Synthesis of structurally diverse and defined bivalent mannosides on saccharide scaffolding

Article abstract of DOI:10.1021/ol047841l

(Chemical Equation Presented) The synthesis of bivalent mannosides by the grafting of α-D-mannopyranoside onto monosaccharide acceptors and conjugation to terephthalic acid or phenylenediamine is described. Computational methods were used to predict acces

Full text of DOI:10.1021/ol047841l

ORGANIC
LETTERS
2005
Vol. 7, No. 2
211-214
Synthesis of Structurally Diverse and
Defined Bivalent Mannosides on
Saccharide Scaffolding
Manuela Tosin,† Sebastien G. Gouin,† and Paul V. Murphy*
Centre for Synthesis and Chemical Biology, Chemistry Department, Conway Institute
of Biomolecular and Biomedical Research, UniVersity College Dublin,
Belfield, Dublin 4, Ireland
paul.V.murphy@ucd.ie
Received October 19, 2004
ABSTRACT
The synthesis of bivalent mannosides by the grafting of
r-D-mannopyranoside onto monosaccharide acceptors and conjugation to terephthalic
acid or phenylenediamine is described. Computational methods were used to predict accessible orientations and distances between the
mannose units.
Cells communicate through complex interaction of carbo-
hydrate polymers with their receptors.1 Such biopolymers
constitute a high-density coding system. Multivalent carbo-
hydrates2,3 form part of this polymer class and are important
in generating high-affinity binding, mediating cell-cell
recognition, adhesion, and modulation of signal transduction.
Synthetic multivalent ligands have proven to be useful in
defining new biological mechanisms.4 Mechanisms of bind-
ing of such ligands to receptors are diverse; for example,
cross-linking of lectins by multivalent ligands5 as well as
chelate effects operate. Opportunities exist for the develop-
ment of therapeutics6 and vaccines.7 Small glycoclusters can
exhibit interesting properties. For example a synthetic com-
pound can be identified, from a series of rigidified multi-
valent ligands, each exposing the same headgroup (lactose),
that exhibits selective blocking of one galectin when evalu-
ated against a panel of galectins.8 This suggests more
generally that detailed three-dimensional structure-activity
relationships of multivalent ligands will be interesting. These
have not been explored to date, although crystal structures
of ligand-receptor complexes are known and relationships
(6) (a) Simanek, E. E.; McGarvey, G. J.; Jablonski, J. A.; Wong, C.-H.
Chem. ReV. 1998, 98, 833 and references cited therein. (b) Mowery, P.;
Yang, Z. Q.; Gordon, E. J.; Dwir, O.; Spencer, G.; Alon, R.; Kiessling, L.
L. Chem. Biol. 2004, 11 725. (c) Kitov, P. I.; Sadowska, J. M.; Mulvey,
G.; Armstrong, G. D.; Ling, H.; Pannu, N. S.; Read, R. J.; Bundle, D. R.
Nature 2000, 403, 669.
(7) (a) Liebe, B.; Kunz, H. Angew. Chem., Int. Ed. Engl. 1997, 36, 618.
(b) Hummel, G.; Schmidt, R. R. Tetrahedron Lett. 1997, 38, 1173. (c)
Ragupathi, G.; Coltart, D. M.; Williams, L. J.; Koide, F.; Kagan, E.; Allen,
J.; Harris, C.; Glunz, P. W.; Livingston, P. O.; Danishefsky, S. J. Proc.
Natl. Acad. Sci. U. S.A. 2002, 99, 13699.
† These authors made an equal contribution.
(1) Gabius, H.-J.; Siebert, H. C.; Andre, S.; Jiminez-Barbero, J.; Rudiger,
H. ChemBioChem 2004, 5, 740.
(2) Lee, Y. V.; Townsend, R. R.; Hardy, M. R.; Lo¨nngren, J.; Arnarp,
J.; Haraldsson, M.; Lo¨nn, H. J. Biol. Chem. 1983, 258, 199.
(3) Mammen, M.; Choi, S.-K.; Whitesides, G. M. Angew. Chem., Int.
Ed. 1998, 37, 2754.
(4) Gestwicki, J. E.; Kiessling, L. L. Nature 2002, 415, 81.
(5) (a) Andre´, S.; Ortega, P. J. C.; Perez, M. A.; Roy, R.; Gabius, H.-J.
Glycobiology 1999, 9, 1253. (b) Brewer, C. F. Biochim. Biophys. Acta 2002,
1572, 255.
(8) (a) Vrasidas, I.; Andre, S.; Valentini, P.; Bock, C.; Lensch, M.;
Kaltner, H.; Liskamp, R. M. J.; Gabius, H.-J.; Pieters, R. J. Org. Biomol.
Chem. 2003, 1, 803-810. (b) Andre´, S.; Liu, B.; Gabius, H.-J.; Roy, R.
Org. Biomol. Chem. 2003, 1, 3909.
10.1021/ol047841l CCC: $30.25
© 2005 American Chemical Society
Published on Web 12/24/2004

between architecture and biological activity have been
described (e.g., small clusters versus dendrimers versus
polymers). For many glycocluster structures, it may be
difficult to define the bioactive conformations that the ligands
adopt if flexible scaffolding is used for display of the
recognition component. Herein we describe synthesis of
ligands with potential to cross-link mannose receptors and
exploration of their conformation by computational methods.
Scaffolds9,10 based on glycosylamides 1 (Figure 1) have
been synthesized and their structure investigated.11,12 We
Figure 2. Top graphs show mannose orientation A plotted vs
mannose orientation B for 1000 conformers of 2-4 sampled during
stochastic dynamics simulations. The profile for 2 is blue, that for
3 is green, and that for 4 is red. The distance between mannose
residues (Å) as a function of terephthalamide torsion is shown
bottom left. The mannose orientation B as a function of tere-
phthalamide torsion for 4 is shown at the bottom right. The
Man-Man distance for 3 is 16-20 Å and that for 4 is 10-14 Å,
when similar orientations of mannose are found for 3 and 4 (i.e.,
when terephthalamide torsion is 0 ( 60°).
Figure 1. Two-dimensional structures of 1-5. Red sphere ) R-D-
mannopyranoside.
methods (Macromodel 8.5)14 before synthesis of the com-
pounds. All calculations were carried out using the GB/SA
solvation model15 for water and the OPLS-AA force field.16
First, the favored angles for the dihedrals Φ and Ψ17 for the
glycosidic linkage between the mannose and glucose/
glucuronic acid residues were calculated by systematic
exploration of Φ and Ψ space and also by conformational
searching techniques, of model disaccharides.18,19 The gly-
cosidic torsion angles for the lowest energy structures agreed
with related disaccharides calculated previously.20
envisaged grafting the R-D-mannopyranoside headgroup onto
hydroxyl groups of the core structure 1 or its related glucose
analogue to generate 2-4 (Figure 2) with potential to cross-
link mannose receptors. Because of a limited number of
degrees of conformational freedom, it is possible to predict
or determine the spatial relationships between their mannose
residues. In addition, we designed 5, a dimer built on a
phenylenediamine unit.13 A prediction of the preferred
conformations of 2-4 was first undertaken by computational
(13) Tosin, M.; Mu¨ller-Bunz, H.; Murphy, P. V. Chem. Commun. 2004,
494.
(14) Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.;
Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput.
Chem. 1990, 11, 440.
(9) Hirschmann, R.; Nicolaou, K. C.; Pietranico, S.; Salvino, J.; Leahy,
E. M.; Sprengeler, P. A.; Furst, G.; Smith, A. B., III; Strader, C. D.; Cascieri,
M. A.; Candelore, M. R.; Donaldson, C.; Vale, W.; Maechler, L. J. Am.
Chem. Soc. 1992, 114, 9217.
(10) Wunberg, T.; Kallus, C.; Opatz, T.; Henke, S.; Schmidt, W.; Kunz,
H. Angew. Chem., Int. Ed. 1998, 37, 2503.
(11) Murphy, P. V.; Bradley, H.; Tosin, M.; Pitt, N.; Fitzpatrick, G. M.;
Glass, W. K. J. Org. Chem. 2003, 68, 5693.
(15) Still, W. C.; Tempczyk, A.; Hawley, R. C.; Hendrickson, T. J. Am.
Chem. Soc. 1990, 112, 6127.
(16) Jorgensen, W. L.; Maxwell, D. S.; Tirado-Rives, J. J. Am. Chem.
Soc. 1996, 118, 11225.
(12) Avalos, M.; Babiano, R.; Carretero, M. J.; Cintas, P.; Higes, F. J.;
Jime´nez, J. L.; Palacios, J. C. Tetrahedron 1998, 54, 615.
(17) Glycosidic torsion Φ is defined as H1-C1-Oxc-Cx, and Ψ is
defined as C1-Ox-Cx-Hx•.
212
Org. Lett., Vol. 7, No. 2, 2005

Scheme 1
Scheme 2
for each molecule. It is apparent that, even if 2-4 can access
conformations where the mannose orientations are similar,
the distance between the mannose residues still differs. For
example, the profiles for 3 (green) and 4 (red) have regions
of overlap when the mannose orientation A is between -180
and 0° and when mannose orientation B ) 0-120°; the plot
that shows the relationship between mannose orientation B
and the terephthalamide torsion indicates that this arrange-
ment occurs when the terephthalamide torsion is between
-60 and 60°. However, in this case the distance between
mannose residues for 3 is 16-20 Å, whereas for 4 it is
10-14 Å.
The synthesis of novel bivalent structures was undertaken
having established that they would have distinct mannose
presentations. We have assumed that the presentation of
mannosides on glucuronic acid or glucose-based scaffolds
would be equivalent and chose our targets on the basis of
the envisaged ease of synthesis. The synthesis of 2 was
achieved from 6 (Scheme 1).25 Saponification of 6 followed
Experimental evidence suggests that the (Z)-anti confor-
mation is preferred for the amide of type 1,21 and a pre-
sumption was made that this would also be the case for
2-4.22 In addition, we assumed that two arrangements, simi-
lar to 1a and 1b, where the carbonyl groups are coplanar
(or close to coplanar) with the aromatic ring would be acces-
sible to 2-4.23 Models were thus built for the divalent
compounds and stochastic dynamics simulations carried out
to explore conformational space accessible to 2-4.24 Struc-
tures (1000) were sampled during the course of simulations
and used for the analysis. What is of interest is the relative
spatial orientation of the two mannose residues and the
distance between these units. These were profiled as outlined
in Figure 2. The plots predict that presentations of mannose
residues, in terms of orientation and distance, are diverse
(18) Both dihedral drive and conformational search SUMM methods in
Macromodel 8.5 were used. Structures of model disaccharides are provided
in Supporting Information.
(24) Stochastic Dynamics Procedures. A temperature of 300 K, a time
step of 1.5 fs, an equilibration time of 1.0 ps, and a simulation time of 5 ns
were used, and 500 structures were sampled over the duration of a 5 ns
simulation. Two simulations were carried out for each compound; one was
started from a low-energy structure where the carbonyl groups were cis
(OdC-CdO torsion ) 0°, similar to 1a) and one from a structure where
the carbonyl groups were trans (OdC-CdO torsion ) 180°, similar to
1b). The results of both simulations were combined for the analysis shown
in Figure 2. The glycosylamide torsions (H1-C1-N-H) were constrained
to 180 ( 5°, and a force constant of 2500 was applied. Profiles of the
glycosidic torsion angles and glycosylamide torsion and terephthalamide
torsion for the sampled structures were consistent with expected preferred
conformers for 2-4.
(19) For a review on theoretical approaches to prediction of oligosac-
charide structure, see: Imberty, A.; Pe´rez, S. Chem. ReV. 2000, 100, 4567.
(20) Previous computational work on related disaccharides was carried
out using the HSEA program. See: Jansson, P.-E.; Kenne, L.; Persson, K.;
Widmalm, G. J. Chem. Soc., Perkin Trans. 1 1990, 591.
(21) Anti conformation is defined as a conformational isomer with the
dihedral angle H1-C1-N-H ) 180 ( 90°. Z refers to the amide
configuration.
(22) In addition to previous studies, glycosylamides prepared in our
laboratory, have (Z)-anti conformation in X-ray crystal structures. Rawe,
S.; Murphy, P. V. Private communication.
(23) Both arrangements of carbonyl groups are generated during con-
formational searching protocols to locate low-energy structures.
(25) Gyo¨rgydea´k, Z.; Thiem, J. Carbohydr. Res. 1995, 268, 85.
Org. Lett., Vol. 7, No. 2, 2005
213

Scheme 3
Scheme 4
by reaction with acetic anhydride gave the 6,3-lactone 7. This
lactone reacted readily with alcohols to give 8 or 9. The
Schmidt glycosidation of 8 with imidate 1026 gave 11, which
was subsequently converted, in very low yield,27 to the dimer
12 on reaction with terephthaloyl chloride promoted by a
polymer-supported aryl phosphine. Removal of the protecting
groups from 12 gave 2.
The tetraacetate 1328 was reacted with 10 as above to give
14. This was converted to azide 15 using SnCl4 and azi-
dotrimethylsilane. The amine 16 was prepared from 15 by
catalytic hydrogenation, and its subsequent reaction with
terephthaloyl chloride in the presence of triethylamine in THF
gave the dimer 17 (28%). This was converted to 3 by
Zemplen deacetylation (Scheme 2). The synthesis of 4 was
achieved via the tribenzoate 18,29 which on glycosidation
gave disaccharide 19. Acetolysis gave 20, and this product
was converted to amide 21 via a glycosyl azide. Reaction
of 21 with terephthaloyl chloride in THF in the presence of
triethylamine followed by removal of the protecting groups
gave 4 (Scheme 3).
of 23 with p-phenylenediamine promoted by HBTU/HOBt
gave, after prolonged reaction time, the protected divalent
compound. Removal of the acetate protecting groups by the
standard methods (NaOMe/MeOH, LiOH, hydrazine) was
unexpectedly problematic and gave a mixture containing
unidentified products, but the use of dibutyltin oxide32 in
methanol proved to be satisfactory and gave 5.33
In summary, we described synthesis of structurally diverse
bivalent mannosides on saccharide scaffolding. Mannose-
mannose orientations and distances were determined by
location on the scaffold and preferred glycosidic and tere-
phthalamide torsions. Each compound showed a distinct
three-dimensional structural profile. A comparison of the
biological properties of 2-5 will be reported in due course.
Acknowledgment. Funding was provided by Enterprise
Ireland, IRCSET, the Program for Research in Third-Level
Institutions (PRTLI), administered by the HEA (of Ireland).
P.V.M. thanks Science Foundation Ireland for a Programme
Investigator Grant.
The divalent mannoside 5 was prepared as shown in
Scheme 4.30 The Schmidt glycoside coupling reaction of 9
with 10 gave 22. The allyl ester protecting group was
removed by palladium catalysis31 to give acid 23. Coupling
Supporting Information Available: Analytical data for
1
2-5 and H and 13C-NMR spectra for all new compounds
(26) Upreti, M.; Ruhela, D.; Vishwakarma, R. A. Tetrahedron 2000, 56,
6577.
(27) Better yields (∼30%) are obtained if PPh3 is used, but purification
is more difficult due to competing formation of a soluble iminophosphorane.
The iminophosporane is covalently linked to polymer-based supporting
reagent, which is removed by filtration.
and selected additional conformational analysis on 2-4. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(28) Li, K.; Helm, R. F. Carbohydr. Res. 1995, 273, 249.
(29) Lee, G. S.; Lee, Y.-J.; Choi, S. Y.; Park, Y. S.; Yoon, K. B. J. Am.
Chem. Soc. 2000, 122, 12151.
(30) Conformational profile of 5 will be discussed in a subsequent paper.
(31) Kunz, H.; Waldmann, H. Angew. Chem., Int. Ed. Engl. 1984, 23,
71.
OL047841L
(32) Liu, H.-M.; Yan, X.; Li, W.; Huang, C. Carbohydr. Res. 2002, 337,
1793.
(33) Compounds 2-5 were purified by reverse-phase HPLC.
214
Org. Lett., Vol. 7, No. 2, 2005