Expeditive synthesis of glycodendrimer scaffolds based on versatile TRIS and mannoside derivatives

Article abstract of DOI:10.1021/jo8008935

(Chemical Equation Presented) A new family of glycodendrimer scaffolds containing 12 and 18 peripheral α-D-mannopyranosidic units has been synthesized by Cu(I)-catalyzed [1,3]-dipolar cycloadditions using sulfurated dendritic scaffolds bearing alkyne functionalities and novel TRIS derivatives.

Full text of DOI:10.1021/jo8008935

uropathogenic E. coli FimH possesses a carbohydrate recogni-
tion domain (CRD) at its tip, which can accommodate one R-D-
mannopyrannoside residue.3
Expeditive Synthesis of Glycodendrimer Scaffolds
Based on Versatile TRIS and Mannoside
Derivatives
For our ongoing research program aimed at the study of
mannose protein binding with the synthesis and biological
evaluation of multivalent glycomimetic inhibitors against bacte-
rial adhesion,4,5 original dendritic architectures are needed.
Hence, the straightforward preparation of a new family of
mannosylated dendrimers is proposed in order to pursue the
systematic study concerning the modulation of critical structural
parameters toward high specificity. The proposed strategy is
based on the synthesis of sulfurated and polypropargylated
versatile dendritic cores onto which mannoside and other sugar
moieties will be efficiently attached by a Cu(I)-catalyzed azide-
alkyne [1,3]-dipolar cycloaddition reaction (CuAAC),6 in high
yields and with complete regioselectivity. The construction of
these dendritic cores emanates from the use of thioacetylated
derivatives of pentaerythritol and tetra- or hexasubstituted
benzene engaged in nucleophilic substitution or 1,4-conjugate
addition to afford adequately functionalized novel TRIS (tr-
is(hydroxymethyl)aminomethane) derivatives. Although pen-
taerythritol has recently been successfully used for the con-
struction of highly branched mannopyranoside structures which
presented nanomolar affinities to E. coli,7 tetrakis- and hexak-
is(bromomethyl)benzene represent less common compounds for
the synthesis of glycodendrimers. Since the synthesis of
“arborols” proposed by Newkome et al. in 1985,8 the widespread
use of TRIS and its AB3-monomer derivatives afforded highly
functionalized structures, taking advantage of its geometry for
a rapid dendritic growth. This scaffold has been used for the
accelerated synthesis of dense carbohydrate-containing den-
drimers9 and more recently for the efficient syntheses of
glycodendrons presented as potential carbohydrate vaccine
candidates and antiviral agents.10 Consequently, the present work
deals with original multivalent dendritic architectures that
Yoann M. Chabre, Christiane Contino-Pe´pin,
Virginie Placide, Tze Chieh Shiao, and Rene´ Roy*
Department of Chemistry, UniVersite´ du Que´bec a` Montre´al,
P.O. Box 8888, Succ. Centre-Ville Montre´al, Que´bec,
Canada H3C 3P8
roy.rene@uqam.ca
ReceiVed April 24, 2008
A new family of glycodendrimer scaffolds containing 12 and
18 peripheral R-D-mannopyranosidic units has been synthe-
sized by Cu(I)-catalyzed [1,3]-dipolar cycloadditions using
sulfurated dendritic scaffolds bearing alkyne functionalities
and novel TRIS derivatives.
(2) Hung, C.-S.; Bouckaert, J.; Hung, D.; Pinkner, J.; Widberg, C.; DeFusco,
A.; Auguste, C. G.; Strouse, R.; Langerman, S.; Waksman, G.; Hultgren, S. J.
Mol. Microbiol. 2002, 44, 903–915.
Multiple carbohydrate-protein interactions are responsible,
at the molecular level, for several critical biological events such
as cellular adhesion and recognition, physiological function
regulation, and pathogenic infections. In spite of the weakness
of these fundamental interactions in terms of affinity and
selectivity on a per-saccharide basis, these attractive forces are
dramatically 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”.1 In the case of
pathogenic infections, a critical step in host-tissue colonization
and biofilm formation is achieved through bacterial adhesion
commonly mediated by carbohydrate-binding lectins expressed
on or shed from bacterial surfaces. One striking example is
found in type 1 fimbriae from Escherichia coli that are the most
common type of adhesive appendages in several enterobacteria
and mediate mannose-specific adhesion of uroepithelial cells
via the 30 kDa lectin-like subunit FimH.2 X-ray crystal-structure
studies have recently revealed that the lectin domain of
(3) Bouckaert, J.; Berglund, J.; Schembri, M.; De Genst, E.; Cools, L.;
Wuhrer, M.; Hung, C. S.; Pinkner, J.; Slattergard, R.; Zavialov, A.; Choudhary,
D.; Langerman, S.; Hultgren, S. J.; Wyns, L.; Klemm, P.; Oscarson, S.; Knight,
S. D.; De Greve, H. Mol. Microbiol. 2005, 55, 441–455.
(4) Nagahori, N.; Lee, R. T.; Nishimura, S. I.; Page´, D.; Roy, R.; Lee, Y. C.
ChemBioChem 2002, 3, 836–844. Roy, R. Trends Glycosci. Glycotechnol. 2003,
15, 291–310.
(5) For specific applications of mannosylated dendrimers: (a) Touaibia, M.;
Roy, R. Mini-ReV. Med. Chem. 2007, 7, 1270–1283. (b) Touaibia, M.; Roy, R.
Carbohydrate-Protein and Carbohydrate-Carbohydrate Interactions. In Compre-
hensiVe Glycoscience; Kamerling, J. P. Ed.; Elsevier: Amsterdam, 2007; Vol.
3, pp 821-870.
(6) (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)
Antoni, P.; Nystro¨m, D.; Hawker, C. J.; Hult, A.; Malkoch, M. Chem. Commun.
2007, 2249–2251. (d) Dedola, S.; Nepogodiev, S. A.; Field, R. A. Org. Biomol.
Chem. 2007, 5, 1006–1017.
(7) (a) Touaibia, M.; Shiao, T. C.; Papadopoulos, A.; Vaucher, J.; Wang,
Q.; Benhamioud, K.; Roy, R. Chem. Commun. 2007, 380–382. (b) Touaibia,
M.; Wellens, A.; Shiao, T. C.; Wang, Q.; Sirois, S.; Bouckaert, J.; Roy, R.
ChemMedChem 2007, 2, 1190–1201.
(8) Newkome, G. R.; Yao, Z.-Q.; Baker, G. R.; Gupta, V. K. J. Org. Chem.
1985, 50, 2003–2004.
(1) (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, 2755–2794.
(9) Ashton, P. R.; Boyd, S. E.; Brown, C. L.; Jayaraman, N.; Stoddart, J. F.
Angew. Chem., Int. Ed. 1997, 36, 732–735.
5602 J. Org. Chem. 2008, 73, 5602–5605
10.1021/jo8008935 CCC: $40.75 2008 American Chemical Society
Published on Web 06/24/2008

SCHEME 1. Synthesis of Thioacetylated Cores
SCHEME 3. Synthesis of Polypropargylated Dendritic
Cores
SCHEME 2. Synthesis of Propargylated TRIS Derivatives
in 65% yield. The quantitative removal of the Boc protecting
group with TFA at room temperature and the use of the
corresponding salt without further purification in a coupling
reaction with bromoacetyl chloride in the presence of DIPEA
afforded the desired compound 8 (65%).
As mentioned before, the critical step of our dendritic growth
resides in the coupling reaction between thioacetylated cores
and TRIS derivatives 5 and 8. Conditions were optimized to
afford satisfactory thioalkylation and were established for a 1,4-
conjugate addition reaction between pentaerythritol tetraacetate
(1) and the acryloylated derivative 5 (see Table 1 in the
Supporting Information for optimization conditions).
The first attempt, concerning the use of basic conditions
classically encountered for deacetylation reactions (NEt3, MeOH)
for in situ formation of thiolate intermediates, failed. We next
turned our attention to reductive conditions (NaBH4), following
Braga’s procedure, described as an efficient sulfur addition
reaction from thiocetate compounds on alkyl halides.14In this
case, the desired dodecapropargylated compound 9 was obtained
in rather low yield (29%), in spite of solvent variation. Finally,
our efforts were successfully directed toward the use of
combined basic and reductive conditions proposed by Fall et
al.15 (NaOH, NaBH4 in EtOH) for efficient in situ alkylation in
a reduced reaction time (64%). Surprisingly, yields were not
improved with prolonged reaction time and no desired com-
pound was observed with the use of THF as solvent. The
optimized conditions described above were used for the efficient
synthesis of the aromatic dodecapropargylated compound 10
(65%) (Scheme 3). Unfortunately, attempts to obtain an octa-
decapropargylated derivative from the hexathioacetate core 3
failed.
represent ideal candidates for our ongoing systematic study to
design efficient but specific inhibitors of viral or bacterial
infections.
The synthesis of thioacetylated aliphatic or aromatic cores,
from commercially available pentaerythritol tetrabromide and
1,2,4,5-tetrakis- or hexakis(bromomethyl)benzene, respectively,
constitutes the initial step of the dendritic growth. The use of
potassium thioacetate in DMF at room temperature allowed the
desired bromide displacement to afford the known pentaeryth-
ritol tetrathioacetate11 (1) and the original aromatic derivatives
2 and 3 in an efficient way and with reduced reaction times
(Scheme 1).
Coupling reactions between these sulfurated cores and TRIS
moieties containing complementary functions acting as elec-
trophiles have been investigated. As shown in Scheme 2, two
closely related strategies have been employed to introduce
suitable functions on TRIS, leading to the desired new tripro-
pargylated compounds 5 and 8 in a concise and efficient way.
N-Acryloyltris[(propargyloxy)methyl]aminomethane (5) has been
synthesized in a two-step orthogonal sequence (thus avoiding
protection/deprotection reactions), from the initial selective
acryloylation of TRIS in methanolic potassium hydroxide media
(4, 89%), according to the procedure described by Pucci et al.,12
followed by propargylation under basic conditions (KOH, DMF)
in 75% yield. The synthetic way to obtain 2-bromoacetamidot-
ris[(propargyloxy)methyl]aminomethane (8) was based on the
almost quantitative synthesis of the N-Boc TRIS derivative 6.13
Propargylation under the above conditions afforded N-(tert-
butyloxycarbonyl)tris[(propargyloxy)methyl]aminomethane (7)
Interestingly, we found that, under the same experimental
conditions, in situ reduction of cores 1 and 2 and even 3
followed by an SN2 reaction with bromide derivative 8 afforded
the corresponding dodeca- and octadecapropargylated dendritic
scaffolds 11-13, respectively, in excellent yields (Scheme 3).
The almost quantitative yields obtained for the SN2 reaction in
(10) Wang, S.-K.; Liang, P.-H.; Astronomo, R. D.; Hsu, T.-L.; Hsieh, S.-L.;
Burton, D. R.; Wong, C.-H. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 3690–
3695.
(11) Wan, Y.; Mitkin, O.; Barnhurst, L.; Kurchan, A.; Kutateladze, A. Org.
Lett. 2000, 2, 3817–3819.
(12) Pucci, B.; Maurizis, J. C.; Pavia, A. A. Eur. Polym. J. 1991, 27, 1101–
(14) Braga, A. L.; Appelt, H. R.; Schneider, P. H.; Silveira, C. C.;
Wessjohann, L. A. Tetrahedron: Asymmetry 1999, 10, 1733–1738.
(15) Fall, Y.; Diouf, O.; Go´mez, G.; Bolan˜o, T. Tetrahedron Lett. 2001, 44,
6069–6072.
1106.
(13) Segura, M.; Sansone, F.; Casnati, A.; Ungaro, R. Synthesis 2001, 14,
2105–2112.
J. Org. Chem. Vol. 73, No. 14, 2008 5603

SCHEME 4. Synthesis of Mannosylated Dendrimers
comparison to those observed for 1,4-conjugate addition might
be explained by the absence of possible retro-Michael reactions.
Furthermore, faster relative kinetics for the nucleophilic sub-
stitution may compete efficiently against the likelihood of thiol
oxidation to disulfide, particularly for reactions involving the
hexasubstituted derivative 3. The symmetry of these novel stable
polypropargylated scaffolds described above, associated with
reactions provided four G(0) dendrimers in good to excellent
yields containing 12 acetylated D-mannopyrannoside residues
(15, 17, 19, and 21) and one exhibiting 18 peripheral protected
glycans (23) (Scheme 4). Generally, the conditions under which
the Cu(I) catalyst was generated in situ afforded better yields
than those using Cu(I) species (CuI) directly. In all cases,
1
analysis of the H NMR spectra of the mannoside dendrimers
1
the presence of the characteristic H and 13C NMR, IR, and
revealed calculated integrations for the triazole protons (δ 7.78
ppm) respective to the anomeric protons (δ 4.82 ppm), and
complete disappearance of the acetylenic signals (δ 2.49 ppm),
thus confirming together with HRMS completion of the multiple
one-pot cycloadditions. De-O-acetylation under slightly modified
Zemple´n conditions (NaOMe, MeOH/CH2Cl2/THF) furnished
the desired dodeca- and octadecavalent glycodendrimers 16, 18,
20, 22, and 24, respectively, in excellent yields.
MS signals, facilitated their unequivocal characterization. Thus,
their treatment with the known 2-azidoethyl-2,3,4,6-tetra-O-
acetyl-R-D-mannopyranoside16 (14) using Cu(I)-catalyzed click
(16) (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.; GarciaCastro, R.; Bernad, A.;
Rojo, J. Bioconjugate Chem. 2003, 14, 817–823.
5604 J. Org. Chem. Vol. 73, No. 14, 2008

reduced pressure, and the resulting crude material was chromato-
graphed on a silica gel column.
The functionalized dendrimer 24, presenting six adjacent
thioacetyl groups around a central benzene core, may provide
an interesting architecture and carbohydrate orientation on its
own, given the possibility of the side chains to be situated
alternately above and below the plane of the central benzene
residue.17 Hence, the global relaxed peripheral sugar density
and specific 2-fold tripodal orientation of the carbohydrate
moieties may constitute another fundamental parameter to
exploit in the study of various protein binding processes.
In conclusion, we have developed an expeditive and system-
atic route toward the synthesis of a series of dodeca- to
octadecamannopyranoside G(0) dendrimers by using triazole
chemistry and building from the corresponding stable polypro-
pargylated scaffolds. Two of these latter species (9 and 10) were
obtained according to an efficient orthogonal and chemoselective
sequence, and all were synthesized with simple and generally
high yielding reactions. Work is in progress on the synthesis of
the corresponding higher generation dendritic architectures
possessing optimized valencies and topologies. Indeed, studies
have shown that a plateau of inhibitory potency was reached
as a function of dendrimer scaffolding generation, above which
the binding interactions begin to diminish. The surface conges-
tion induced inaccessibility of individual sugars toward their
protein receptors.18 Moreover, it has been observed that the
structures and valency represented critical factors in the
optimization of individual binding interactions.19 In order to
confirm those important observations with our structures,
biological studies are currently under investigation to estimate
their therapeutic potential, notably for the inhibition of adhesion
of E. coli toward urothelial infections and other mannoside
binding proteins such as DC-SIGN.5
Data for 13: eluent for column chromatography CH2Cl2/EtOAc,
80:20 to 75:25 (v/v); yield 88%; colorless oil; Rf ) 0.53 (CH2Cl2/
EtOAc 70:30 (v/v)); νmax(NaCl)/cm-1 3310 s (br), 3005 s, 2910 s
(br), 2113 s, 1673 s, 1652 s, 1540 m, 1472 m, 1250 s, 1146 s,
796 s (br); δH (300 MHz; CDCl3) 2.49 (t, 18H, J 2.4, Ct CH),
3.37 (s, 12H, SCH2CO), 3.86 (s, 36H, CH2O), 4.15 (s, 12H,
CarCH2S), 4.16 (d, 24H, J ) 2.4 Hz, OCH2Ct CH), 6.70 (br s, 6H,
NH); δC (75.5 MHz; CDCl3) 31.6, 37.7, 58.7, 59.5, 68.4, 75.1, 79.7,
136.1, 168.8; m/z (TOF+ HRMS) for C102H120N6O24S6 1003.341 19
[M + 2H]2+, found 1003.340 69.
Sample Procedure for the Preparation of Dendrimer 23. To
a solution of 13 (20 mg, 0.010 mmol, 1.0 equiv) in a 1:1 (v/v)
mixture of water and tetrahydrofuran (3.0 mL) were added
2′-azidoethyl-2,3,4,6-tetra-O-acetyl-R-D-mannopyranoside (14; 93.6
mg, 0.224 mmol, 22.5 equiv), CuSO4 ·5H2O (15.0 mg, 0.060 mmol,
6.0 equiv) and sodium ascorbate (11.8 mg, 0.060 mmol, 6.0 equiv).
While it was stirred, the mixture was first heated to 55 °C for 6 h
and left at room temperature for an additional 18 h. Ethyl acetate
(15 mL) was added, and the solution was washed with saturated
aqueous NH4Cl (2 × 10 mL), water (10 mL), and brine (5 mL).
Organics were collected, dried over Na2SO4, and concentrated to
dryness in vacuo.
Data for 23: eluent for column chromatography CH2Cl2/MeOH,
100:0 to 90:10 (v/v)); yield 78%; white solid; Rf ) 0.45 (CH2Cl2/
MeOH 93:7 (v/v)); mp 87-90 °C; δH (300 MHz; CDCl3) 1.95,
2.01, 2.07, 2.11 (4 × s, 216H, COCH3), 3.37 (s, 12H, SCH2CO),
3.67 (br s, 18H, H5), 3.81 (s, 36H, CqCH2O), 3.90 (m, 36H,
OCH2CH2), 4.05 (s, 12H, CarCH2S), 4.06 (m, 18H, H6b), 4.18 (m,
18H, H6a), 4.56 (br s, 36H, CHdCCH2), 4.57 (m, 36H,
OCH2CH2N), 4.82 (br s, 18H, H1), 5.18-5.30 (m, 54H, H2, H3,
H4), 7.08 (br s, 6H, NH), 7.80 (br s, 18H, CHdC); δC (75.5 MHz;
CDCl3) 20.6, 20.7, 20.7, 20.8, 31.3, 37.2, 49.4, 62.1, 64.6, 65.6,
66.3, 68.7, 69.2, 97.4, 124.0, 136.3, 144.8, 169.6, 169.8, 169.9,
170.5, 170.7; m/z (TOF+ HRMS) for C390H534N60O204S6 2379.296 78
[M + 4H]4+, found 2379.301 30.
Experimental Section
Octadecapropargylated Derivative 13. To a solution of the
hexa(thioacetylated) core 3 (20.0 mg, 0.033 mmol, 1.0 equiv) in
dry EtOH (2.0 mL) were added at room temperature finely ground
NaOH (15.8 mg, 0.395 mmol, 12.0 equiv) and NaBH4 (15.0 mg,
0.395 mmol, 12.0 equiv) under a nitrogen atmosphere. After 5 min,
2-bromoacetamido[tris(propargyloxy)methyl]aminomethane (8; 91.6
mg, 0.257 mmol, 7.8 equiv) was added to the solution, which was
then warmed to 40 °C for 3 h. The solvent was removed under
Acknowledgment. This work was supported by a grant from
the Natural Science and Engineering Research Council of
Canada (NSERC) to R.R. Y.M.C. is thankful to the FQRNT
for a postdoctoral scholarship. We also thank Dr. B. Pucci for
suggesting compound 4.
Supporting Information Available: Text and figures giving
experimental details for the synthesis of compounds 1-13 and
(17) Hardy, A. D. U.; MacNicol, D. D.; Swanson, S.; Wilson, D. R.
Tetrahedron Lett. 1978, 38, 3579–3582. (b) D’Aleo, A.; Williams, R. M.;
Osswald, F.; Edamana, P.; Hahn, U.; van Heyst, J.; Tichelaar, F. D.; Vo¨gtle, F.;
De Cola, L. AdV. Funct. Mater. 2004, 14, 1167–1177.
1
15-24 and H and 13C NMR spectra of compounds 2, 3, and
5-24. This material is available free of charge via the Internet
at http://pubs.acs.org.
(18) Roy, R. Top. Curr. Chem. 1997, 187, 241–274.
(19) Andre´, S.; Cejas Ortega, P. J.; Alamino Perez, M.; Roy, R.; Gabius,
H.-J. Glycobiology 1999, 9, 1253–1261.
JO8008935
J. Org. Chem. Vol. 73, No. 14, 2008 5605