Facile synthesis of size dependent Ru(ii)-carbohydrate dendrimers via click chemistry

Article abstract of DOI:10.1039/b925113h

A facile and flexible approach for the preparation of Ru(ii) complexes containing different carbohydrates based on the Cu(ii)-catalyzed Huisgen-[3+2] cycloaddition is described.

Full text of DOI:10.1039/b925113h

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Facile synthesis of size dependent Ru(II)–carbohydrate
dendrimers via click chemistryw
ab
Raghavendra Kikkeri,ab Xinyu Liu,z Alexander Adibekian,yab Yu-Hsuan Tsaiab
and Peter H. Seeberger*ab
Received (in Cambridge, UK) 1st December 2009, Accepted 5th February 2010
First published as an Advance Article on the web 24th February 2010
DOI: 10.1039/b925113h
A facile and flexible approach for the preparation of Ru(II)
complexes containing different carbohydrates based on the
Cu(II)-catalyzed Huisgen-[3+2] cycloaddition is described.
rapid synthesis of Ru(II)–glycodendrimers via ‘‘click chemistry’’.
Carbohydrate dendrimers containing an azido-linker and
Ru(II)–acetylene complexes are prepared separately. Sub-
sequent Huisgen-[3+2] cycloaddition, followed by the
removal of protecting groups on carbohydrate moieties, will
provide access to the desired complexes in a straightforward,
modular fashion. Tripodal sugar-azides 14 and 15 were
prepared from tris(hydroxylmethyl)aminomethane 9 (Scheme 1).
Coupling of 9 with acrylonitrile, followed by treatment with
HCl in ethanol and amide bond formation with 5-bromo
valeric acid, yielded 12. Nucleophilic substitution of bromide
by azide, saponification and esterification with pentafluoro-
phenol afforded 13 in 86% yield. The same procedure was
utilized to obtain activated ester 13a. Pentafluorophenol ester
13 was further reacted with 2-aminoethoxy per-acetylated
mannose and mannose-tripod to obtain first and second
generation azide mannose dendrons 14 and 15 respectively
(Scheme 1). Tripod 17 was prepared by mixing active ester 13a
with propargyl amine.
Lectins are carbohydrate binding proteins involved in cell–cell
interactions that are the basis of a variety of biological
processes, such as cell adhesion and migration, phagocytosis,
cell differentiation and apoptosis.1 Moreover, carbohydrates
that selectively bind lectins have been used for imaging and
delivery.2 The interest in lectin–carbohydrate interactions has
increased considerably. Monomeric binding affinities for
carbohydrate–lectin interactions are typically in the milli- to
micromolar range. Multivalent presentation of carbohydrates,
as is the case on the cell surface, overcomes the problem and
can be mimicked using glycodendrimers.3 A host of glyco-
dendrimers has been prepared4–6 and explored in different
applications.3 The facile, Cu(II)-assisted Huisgen-[3+2] cyclo-
addition has been applied to attach carbohydrates to
PMMAM dendrimers,4b nanoparticles5 and dendrimers
formed via a self-assembly process6 in order to construct
glycodendrimers. Glycodendrimers designed for biological
assays should ideally contain means to optically, electro-
chemically or gravimetrically detect them in biological systems.
We previously described Ru(II)–glycodendrimers as strong
optical and electrochemical probes.7 A general synthetic
protocol to rapidly and effectively synthesize structurally
diverse Ru(II)–glycodendrimer complexes remained elusive.
Here, we report the design and synthesis of different
generations of Ru(II)–glycodendrimers (1–8) bearing varying
numbers of carbohydrates (Fig. 1). Increased carbohydrate
density around the Ru(bipy)3 core strongly influences the photo-
physical and colloidal aggregation properties of the dendrimers
compared to organic fluorescent glycodendrimers (Table 1).
Ru(II)–alkynes are readily accessible in high yields using
inexpensive starting materials and are an ideal basis for the
Ru(II)–alkyne derivatives 21 and 24 were readily prepared
from 4,40-dimethyl bipyridine 18 (Scheme 2). Oxidation with
chromic acid and coupling with 17 after Boc-cleavage in the
presence of triethyl amine followed by complexation with
cis-Ru(bipy)2Cl2 yielded complex 21. Similarly, 24 was
synthesized by selenium dioxide oxidation of 18 to the
monocarboxylic acid derivative followed by coupling with
propargyl amine and complexation with RuCl3. Finally, 21
or 24 and fluorescein alkyne were coupled with stoichiometric
amounts of dendrons 14 and 15 via the [3+2] cycloaddition
reaction. Deacetylation yielded dendrimers 1–8. When a
similar method was applied to the synthesis of a Ru(II)
complex substituted with 54 sugars, a mixture of 27 and 36
mannose substituted Ru(II) dendrimers was obtained. Steric
effects are likely responsible for the failure to obtain the fully
glycosylated dendrimer. These results indicate that the
Ru(II)-complex is a facile template to obtain varying carbo-
hydrate densities when compared to the fluorescein probe. The
optical properties of Ru(II)-complexes 1–8 were investigated
at room temperature. Complexes 1–7 showed a maximum
emission at 643–648 nm. The quantum yield of 7 is almost
twice that of complexes 1–6, thus demonstrating size
dependent optical dendrimer properties. The quantum yield
of 8 is almost comparable to the fluorescein dye.
a Max-Planck-Institute of Colloids and Interfaces, Department of
Biomolecular Systems, Am Muhlenberg 1, 14476 Potsdam, Germany
¨
b Freie Universitat Berlin, Institute of Chemistry and Biochemistry,
¨
Arnimallee 22, 14195 Berlin, Germany.
E-mail: peter.seeberger@mpikg.mpg.de
w Electronic supplementary information (ESI) available: Full experi-
mental details for synthesis of metal-dendrimers and its optical and
turbidity studies. See DOI: 10.1039/b925113h
z Current address: Harvard Medical School, Department of Biological
Chemistry and Molecular Pharmacology, 240 Longwood Avenue,
Boston, MA 02115, USA.
y Current address: The Scripps Research Institute, SR107, 10550
North Torrey Pines Road, La Jolla, CA 92037, USA.
After assessing the optical properties, lectin–carbohydrate
interactions between galanthus nivilis agglutinin (GNA),
concanavalin A (ConA) lectin and a-manno- and b-gluco-
pyranosides were investigated.8 GNA and ConA have twelve
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Chem. Commun., 2010, 46, 2197–2199 | 2197

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Scheme 1 Synthesis of sugar-tripod azides and fluorescein dendrimer
8. (a) Acrylonitrile/NaOH (40%); (b) conc HCl/EtOH, 51%;
(c) 5-bromo-valeric acid/DIC/HOBT/DCM, 69%; (d) NaN3/DMF,
72%; NaOH/MeOH; pentafluorophenol/DIC/HOBT/DCM, 86%;
(e) 2-(tert-butoxycarbonyl amino)ethoxy-2,3,4,6-tetra-O-acetyl-a-D-
mannoside/DCM/TEA, 71%. (f) Acrylonitrile/NaOH (40%), conc
HCl/EtOH, 51%; N-Boc-b-Ala/DIC/HOBT/DCM, 63%; pentafluoro-
phenol/DIC/DCM, 71%; (g) propargyl amine/TEA/DCM, 77%;
(h) tripod-mannose7b/DCM/TEA, 47%; (i) CuSO4/ascorbic acid/
THF : H2O (1 : 1), 12 h, 84%; NaOMe/MeOH, 61%.
and four binding sites for mannose respectively.9 In HEPES
buffer binding of complexes 1–8 correlates with turbidity and
increased density of the mannose and glucose sugars (Fig. 2).
As expected, complex 2 containing b-galactose did not result
in turbidity, whereas 7 resulted in highly turbid solutions. All
other complexes showed weak turbidity. Similarly, ConA
showed strong turbidity with complex 7 and no aggregation
with any other complex. In order to demonstrate that the
turbidity is due to specific carbohydrate–protein interactions,
excess mannose was added to inhibit dendrimer–GNA
binding.
Fig. 1 Structures of Ru(II)-complexes 1–8.
Table 1 Photophysical properties of dendrimers 1, 6, 7 and 8.
Dendrimer size was calculated for the minimum energy conformation
of the glycodendrimers using MM2 Chem 3D structure and
Mercury programs
In conclusion, we have developed an efficient synthetic route
to non-bleaching, high quantum yield fluorescent Ru(II)–
carbohydrate dendrimers using the copper catalyzed
Huisgen-[3+2] cycloaddition. Lectin binding affinity and
optical properties of the metallo-glycodendrimers can be
readily tuned by changing the number of carbohydrate
moieties. Specific carbohydrate–protein interactions of these
Compound
lmax/nm
QY (F)
Size/A
1
6
7
8
645
648
648
548
0.061–0.066
0.098
0.125
B12–19
B36–56
B68–86
B13–21
0.058
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We thank the Max-Planck Society and the DAAD
(fellowship to Y.H.T.) for financial support.
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Scheme 2 Synthesis of Ru(II)-cores 21 and 24 as well as formation of
metal glycodendrimers 1–7. (a) CrO3/conc H2SO4, 98%; (b) SeO2/conc
H2SO4, 98%; (c) comp 17/TEA/DCM, 12 h, 69%; cis-Ru(bipy)2Cl2/
EtOH, 12 h, 51%; (d) propargyl amine/DCM/TEA, 76%; RuCl3/
EtOH/AcOH, 12 h, 39%; (e) CuSO4/ascorbic acid/THF : H2O (1 : 1)
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Fig. 2 Turbidity assay: absorption change of complexes 1, 3 and 4
(’), 2 (m), 5, 6 and 8 (dotted line) and 7 (straight line) at 530 nm upon
addition of GNA (1 mg mLꢁ1). Mannose (100 mM) was added to all
complexes after 25 min.
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Chem. Commun., 2010, 46, 2197–2199 | 2199