Hexaphenylbenzene as a rigid template for the straightforward syntheses of "star-shaped" glycodendrimers

Article abstract of DOI:10.1021/jo102215y

Original glycodendrimers emanating from propargylated hexaphenylbenzene cores and containing up to 54 peripheral sugar ligands have been synthesized by Cu(I)-catalyzed [1,3]-dipolar cycloadditions using both convergent and divergent approaches.

Full text of DOI:10.1021/jo102215y

pubs.acs.org/joc
toward antiinfective strategies to their use in photodynamic
Hexaphenylbenzene as a Rigid Template for the
Straightforward Syntheses of “Star-Shaped”
Glycodendrimers
therapy or as vaccines, adjuvants, immunotherapy, and anti-
angiogenic agents.3 As part of our ongoing research program
aimed at the understanding of initial steps involved in infec-
tious phenomenon via saccharide-lectin recognition processes,
the elaboration and biological evaluation of multivalent
glycomimetic inhibitors are needed.3-5 Hence, the straight-
forward preparation of a new family of multivalent glyco-
sylated architectures built around a more rigid hexaphenyl-
benzene scaffold is proposed. This additional core will help
pursuing our systematic investigations regarding the roles
played by the subtle but critical modulations of structural
parameters responsible for high avidity, with specific and
tailored presentation of peripheral recognition moieties.3 By
virtue of their flexible scaffolds and linkers, previous glyco-
dendrimers showed negative cooperativity upon binding to
lectins as shown by their increasingly contributing entropic
lost.6 Hence, by designing rigidified scaffolds with less flexible
conformational and translational mobilities, we aimed at
counterbalancing the negative effects observed by flexible
scaffolds such as PAMAM, PPI, and polyesters.3 Note-
worthy is the fact that water-soluble dendrimers emanating
from aromatic shape-persistent and rigid scaffolds can pro-
vide interesting properties including host-guest molecular
recognition,7 and induce autoassembly mechanisms respon-
sible for the amplification of the inhibition.8 Furthermore,
the inherent rigidity of the distorted but symmetrical inner
aromatic system, as observed from X-ray crystals of hexakis-
(4-hydroxyphenyl)benzene 1,9 can afford unique homogeneous
epitopes’ spatial orientation. More specifically, although few
examples of glycoclusters organized around a radial hexa-
phenylbenzene core have already been described, synthetic
methodologies remain limited to the [2 þ 2 þ 2] dicobalt
octacarbonyl-catalyzed cyclotrimerizations.10 Those strate-
gies strictly require preliminary preparations of diglycosy-
lated aromatic alkynes. Consequently, the cumbersome
syntheses of these precursors provide rather limited access
to “molecular asterisks” of higher paucivalency.
Yoann M. Chabre,† Patrick P. Brisebois,† Leıla Abbassi,†
¨
Sheena C. Kerr,‡ John V. Fahy,‡ Isabelle Marcotte,† and
,†
ꢀ
Rene Roy*
†
ꢀ
ꢀ
ꢁ
ꢀ
Department of Chemistry, Universite du Quebec a Montreal,
P.O. Box 8888, Succ. Centre-Ville Montreꢀal, QC, Canada
H3C 3P8, and ‡Cardiovascular Research Institute, University
of California, 505 Parnassus Avenue, Box 0111 San Francisco,
California 94143, United States
roy.rene@uqam.ca
Received November 5, 2010
Original glycodendrimers emanating from propargylated
hexaphenylbenzene cores and containing up to 54 per-
ipheral sugar ligands have been synthesized by Cu(I)-
catalyzed [1,3]-dipolar cycloadditions using both conver-
gent and divergent approaches.
Several important biological events such as cellular adhe-
sion and recognition, regulation of physiological functions,
and pathogenic infections are mediated, at the molecular
level, by multiple carbohydrate-protein interactions.1 More
specifically, bacterial adhesion processes are commonly
achieved through carbohydrate-binding lectins expressed
on or shed from bacterial surfaces that can also be involved
in host-tissue colonization and biofilm formation. In spite of
the weaknesses of these key binding interactions in terms
of avidity and selectivity on a per-saccharide basis, these
attractive forces are dramaticaly and naturally reinforced by
the presence of multiple copies of both the ligands and the
receptors that are engaged in a phenomena known as the
“glycoside cluster or dendritic effect”.2
In light of these considerations, we report herein the
design of original polypropargylated scaffolds obtained via a
(3) Chabre, Y. M.; Roy, R. Adv. Carbohydr. Chem. Biochem. 2010, 63,
165–393.
ꢀ
(4) (a) Nagahori, N.; Lee, R. T.; Nishimura, S. I.; Page, D.; Roy, R.; Lee,
Y. C. ChemBioChem 2002, 3, 836–844. (b) Roy, R. Trends Glycosci.
Glycotechnol. 2003, 15, 291–310. (c) Touaibia, M.; Roy, R. Carbohydrate-
Protein and Carbohydrate-Carbohydrate Interactions. In Comprehensive
Glycoscience; Kamerling, J.-P., Ed.; Elsevier: Boston, MA, 2007; Vol. 3, pp
821-870.
(5) For reviews on glycoclusters and glycodendrimers: (a) Chabre, Y. M.;
Roy, R. Curr. Top. Med. Chem. 2008, 8, 1237–1285. (b) Imberty, A.; Chabre,
Y. M.; Roy, R. Chem.;Eur. J. 2008, 14, 7490–7499. (c) Li, Y.; Cheng, Y.;
Xu, T. Curr. Drug Discovery Technol. 2007, 4, 246–254.
ꢀ
(6) Dam, T. K.; Roy, R.; Page, D.; Brewer, C. F. Biochemistry 2002, 41,
1359–1363.
Biological applications of synthetic poly-glycosylated de-
rivatives span from specific screening and targeting of lectins
€
(7) Sakamoto, J.; Mullen, K. Org. Lett. 2004, 6, 4277–4280.
(8) Sleiman, M.; Varrot, A.; Raimundo, J.-M.; Gingras, M.; Goekjian,
P. G. Chem. Commun. 2008, 6507–6509.
(9) Kobayashi, K.; Shirasaka, T.; Sato, A.; Horn, E.; Furukawa, N.
Angew. Chem., Int. Ed. 1999, 38, 3483–3486.
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(2) (a) Lee, Y. C.; Lee, R. T. Acc. Chem. Res. 1995, 28, 321–327. (b) Roy,
R. Curr. Opin. Struct. Biol. 1996, 6, 692–702. (c) Mammen, M.; Choi, S. K.;
Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 2754–2794. (d) Lundquist,
J. J.; Toone, E. J. Chem. Rev. 2002, 102, 555–578.
(10) (a) Roy, R.; Corazon Trono, M.; Giguere, D. ACS Symp. Ser. 2005,
896, 137–150. (b) Dominique, R.; Liu, B.; Das, S. K.; Roy, R. Synthesis 2000,
6, 862–868. (c) Kaufman, R. J.; Sidhu, R. S. J. Org. Chem. 1982, 47, 4941–
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724 J. Org. Chem. 2011, 76, 724–727
Published on Web 12/29/2010
DOI: 10.1021/jo102215y
r
2010 American Chemical Society

Chabre et al.
JOCNote
SCHEME 1. Synthesis of Polypropargylated Hexaphenylben-
zene Derivatives 2 and 4
SCHEME 2. Structures of Glycosyl Azides 5 and 6 and
Synthesis of Dendronized Derivatives 9 and 10
2-azidoethyl 2,3,4,6-tetra-O-acetyl-R-D-mannopyranoside
514 and lactosyl azide 615 containing the necessary comple-
mentary function, were prepared (Scheme 2).
To access higher multivalent functionalities, dendroniza-
tions of sugar azides were achieved from tripropargylated
synthon 3 on which were “clicked” sugar azides 5 or 6 to
afford brominated analogues 7 and 8 in 94 and 84% yield,
respectively. For the necessary convergent strategy, subse-
quent transformation of bromides 7 and 8 into azide deri-
vatives was conveniently carried out in the presence of NaN3
in DMF, yielding desired mannosylated and lactosylated
dendrons 9 and 10, respectively, in more than 91% yields,
both exhibiting an active focal azido function (Scheme 2).
Coupling of monovalent glycosyl azides 5 and 6 on O-
propargylated hexaphenylbenzene core 2 with standard
CuAAc conditions in the presence of substoichiometric
amount of CuSO4 and sodium ascorbate in a homogeneous
THF/water mixture afforded fully substituted star-shaped
hexavalent derivatives 11 and 12 in 79 and 74% yield,
respectively (Scheme 3). De-O-acetylation of lactosylated
postfunctionalization strategy. The corresponding multiva-
lent glycoarchitectures rose from both divergent and con-
vergent accelerated dendritic growth using click chemistry.
In practice, the synthetic sequence was initiated by a double
Stille coupling involving bis(tributylstannyl)acetylene and p-
iodoanisole in the presence of lithium chloride, tetrakis-
(triphenylphosphine)palladium (0), and 2,6-di-tert-butyl-4-
methylphenol in dioxane at reflux, to give the corresponding
symmetrical diarylalkyne in 71% yield (Scheme 1).11
[Co2(CO)8]-catalyzed cyclotrimerization of the resulting
bis(p-methoxyphenyl)acetylene and subsequent methyl ether
deprotection with BBr3 provided efficient access to key hexa-
hydroxylated precursor 1 in 87% yield over two steps.9 Direct
propargylation with propargyl bromide and K2CO3 in DMF
at 65 °C afforded the novel hexafunctionalized analogue 2 in
a satisfactory 82% yield. Multiplication of peripheral propargyl
functions was further addressed with the use of orthogonal
AB3 synthon 2-bromoacetamido-tris[(propargyloxy)methyl]-
aminomethane 3, obtained according to a four-steps sequence
from TRIS (tris(hydroxymethyl)aminomethane) as previously
described.12 The application of similar experimental condi-
tions led to the unprecedented octadecapropargylated scaf-
fold 4 without significant diminution of substitutions’ effi-
ciency (Scheme 1). Completion of the reaction was supported
by analysis of NMR spectra, revealing a unique signal pattern
for extended inner aromatic core consisting of a character-
istic pair of doublets at δ = 6.43 and 6.67 ppm in 1H NMR
and five distinct signals at δ = 113.1, 132.2, 134.2, 140.0, and
154.4 ppm in 13C NMR. Furthermore, expected correspon-
dence between observed integrations for aromatic and acet-
ylenic signals (24 protons vs 18 protons), together with HRMS
analysis confirmed the desired structure. Having key-scaf-
folds 2 (6-mer) and 4 (18-mer) in hands, we next turned our
attention toward the preparation of higher multivalent
architectures containing from 6 to 54 surface carbohydrate
ligands. The introduction of the modified mannopyranoside
or lactopyranoside termini was efficiently achieved using Cu(I)-
catalyzed azide-alkyne [1,3]-dipolar cycloaddition reaction
(CuAAC).13 To this end, known saccharide derivatives,
ꢀ
analogue 12 under usual Zemplen conditions (NaOMe,
MeOH) furnished fully water-soluble “glycoasterisk” 13 in
a satisfactory yield of 77%. Furthermore, a convergent
approach involving treatment of the analogous mannosy-
lated dendron 9 under similar experimental conditions effi-
ciently furnished the extended octadecavalent architecture
14 in a 70% yield.
An accelerated divergent strategy was subsequently ap-
plied from “hypercore”16 4 and acetylated lactosyl azide 6.
Using standard CuAAC conditions resulted in the octadeca-
lactosylated cluster 15 (59%) containing 18 triazole groups
radially distributed in one layer and a different inner space
compared to the mannosylated analogue 14 described above.
This last structural parameter might be important in respect
to the study of the dual rigidity/compaction’s influence
responsible for specific topographical epitope orientation.
Interestingly, although clicked moieties consisted in different
saccharides and valencies, comparable global yields were
obtained for both octadecavalent structures, regardless of
the multiple one pot reactions’ number (70% yield for 14, 6
coupling reactions vs 58% yield for 15, 18 coupling reac-
tions). In fact, both strategies indifferently afforded excellent
partial yields superior or equal to 95%. Transesterification
was then carried out under basic conditions (1 M MeONa in
(11) Cummins, C. H. Tetrahedron Lett. 1994, 35, 857–860.
ꢀ
(12) Chabre, Y. M.; Contino-Pepin, C.; Placide, V.; Shiao, T. C.; Roy, R.
J. Org. Chem. 2008, 73, 5602–5605.
(13) (a) Wu, P.; Malkoch, M.; Hunt, J. N.; Vestberg, R.; Kaltgrad, E.;
Finn, M. G.; Folkin, V. V.; Sharpless, K. B.; Hawker, C. J. Chem. Commun.
2005, 5775–5777. (b) Himo, F.; Lovell, T.; Hilgraf, R.; Rostovtsev, V. V.;
Noodleman, L.; Sharpless, K. B.; Fokin, V. V. J. Am. Chem. Soc. 2005, 127,
210–216. (c) Dedola, S.; Nepogodiev, S. A.; Field, R. A. Org. Biomol. Chem.
2007, 5, 1006–1017. (d) Meldal, M.; Tornoe, C. W. Chem. Rev. 2008, 108,
(14) (a) Chernyak, A. Y.; Sharma, G. V.; Kononov, L. O.; Krishna, P. R.;
Levinsky, A. B.; Kochetchov, N. K.; Rao, A. V. R. Carbohydr. Res. 1992,
223, 303–309. (b) Arce, E.; Nieto, P. M.; Diaz, V.; Garcia Castro, R.; Bernad,
A.; Rojo, J. Bioconjugate Chem. 2003, 14, 817–823.
(15) Tropper, F. D.; Andersson, F. O.; Braun, S.; Roy, R. Synthesis 1992,
618–620.
€
2952–3015. (e) Aragao-Leoneti, V.; Campo, V. L.; Gomes, A. S.; Field,
R. A.; Carvalho, I. Tetrahedron 2010, 66, 9475–9492.
ꢀ
(16) Wooley, K. L.; Hawker, C. J.; Frechet, J. M. J. J. Am. Chem. Soc.
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J. Org. Chem. Vol. 76, No. 2, 2011 725

Chabre et al.
JOCNote
SCHEME 3. Elaboration of Hexavalent (11, 12, and 13) and Octadecavalent (14, 15, and 16) Glycosylated Architectures Together with
Glycodendrimer 17 Containing 54 Mannopyranoside Termini
TABLE 1. High- and Low-Resolution ESIþ-MS Experiments for
Multivalent Glycosylated Compounds 11-17
MeOH) to afford fully hydroxylated lactocluster 16 in a
satisfactory yield (80%). Application of the corresponding
compound
(valency)
calculated
mass (m/z)
experimental
mass
accelerated methodology involving coupling of dendritic
core 4 and mannosylated dendron 9 furnished G(1)-manno-
dendrimer 17 (48%) exhibiting 54 protected mannoside
termini and containing two distinct TRIS layers in internal
and middle sections of the dendritic platform. As observed
previously, classical click chemistry conditions, using only an
additional short initial heating period, gave access to hyper-
functionalized and packed structure in 48% yield, corre-
sponding to an excellent 96% yield per individual coupling
reactions and corroborating the reliability of the accelerated
dendritic growths developed in this study.
entry
1
2
3
4
5
6
7
11 (6)
12 (6)
13 (6)
14 (18)
15 (18)
16 (18)
17 (54)
1681.57138 ([M þ 2H]2þ
1609.49995 ([M þ 3H]3þ
1531.52443 ([M þ 2H]2þ
)
)
)
1681.56904a
1609.49710a
1531.52785a
2057.27b
2057.32 ([M þ 5H]5þ
3548.68 ([M þ 4H]4þ
)
)
3548.74b
1778.82873 ([M þ 5H]5þ
30551.26 ([M þ H]þ)
)
1778.82137a
30550.70b
aHigh-resolution mass spectrometry experiment with theoretical
mass calculated from exact mass. bLow-resolution mass spectrometry
experiment with theoretical mass calculated from molecular weight.
In all cases, analysis of the 1H and 13C NMR spectra of the
multivalent glycosylated derivatives revealed expected inte-
grations for the triazole protons respective to the internal
aromatic and outer anomeric protons, complete disappear-
ance of the acetylenic signals (δ 3.47 ppm for 2, and δ 2.40
ppm for 4, in 1H NMR), thus, confirming together with mass
spectrometry and infrared spectroscopy unequivocal com-
pletion of the multiple one-pot click reactions (Table 1).
In conclusion, we have developed an expeditive and
systematic route toward the synthesis of original glycoclus-
ters and G(1)-glycodendrimers built from circular and aro-
matic polypropargylated scaffolds. All the multivalent
glycoconjugates were obtained with simple and generally
high yielding reactions including nucleophilic substitutions
to generate synthons and CuAAC reactions to covalently
726 J. Org. Chem. Vol. 76, No. 2, 2011

Chabre et al.
JOCNote
attached saccharidic moieties. To this end, two straightfor-
ward synthetic pathways have been addressed and generated
high valency derivatives containing up to 54 peripheral
epitopes in satisfactory yields. Interestingly, the hydropho-
bic character of the inner aromatic core has also been
counterbalanced by the hydroxylated sugar derivatives,
affording fully water-soluble derivatives. This critical prop-
erty combined with the specific spatial presentation of the
termini radiating from these “star-shaped” structures have
actually encouraged the evaluation of their binding proper-
ties on relevant lectins. Thus, promising preliminary data for
lactosylated dendrimers 13 and 16 already highlighted their
strong inhibitory potencies against galectin-3 binding to
mucins with a marked multivalent effect and low micromolar
values for octadecavalent derivative 16. In addition, results
compared well with previously studied lactosylated PA-
MAM dendrimers17 and detailed experimental procedures
will be reported in due course.
(d, J = 8.7 Hz, CHar, 12H), 4.19-4.09 (m, OCH2CdO, CqCH2O,
48H), 3.81 (br s, OCH2CtCH, 36H), 2.40 (br s, OCH2CtCH,
18H); 13C NMR (150 MHz, CDCl3, δ ppm): 167.7 (CONH),
154.4 (Cq-arO), 140.0 (Car-central), 134.2 (CqdCH), 132.2
(Car-centralCqdCH), 113.1 (OCq-ardCH), 79.4 (OCH2CtCH),
74.8 (OCH2CtCH), 68.2 (CqCH2O), 66.8 (Cq), 59.0 (OCH2Cd
O), 58.5 (OCH2CtCH); m/z (ESIþ HRMS) for C132H132N6-
O30 = 761.30687 [M þ 3H]3þ, found 761.30919; 1141.45667
[M þ 2H]2þ, found 1141.45772; 1163.43861 [M þ 2Na]2þ, found
1163.43813.
ꢀ
General Procedure for Zemplen Reaction. Acetylated cluster
or dendrimer was dissolved in dry MeOH and a solution of
sodium methoxide (1 M in MeOH, 5 μL per 30 min period) was
added until slow precipitation of the product. Additional 50 μL
of the sodium methoxide solution was injected and the mixture
was stirred overnight. Ten milliliters of MeOH was then added
and the mixture was transferred into a centrifuge tube (15 mL).
After a first centrifugation, solvent was removed and the
remaining white solid was subsequently resuspended with 15
mL of MeOH ancentrifugeded (4 times). Water was finally
added for entire solubilization and the solution was neutralized
by addition of ion-exchange resin (Amberlite IR 120 Hþ) until
pH 6-7. After filtration, water was removed in vacuo with rotary
evaporator. The residue was then lyophilized to yield the fully
deprotected and hydrosoluble glycocluster or glycodendrimer.
Experimental Section
General Procedure for Functionalization of 1. To a solution of
1 (1.0 equiv, C = [0.04 M]) and K2CO3 (7.2 equiv) in dry DMF
was added corresponding halide (9.0 equiv) over a 10 min period
and under a nitrogen atmosphere. The mixture was stirred and
warmed at 65-80 °C for additional 20-40 h. The resulting
yellowish residue was diluted with ethyl acetate (10 mL) and
washed with two 20 mL portions of water, 10 mL of brine, and
dried over Na2SO4. Filtration and evaporation of the solvent
afforded crude products which were subsequently purified
either by precipitation (for 2) or by silica gel column chroma-
tography (for 4).
1
Data for Derivative 16. H NMR (600 MHz, D2O, δ ppm):
7.96 (s, Htriazole, 18H), 6.48 (br s, CHar, 12H), 6.15 (br s, CHar,
12H), 5.56 (dapp, H1gal, 18H), 4.37-4.30 (m, H1glu, OCH2Ctriazole
54H), 4.08 (br s, OCH2CO, 12H), 3.89 (tapp, H2glu, 18H),
3.83-3.41 (m, CqCH2O, H2gal, H3gal, H3glu, H4gal, H4glu, H5gal
5glu, H6gal, H6glu, 234H); 13C NMR (150 MHz, D2O, δ ppm):
,
,
H
169.8 (CONH), 154.8 (Cq-arO), 143.9 (Ctriazole), 139.7 (Car-central),
133.9 (CqdCH), 131.9 (Car-centralCqdCH), 123.8 (CHtriazole),
112.4 (OCq-ardCH), 102.5 (C1gal), 86.9 (C1glu), 77.3 (C4glu),
77.0 (C3glu), 75.0 (C5glu), 74.1 (C2glu), 72.1 (C3gal), 71.7 (C5gal),
70.5 (C2gal), 68.2 (C4gal), 67.8 (CqCH2O), 65.8 (OCH2CO), 63.2
(OCH2Ctriazole), 60.6 (C6gal), 59.3 (C6glu) 59.1 (Cq); m/z (ESIþ
HRMS) for C348H510N60O210 = 1778.82873 [M þ 5H]5þ, found
1778.82137.
Data for Hexapropargylated Derivative 2. Obtained from
precipitation, with a minimum amount of ethyl acetate and then
cold hexanes, as an off-white solid (89.0 mg, 0.141 mmol) with a
82% yield. Mp >250 °C (decomposition); 1H NMR (600 MHz,
DMSO-d6, δ ppm): 6.75 (d, J = 8.7 Hz, CHar, 12H), 6.47 (d, J =
8.7 Hz, CHar, 12H), 4.54 (d, J = 1.9 Hz, OCH2CtCH, 12H),
3.44 (t, J = 1.9 Hz, OCH2CtCH, 6H); 13C NMR (150 MHz,
DMSO-d6, δ ppm): 154.5 (Cq-arO), 139.9 (Car central), 133.5
(CqdCH), 131.8 (Car-centralCqdCH), 112.9 (OCq-ardCH), 79.2
(OCH2CtCH), 77.9 (OCH2CtCH), 55.2 (OCH2CtCH); m/z
(ESIþ HRMS) for C60H42O6 = 859.30542 [M þ H]þ, found
859.30267; 881.28736 [M þ Na]þ, found 881.28463.
Acknowledgment. This work was supported by the Natural
Science and Engineering Research Council of Canada
(NSERC) (RR). P.P.B. is thankful to the CIHR for a
training program scholarship in Chemical Biology. Dr. A.
ꢀ
ꢀ
Furtos and M.-C. Tang (Universite de Montreal, QC,
Canada) are also acknowledged for mass spectrometry
analyses.
Data for Octadecapropargylated Derivative 4. Obtained from
purification by silica gel chromatography (eluent: CH2Cl2/MeOH
98:2) (57.0 mg, 0.0250 mmol) in a 79% yield, as a yellowish oil.
Rf = 0.41 CH2Cl2/MeOH (95:5); 1H NMR (600 MHz, CDCl3, δ
ppm): 6.81 (br s, 6H, NH), 6.67 (d, J = 8.7 Hz, CHar, 12H), 6.44
Supporting Information Available: Experimental details for
the synthesis of compounds 1-4, 7-17. List of NMR spectra
(1H, 13C, COSY), FT-IR spectra, and MS experiments for
compounds 1, 2, 4, 7-17. This material is available free of
charge via the Internet at http://pubs.acs.org.
ꢀ
(17) Andre, S.; Cejas Ortega, P. J.; Alamino Perez, M.; Roy, R.; Gabius.,
H.-J. Glycobiology 1999, 9, 1253–1261.
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