Multivalent, bifunctional dendrimers prepared by click chemistry

Article abstract of DOI:10.1039/b512021g

Unsymmetrical dendrimers, containing both mannose binding units and coumarin fluorescent units, have been prepared using click chemistry and shown to be highly efficient, dual-purpose recognition/detection agents for the inhibition of hemagglutination. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b512021g

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Multivalent, bifunctional dendrimers prepared by click chemistry{
a
Peng Wu, Michael Malkoch, Jasmine N. Hunt, Robert Vestberg, Eiton Kaltgrad, M. G. Finn,*
b
b
b
a
a
a
Valery V. Fokin,* K. Barry Sharpless* and Craig J. Hawker*
a
b
Received (in Berkeley, CA, USA) 24th August 2005, Accepted 21st October 2005
First published as an Advance Article on the web 4th November 2005
DOI: 10.1039/b512021g
Unsymmetrical dendrimers, containing both mannose binding
units and coumarin fluorescent units, have been prepared using
click chemistry and shown to be highly efficient, dual-
purpose recognition/detection agents for the inhibition of
hemagglutination.
block and the resulting anhydride 1 provides easy access to both
the alkyne 2 and the azide 3 by condensation with the appropriate
10
alcohol. Removal of the acetonide protecting groups and
subsequent acylation with the anhydride 1 allows for facile
generation growth of the dendritic blocks containing either a single
acetylene 4 or azide group 5 at the focal point. The resulting
dendrons can be rendered reactive and hydrophilic by deprotection
or kept protected and hydrophobic by retention of the acetonide
caps.
The high functional group density at the chain ends of dendrimers,
coupled with unprecedented control over molecular structure,
makes these synthetic materials extremely attractive candidates for
1
a variety of surface active applications. One of the most promising
To facilitate these studies and allow preparation of a library of
structures, two series of dendrons up to the 4th generation were
synthesized in high yield and purity (Scheme 1). Coupling of the
differentiated dendritic blocks containing a variety of chain end
is to exploit the numerous chain end groups as multivalent binding
2
sites for interaction with biological receptors and cell surfaces in
3
the construction of targeted drug delivery systems. This concept
4
has been exploited by Cloninger et al. in a series of pioneering
studies designed to alter and control the strength of lectin binding
by varying the generation number of a series of mannose
functionalized PAMAM dendrimers. For future practical applica-
tions, at least 3 functional units are required: a targeting moiety, a
medicinally active agent (drug), and a diagnostic label such as a
fluorescent dye, each attached at a specific position within the
nanostructure. While a number of approaches have been reported
5
for combining all of these elements into a single system, the
structural control and monodispersity of dendrimer-based
6
macromolecules promise superior performance.
A general
strategy for the facile synthesis of chemically differentiated
dendrimers which allows for the introduction of functional groups
at defined locations has however not been reported. To address
7
this challenge, the synthesis of dendritic block copolymers in
which two distinct clusters of functionality (targeting and
detection) are placed at the chain ends in a controlled fashion is
described. The key chemical transformation which allows simple
and facile preparation of these dual-purpose, multifunctional
materials is the copper(I)-catalyzed azide–alkyne cycloaddition, a
8
premier ‘‘click’’ reaction. The high efficiency of this process is
exploited to couple the dendritic blocks together, while its
tolerance of a wide variety of functional groups allows the
introduction of reactive units at the periphery without the use of
9
protecting groups.
The synthetic approach selected is based on 2,2-bis(hydroxy-
methyl)propionic acid (bis-MPA) as a biocompatible building
a
The Scripps Research Institute, Department of Chemistry, BCC-315
10550 N. Torrey Pines Rd., La Jolla, CA 92037, USA.
E-mail: sharples@scripps.edu; Fax: +1 858 784 7562;
Tel: +1 858 784 7515
Materials Research Laboratories, University of California, Santa
b
Barbara, CA 93106, USA. E-mail: hawker@mrl.ucsb.edu;
Tel: +1 805 893 7161
{ Electronic supplementary information (ESI) available: Synthesis and
characterization of dendritic macromolecules. See DOI: 10.1039/b512021g
Scheme 1 Synthesis of hydrophilic and hydrophobic dendrons with
either acetylene 4 or azide 5 groups at the focal point.
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functional groups proceeded smoothly under the copper(I)-
catalysis conditions. Thus, reaction of (HO) –[G-3]–Az, 6, and
8
(
An)
4
–[G-3]–Acet, 7, in dry THF in the presence of catalytic
Br] and N,N-diisopropylethylamine furnished the
[Cu(PPh )
3 3
diblock dendrimer 8 in 92% yield after purification (Scheme 2).
The orthogonality of this process is demonstrated by the ability to
employ both the hydroxy and the acetonide terminated bis-MPA
dendrons with no unwanted side reactions occurring at the
numerous chain ends. The efficiency of this polymer coupling
reaction was further proved by GPC, NMR and MALDI analysis,
+
11
the latter showing a single peak at 1985 (MH ) for 8. Using the
same methodology, a series of amphiphilic dendrimers from
generation 1 to 4 were prepared from dendrons of varied sizes. For
example, the asymmetrical structure 9, in which both the size and
functionality of the dendron is varied, was assembled from a
hydroxy functionalized [G-1]–azide and an acetonide functiona-
lized [G-4]–acetylene.
The modular nature of this synthetic strategy and the chemical
stability of triazoles, azides and acetylenes allows for the efficient
introduction of functional groups at different stages of the process.
For example, the asymmetrical dendrimer (An)16–[G-4]–[G-1]–
2
(OH) , 9, was first decorated with alkyne groups by esterification
of the two free hydroxyls with pent-4-ynoic anhydride (Scheme 3).
Removal of the acetonide protecting groups on the 4th generation
dendritic block of 10 revealed 16 reactive hydroxy groups (11)
Scheme 2 Synthesis of unsymmetrical dendrimer
8 containing a
12
chemically differentiated surface.
followed by attachment of the two 7-diethylaminocoumarin dyes
Scheme 3 Synthesis of multivalent, asymmetrical dendrimer 14 containing 16 mannose units and 2 coumarin chromophores.
5
776 | Chem. Commun., 2005, 5775–5777
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the introduction of mannose and coumarin unit derivatives to the
periphery of individual blocks allowed preparation of agents with
dual function, recognition and detection, which may prove useful
in identification and treatment of pathological conditions via
multivalent interactions.
Financial support of this work by the National Institutes of
Health through a Program of Excellence in Nanotechnology
Grant (1 U01 HL080729-01) and the National Institute of General
Medical Sciences (GM 28384), National Science Foundation
(UCSB MRSEC, DMR-0520415, DMR-0301833, CHE-0514031),
the W. M. Keck Foundation, the Skaggs Institute for Chemical
Biology, Skaggs predoctoral fellowship (P.W.), and the Sweden-
American Foundation (M.M.) is gratefully acknowledged. For
synthetic help and fruitful discussions, thanks to D. Thayer and
S. Narayan.
Fig. 1 MALDI mass spectrum of the differentiated, dye-labelled
dendrimer 13 showing efficient functionalization and monodispersity.
Notes and references
to the alkyne units resulting in 12. After introduction of the
1
6 alkynes (via esterification with the anhydride of pentynoic acid),
1
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unprotected 2-azidoethyl a-D-mannopyranoside in THF–water
to furnish the desired asymmetrical, dual functionalized dendrimer,
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14. Complete characterization of the asymmetric dendrimers by
GPC, NMR and MALDI spectroscopy showed essentially
monodisperse materials with quantitative functionalization of the
chain ends after every step. For example, the acetylene
functionalized, fluorescently labelled dendrimer 13 showed a single
2
3
4
+
+
molecular ion (MH = 4184; MNa = 4206) in the MALDI
spectrum which correlates with 2 coumarin and 16 acetylene chain
end groups (Fig. 1). Designed to bear peripheral groups for
polyvalent binding (mannose) and fluorescent dyes (coumarin) for
visualization/diagnostic purposes, this macromolecular structure is
an example of the sophisticated, multifunctional nanomaterials
that can be constructed in a stepwise, yet facile manner using Click
5
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13
methodology.
The performance of the mannosylated dendrimer was evaluated
in the standard hemagglutination assay using the mannose binding
14
protein concanvalin A and rabbit red blood cells. Dendrimer 14
exhibited 240-fold greater potency than monomeric mannose,
corresponding to a relative activity of 15 per sugar moiety when
compared to mannose (activity = 1). This demonstrates the
synergistic benefit provided by the multivalent, dendritic array of
receptor groups. A complete study of polyvalent affinity vs.
dendrimer size and generation number is underway and will be
described in the future.
8 V. V. Rostovsev, L. G. Green, V. V. Fokin and K. B. Sharpless, Angew.
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10 M. Malkoch, E. Malmstr o¨ m and A. Hult, Macromolecules, 2002, 35,
In summary, copper(I)-catalyzed azide–acetylene cycloaddi-
8307–8314.
11 See supporting information.
15
tion has proven to be a powerful tool for both the preparation
of unsymmetric diblock dendrimers and for efficient differentiation
of the dendritic chain end groups. By preparing dendrons with
unique acetylenic and azide groups at the focal point, highly
efficient coupling of these blocks was achieved through the
1
1
2 L. Zhu, V. M. Lynch and E. V. Anslyn, Tetrahedron, 2004, 60, 7267.
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1
formation of
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modification and sequential differentiation of the chain ends by
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 5775–5777 | 5777