen e (11). Compound 10 (205 mg, 0.30 mmol) was treated
with 50% TFA-CH2Cl2 (5 mL) for 1 h and evaporated under
reduced pressure to give 11 as a hygroscopic solid which was
used in the next step without further purification: FABMS
1,3-B is [3-[4-[3-(2,3,4,6-t e t r a -O -a c e t y l-â-D -g lu c o p y -
r a n osyl)t h iou r eid om et h yl]b en zyl]t h iou r eid om et h yl]-
ben zen e (3). The coupling reaction of 15 (73 mg, 0.15 mmol)
and 7 (129 mg, 0.33 mmol) in water-acetone at pH 8, following
the procedure described above for the preparation of 2 and
purification by column chromatography (3:1 EtOAc-petroleum
ether) afforded 3 (140 mg, 73%) as an amorphous solid: Rf
0.25 (3:1 EtOAc-petroleum ether); [R]D + 84.0 (c 1.0, CH2-
1
m/z 493 (100, [M + H]+); H NMR (500 MHz, CD3OD, 313 K)
δ 4.06 (s, 4 H, CH2NH2), 4.69, 4.76 (2 s, each 4 H, CH2NHCS),
7.24-7.37 (m, 12 H, Ph); 13C NMR (125.7 MHz, CD3OD, 313
K) δ 43.0, 47.2, 127.2-140.0, 183.1.
1,3-Bis[3-[4-(ter t-bu toxyca r bon yla m in om eth yl)ben zyl]-
t h iou r eid om et h yl]b en zen e (14). Coupling reaction of 12
(125 mg, 0.3 mmol) and 13 (280 mg, 1.2 mmol) and purification
as above-described for the preparation of 10 afforded 14 (250
mg, 65%) as an amorphous solid: Rf 0.15 (1:1 EtOAc-
petroleum ether); FABMS: m/z 715 (40, [M + Na]+); 1H NMR
(300 MHz, DMSO-d6) δ 1.38 (s, 18 H, CMe3), 4.07 (d, 4 H, J )
6.1 Hz, CH2NHCO), 4.64 (bs, 8 H, CH2NHCS), 7.15-7.28 (m,
12 H, Ph), 7.36 (t, 2 H, J ) 6.1 Hz, NHCO), 7.92 (bs, 4 H,
NHCS); 13C NMR (75.5 MHz, DMSO-d6) δ 28.6, 43.1, 46.9,
77.8, 125.8-139.3, 155.8, 183.0. Anal. Calcd for C36H48N6O4
S2: C, 62.40; H, 6.98; N, 12.13. Found: C, 62.41; H, 6.87; N,
11.96.
1
Cl2); FABMS m/z 1293 (10, [M + Na]+); H NMR (500 MHz,
CDCl3) δ 2.02, 1.99, 1.98, 1.97 (4 s, each 6 H, 8 Ac), 3.78 (m,
2 H, H-5), 3.95 (bd, 2 H, J ) 12.0 Hz, H-6b), 4.17 (bd, 1 H, J
) 12.0 Hz, 2 H-6a), 4.48 (bs, 12 H, CH2), 4.93 (t, 2 H, J ) 9.4
Hz, H-2), 4.98 (t, 2 H, J ) 9.4 Hz, H-4), 5.28 (t, 2 H, J ) 9.4
Hz, H-3), 5.75 (bt, 2 H, J ) 9.4 Hz, H-1), 6.75, 6.88 (2 bs, each
4 H, NH), 7.10-7.28 (m, 12 H, Ph); 13C NMR (75.5 MHz,
CDCl3, 313 K) δ 20.6, 20.4 (MeCO), 47.9, 48.0, 61.7, 68.3, 70.6,
72.9, 73.2, 82.5, 125.1-137.8, 169.6-170.7, 181.5, 183.0. Anal.
Calcd for C56H70N8O18S4: C, 52.90; H, 5.55; N, 8.81. Found:
C, 52.65; H, 5.43; N, 8.75.
Ack n ow led gm en t. We thank the Direccio´n General
de Investigacio´n Cient´ıfica y Te´cnica (Grant No. PPQ
2000-1341) for financial support.
1,3-Bis[3-[4-(a m in om eth yl)ben zyl]th iou r eid om eth yl]-
ben zen e (15). Removal of the Boc groups of 14 (205 mg, 0.30
mmol) with 1:1 TFA-CH2Cl2, as described above for the
preparation of 11, gave 15 as a hygroscopic solid which was
J O001508N