Construction of well-defined multifunctional dendrimers using a trifunctional core

Article abstract of DOI:10.1039/b913139f

A simple synthetic strategy was developed for the synthesis of well-defined multifunctional poly(amide)-based dendrimers using a trifunctional core.

Full text of DOI:10.1039/b913139f

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Construction of well-defined multifunctional dendrimers using
a trifunctional corew
´
Catia Ornelas and Marcus Weck*
Received (in Austin, TX, USA) 2nd July 2009, Accepted 28th July 2009
First published as an Advance Article on the web 18th August 2009
DOI: 10.1039/b913139f
A simple synthetic strategy was developed for the synthesis of
well-defined multifunctional poly(amide)-based dendrimers using
a trifunctional core.
the synthesis of well-defined bifunctional dendrimers either by
combination of two different functionalized dendrons12,13 or
by using ABxCy-type dendrons,4,14–16 where x Z 1 and y = 1,
and A is the functional group at the focal point. These
strategies, however, are limited to two functionalities and
do not allow for the controlled incorporation of multiple
functionalities. Introduction of more than two functionalities
was only achieved for benzyl ether-based17,18 and triazine-based19
dendrimers. We have reported the synthesis of trifunctional
poly(amide)-based dendrimers containing 16 protected acid
groups, one azide and one aldehyde groups and have
demonstrated their orthogonal functionalization by using the
copper-catalyzed 1,3 dipolar cycloaddition (CuAAC) and
Schiff base coupling.20,21 While highly successful, our initial
strategy requires extensive synthesis potentially limiting
the general use of this strategy. Here we introduce a new
concept towards multifunctionalized dendrimers starting
with a trifunctional core which can be monofunctionalized
selectively. Dendrons bearing different functionalities can be
added to the trifunctional core, affording a well-defined
multifunctional dendrimer. To demonstrate our new strategy,
we describe the synthesis of a dendrimer containing one
fluorescent dye, nine azide groups and nine acid groups that
are available for further functionalization (Scheme 1).
Dendrimers are perfectly branched macromolecules possessing
a high number of active termini that are partially responsible
for their properties and functions.1–3 Synthetic methodologies
for the construction of dendrimers are categorized into either
divergent4 or convergent5 strategies, both consisting of stepwise,
iterative reaction processes based on branched monomers.
Therefore, appropriate selection of monomers, cores, and
mode of assembly ultimately defines the physical properties
of dendrimers, such as size, shape, molecular weights, chemical
properties, solubility, viscosity, polydispersities, thermal behaviors,
and internal and surface functionalization.6
Traditionally, dendrimers are synthesized from ABx monomers,
resulting in symmetrical structures with B terminal groups.4
As dendritic materials migrate into new research areas, the
demand on their structural complexity is increasing. For
example, the use of dendrimers in theranostics requires
multiple functionalities for drug delivery, imaging and
targeting. This has led to the partial elucidation of general
design principles for the relationship between dendrimer
architecture, biocompatibility, retention and drug release.7
The ‘‘ideal’’ dendritic drug carrier for cancer therapy should
contain several functionalities including: (i) hydrophilic groups
such as PEG chains in order to increase water solubility and
biocompatibility; (ii) imaging agents such as fluorescent dyes
or gadolinium complexes in order to monitor the trajectory of
the dendrimer in vitro or/and in vivo; (iii) targeting groups,
such as folic acid, biotin or antibodies, to increase binding
specificity to cancer cells; and finally (iv) the drug, either
encapsulated in the dendritic structure or covalently attached
by hydrolysable bonds.7–11 Pertinent to the realization
of such a complex dendrimer-based drug carrier is the
multifunctionalization of a dendrimer.
The dendrons used in our strategy are poly(amide)-based to
assure biocompatibility and follow the 1 - 3 connectivity
pioneered by Newkome et al.22 The synthesis of the dendrons
was carried out using peptide coupling steps between
commercially available monomers
1 and 2 to yield
dendron 3 (Scheme 2).21,22 After reduction of the nitro group
in 3 affording the dendron amino-nona-ester 4, a PEG spacer
(5) containing a fmoc protected amine was introduced at the
focal point to enhance chemical accessibility of the amine
group and to increase water solubility. This coupling reaction
Strategies to multifunctionalize dendrimers are limited.
Multifunctionalization of PAMAM dendrimers, the leading
dendrimer scaffold used for biological applications, is mainly
achieved using a random statistical approach, i.e. by successive
partial functionalization of the total amine termini9 with a lack
of control over the final structures. There are a few reports on
Molecular Design Institute, Department of Chemistry,
New York University, New York, NY 10003-6688, USA.
E-mail: Marcus.weck@nyu.edu; Tel: +1 212-992-7968
w Electronic supplementary information (ESI) available: Detailed
experimental procedures, characterization data, and 1H NMR and
mass spectra. See DOI: 10.1039/b913139f
Scheme
1 Schematic representation of our strategy towards
well-defined multifunctional dendrimers using dendrons bearing different
functionalities that are selectively attached to a trifunctional core.
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Scheme 2 Synthesis of the poly(amide)-based dendrons.
was performed using HATU as the coupling agent. While
most peptide couplings involving HATU use diisopropylethyl-
amine (DIPEA) as the base, it resulted in fmoc deprotection
during the coupling reaction. Therefore, triethylamine was
successfully used as base for the coupling reactions that
involve molecules containing fmoc groups. Dendron 6 was
purified by column chromatography using silica gel and ethyl
acetate as the eluent. The acid groups of 6 were selectively
deprotected in the presence of the fmoc-amine group on the
focal point, using formic acid, affording dendron 7. The azide
functionality was introduced into the dendron by coupling 7
with 3-aminopropyl azide (8) to obtain dendron 9. Deprotection
of the fmoc-amine group at the focal point was performed in
20 min, using piperidine in DMF at room temperature to yield
the final target structure 10. Dendron 10 was purified by
dialysis against water using a Spectra-Pors MWCO (molecular
weight cut-off) 500–1000 dialysis membrane and was obtained
as a colorless waxy product (Scheme 2). The mass spectrum of
dendron 10 shows its molecular peak (M+H)+ at 1819.4 m/z
(calcd for C73H128N41O15: 1819.1), confirming the expected
structure. Every step of the synthesis was carried out in above
90% yield making the synthetic strategy highly efficient and
usable on the larger scale.
fluorescent dye (fluorescein) via CuAAC, an efficient click
reaction.23 The reaction occurred in an orthogonal fashion
between the alkyne of 15 and the azide derivative of
fluorescein 16 affording compound 17 that was purified by
reverse-phase column chromatography and was obtained as a
yellow solid in 62% yield. Dendron 10 was then attached to
the acid group of 17 using peptide coupling affording, after
purification by dialysis against methanol using a Spectra-Pors
MWCO 2000 dialysis membrane, compound 18 in 72% yield.
The 1H NMR spectrum of 18 shows the signals corresponding
to the fluorescein and the trifunctional core in the aromatic
region, the Boc group at 1.52 ppm, the PEG linker between 3.5
and 4.0 ppm, and the peaks corresponding to the dendron,
including the CH2N3 at 3.36 ppm. After deprotection of the
amine group using formic acid (the disappearance of the Boc
1
protons was followed by H NMR spectroscopy) dendron 20
containing one acid group at the focal point and nine
protected acid groups at the periphery was attached to the
amino group of the trifunctional core. Dendrimer 21 was
obtained in 81% yield after purification by dialysis against
methanol using
a
Spectra-Pors MWCO 2000 dialysis
membrane. Due to the addition of dendron 20 that contains
tBu termini, we observed a significant increase in the hydro-
phobicity of dendrimer 21 compared to 19 resulting in good
solubility in methylene chloride. This fact, together with NMR
spectroscopy and mass spectrometry, confirmed the successful
attachment of the third functionality to the trifunctional core.
The final step of the synthesis was the deprotection of the acid
groups of 21 to afford the multifunctional target dendrimer 22
containing one fluorescent dye, nine azide groups and nine
acid groups. The mass spectrum of 22 shows its molecular ion
peak (M+H)+ at 3524.2 (calcd for C152H219N52O45S: 3524.6)
confirming the structure of the final well-defined multi-
functional dendrimer (see ESIw for experimental details,
Our trifunctional core 15 was synthesized from dimethyl
5-aminoisophthalate 11 in four steps in 44% overall yield
(Scheme 3) and contains (i) an alkyne group suitable for 1,3
dipolar cycloaddition using copper catalysts (CuAAC), (ii) a
carboxylic acid available for amide or ester formation and (iii)
a Boc-protected amine. The structure of core 15 was confirmed
by 1H NMR spectroscopy that shows the aromatic CH at 8.07
and 8.02 ppm, the CH2CRCH at 4.15 ppm, CH2CRCH at
2.62 ppm and the tBu protons of the Boc group at 1.52 ppm,
and by mass spectroscopy showing the molecular ion peak
(MÀH)À at 317.4 m/z (calcd for C15H17N2O5: 317.1).
1
The attachment of the functionalities to the trifunctional
core was carried in a stepwise fashion by first introducing a
characterization data and H NMR and mass spectra of all
compounds).
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Scheme 3 Synthesis of the multifunctional dendrimer 22.
In conclusion, in this contribution, we introduce a new
multifunctionalization strategy in dendrimer chemistry. All
steps have been carried out in high yields and allow for the
introduction of a wide variety of functional groups into
dendrimers. We suggest that this strategy permits for synthesis
of highly functionalized dendrimer-based materials for a
number of applications including theranostics. For example,
the dendrimer reported here might potentially be used in anti-
cancer technology by attaching (i) targeting moieties using
CuAAC reaction, (ii) drugs via the unprotected acid groups on
the surface of the dendrimer as well as (iii) different imaging
agents. Therefore, this strategy can be viewed as a ‘‘toolbox’’
of drug delivery carriers by varying the dendron functionalities
and size. Further development of this ‘‘toolbox’’ as well as its
application to anti-cancer studies are currently being carried
out in our group.
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