Efficient synthesis of dendrimers via a thiol-yne and esterification process and their potential application in the delivery of platinum anti-cancer drugs

Article abstract of DOI:10.1039/b910340f

A combination of thiol-yne chemistry and esterification reactions were successfully applied for the preparation of dendrimers with an array of terminal functionalities via the divergent growth strategy; maximizing the number of reactive chain ends whilst

Full text of DOI:10.1039/b910340f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Efficient synthesis of dendrimers via a thiol–yne and esterification
process and their potential application in the delivery of platinum
anti-cancer drugsw
Gaojian Chen, Jatin Kumar, Andrew Gregory and Martina H. Stenzel*
Received (in Cambridge, UK) 27th May 2009, Accepted 21st August 2009
First published as an Advance Article on the web 4th September 2009
DOI: 10.1039/b910340f
A combination of thiol–yne chemistry and esterification reactions
were successfully applied for the preparation of dendrimers with
an array of terminal functionalities via the divergent growth
strategy; maximizing the number of reactive chain ends whilst
minimizing the number of reaction steps required in the process.
reactions.20 Recently another reaction, emerging as an attractive
click process, is the addition of thiols to alkenes, which is
called thiol–ene coupling or a thiol–ene click reaction.21–28 The
applicability of this reaction has been highlighted by Hawker
and co-workers, whereby they invented a new approach to
synthesize dendrimers via the thiol–ene chemistry.25 The process
could be carried out in the absence of solvent; it was initiated
photochemically and negated the need for a metal catalyst.
Akin to the aforementioned thiol–ene reaction, the thiol
addition to alkynes has been exploited as an approach in the
field of organic and organometallic chemistry for small molecular
synthesis since the reaction was discovered in the 1930s.29,30
However, this reaction has not been widely used in the field of
polymer/material synthesis, with only a few articles published
recently,31–36 none of which cite the synthesis of dendrimers.
The process is similar to the thiol–ene click reaction. Like the
thiol–ene reaction it is efficient and does not require a metal
catalyst but, for the thiol–yne system, the reaction proceeds by
reacting two equivalents of thiol with the alkyne, via a
two-step process.37 Therefore, after the reaction, each alkyne
functional group will be combined with two thiols, which
is very attractive and advantageous in the synthesis of
dendrimers.
Dendrimers are extremely well-defined, globular, synthetic
polymers with a number of characteristics which make them
useful in biological systems, especially in the field of drug
delivery.1,2 The nanometric size of dendrimers is beneficial for
their entry into hyper-permeable vasculature, while their high
molecular weight and lymphatic dysfunction cause their
localization, as well as preventing escape via the EPR effect.
In addition, their surfaces can be elegantly modified so that the
drugs can be physically entrapped, encapsulated or conjugated
by covalent bonds, hydrogen bonds or ionic interactions.3
Poly(amidoamine) (PAMAM) dendrimers, the first complete
dendrimer family to be synthesized as an example, are widely
used in different applications,4,5 most notably in drug delivery
and biomedical applications.6 The fine structure of dendrimers
brings with it excellent properties, but unfortunately synthetic
difficulties as well. Dendrimers are synthesized by using
repetitive steps of synthetic protocols, either adopting the
divergent7,8 or convergent9 approach. The synthetic protocols
utilized, however, typically require many sequential reaction
steps in order to obtain a high number of reactive terminal
groups. For example, eight steps are needed to obtain a
traditional dendrimer with forty-eight end-group functionalities.10
To maximize the yield of the desired dendrimers, highly
efficient and orthogonal reactions are required. It is also
favourable if the number of reaction steps can be reduced,
whilst maintaining a high number of functional groups. One
good example of this was reported by Majoral and co-workers
who employed the Staudinger reaction for the efficient synthesis
of dendrimers.11
Herein we report a new strategy for synthesizing dendrimers
utilizing thiol–yne chemistry. The reactions were very efficient
(only ten minutes were required for each thiol–yne reaction)
and only a limited number of steps were needed in order to
produce macromolecules with a high number of terminal
functional groups. For example, a dendrimer with forty-eight
end-group functionalities was constructed in three steps from
the core (Scheme 1). A benzene-based core was chosen for the
dendrimer synthesis with either carboxylic or hydroxyl
functional groups on the chain ends. The variety of different
terminal groups allows the dendrimers to be complexed or
conjugated with a range of different drug molecules. In this
communication a preliminary study was undertaken to test if a
platinum-based drug could be conjugated with the carboxylate
terminated dendrimer.
Copper catalyzed azide–alkyne click (CuAAC) chemistry is
a robust and efficient reaction, which has been successfully
applied to many applications, including dendrimer
synthesis.12–19 Malkoch and co-workers reported accelerated
synthesis of dendrimers via esterification and CuAAC click
The reaction between 1 and 1-thiolglycerol, in the presence
of trace amounts of the photoinitiator 2,2-dimethoxy-2-phenyl-
acetophenone (DMPA), was carried out at room temperature
by irradiation with UV-lamp (l = 365 nm) for ten minutes.
The thiol–yne reaction yielded the first generation of the
dendrimer [G1]-OH12 (2) with twelve hydroxyl functionalities.
The subsequent esterification of 2 with acetylene anhydride (3)10
produced the alkyne end-functional dendrimer [G1]-yne12 (4).
The thiol–yne reaction between (4) and 1-thiolglycerol
Centre for Advanced Macromolecular Design (CAMD), School of
Chemical Science and Engineering, The University of New South
Wales, Sydney, NSW 2052, Australia.
E-mail: M.Stenzel@unsw.edu.au; Tel: +61-293854344
w Electronic supplementary information (ESI) available: Synthetic
procedures; GPC, IR, NMR and MALDI-TOF of dendrimers;
experimental methods. See DOI: 10.1039/b910340f
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6291–6293 | 6291

View Article Online
1
Fig. 1 Representative H NMR spectrum of the dendrimer (5).
Scheme 1 Synthesis of hydroxyl-terminated dendrimer.
Fig. 2 MALDI-TOF spectra of the dendrimers (2), (4) and (5).
afforded the dendrimer (5) with forty-eight hydroxyl
functionalities. It is worth noting that further purification
using chromatography was needed here, after the thiol–yne
reaction, due to the impurities in 1-thiolglycerol. Contrary to
this, the other reaction using thioglycolic acid afforded the
pure product after precipitation. Characterization of the
purified products was conducted using NMR spectroscopy,
gel-permeation chromatography (GPC) and MALDI-TOF
mass spectrometry.
A typical NMR spectrum for the
dendrimer (5) can be seen in Fig. 1. The single peak at 8.43
confirms the symmetrical dendritic framework. The mono-
disperse nature of each generation of the dendrimer was
further demonstrated and complimented by the GPC and
MALDI-TOF data. From the MALDI-TOF results, a strong
signal for each macromolecule is observed in all cases at the
expected molecular weight with only minor contamination
from defect structures (Fig. 2). The GPC traces indicated
that both the hydroxyl and alkyne-terminal dendrimers are
monodisperse, with the expected increase in molecular size as
the generation number is increased (Fig. 3). Dendrimer (5)
was further converted to alkyne functionality and reacted with
1-thiolglycerol to afford a dendrimer (8) with 192 hydroxyl
functional groups. This demonstrates that the thiol–yne
process is a convenient and efficient method to synthesize
dendrimers with fewer steps compared with other conventional
routes. In addition, dynamic light scattering (DLS) has been
used to analyze the size of dendrimers synthesized, which vary
between 2 and 4 nm, similar to the traditional PAMAM
dendrimer (see ESIw).
Fig. 3 GPC traces of the dendrimers (2), (4), (5) and (8).
To further demonstrate the efficiency of the thiol–yne
reaction in the construction of the dendritic backbone with
other chain end functionalities, a second thiol–yne reaction to
introduce carboxyl functionalities onto the chain ends was
carried out. Previous research has shown that anionic carboxyl
functionalized dendrimers have a significantly lower cytotoxicity
compared to the cationic amino-terminated dendrimers1 and
that carboxylated terminal half-generation PAMAM can be
conjugated to platinum-based drugs.38 In this communication,
[G1]-yne12 (4) was reacted with thioglycolic acid to afford a
dendrimer with twenty-four carboxylic acid terminal function-
alities (6). cis-Dichlorodiammineplatinum(II) (CDDP) was
then used to test the viability of the new carboxylic terminal
dendrimer (6) as a carrier (Scheme 2). The amount of CDDP
ꢀc
This journal is The Royal Society of Chemistry 2009
6292 | Chem. Commun., 2009, 6291–6293

View Article Online
5 L. Tao, J. Geng, G. Chen, Y. Xu, V. Ladmiral, G. Mantovani and
D. M. Haddleton, Chem. Commun., 2007, 3441–3443.
6 R. Esfand and D. A. Tomalia, Drug Discovery Today, 2001, 6,
427–436.
7 D. A. Tomalia, H. Baker, J. Dewald, M. Hall, G. Kallos,
S. Martin, J. Roeck, J. Ryder and P. Smith, Polym. J. (Tokyo),
1985, 17, 117–132.
8 G. R. Newkome, Z. Yao, G. R. Baker and V. K. Gupta, J. Org.
Chem., 1985, 50, 2003–2004.
9 C. J. Hawker and J. M. J. Frechet, J. Am. Chem. Soc., 1990, 112,
7638–7647.
10 P. Antoni, Y. Hed, A. Nordberg, D. Nystrom, H. von Holst,
A. Hult and M. Malkoch, Angew. Chem., Int. Ed., 2009, 48,
2126–2130.
11 V. Maraval, A. M. Caminade, J. P. Majoral and J. C. Blais, Angew.
Chem., Int. Ed., 2003, 42, 1822–1826.
12 J.-F. Lutz, Angew. Chem., Int. Ed., 2007, 46, 1018–1025.
13 W. H. Binder and R. Sachsenhofer, Macromol. Rapid Commun.,
2007, 28, 15–54.
14 C. Ornelas, J. Ruiz Aranzaes, E. Cloutet, S. Alves and D. Astruc,
Angew. Chem., Int. Ed., 2007, 46, 872–877.
15 C. N. Urbani, C. A. Bell, M. R. Whittaker and M. J. Monteiro,
Macromolecules, 2008, 41, 1057–1060.
Scheme 2 The chemical structures of the cisplatin (CDDP) and the
carboxylic terminal dendrimer (6).
16 M. J. Joralemon, R. K. O’Reilly, J. B. Matson, A. K. Nugent,
C. J. Hawker and K. L. Wooley, Macromolecules, 2005, 38,
5436–5443.
in the conjugate was determined by a method described in a
previous study,39 and a loading of 1.37 mmol gÀ1 of platinum
was found in reference to standard solutions of free CDDP,
which corresponds to a loading of approximately 86%, based
on the theoretic value of 1.59 mmol gÀ1 whereby all the
platinum species present have been conjugated to the dendrimer.
In summary, the thiol–yne reaction was successfully applied
to the synthesis of dendritic macromolecules via a divergent
approach. Dendrimers with one hundred and ninety-four
hydroxyl functionalities and twenty-four carboxyl groups were
synthesized, maintaining a high number of reactive chain ends
with the bare minimum of reactions. The latter system was
further tested as a possible drug delivery vehicle with the
successful conjugation of the dendrimer with cisplatin. The
metal-free and benign reaction conditions allow for an environ-
mentally friendly process to be developed which is more
attractive for drug delivery related applications. The platinum
conjugation experiment, although only a preliminary study,
has shown a possible application for these macromolecules
hence, further investigations in regard to the dendrimer acting
as a drug carrier are currently being carried out. Most
importantly, we believe this approach featuring the robust
and efficient nature of thiol–yne reaction will not only benefit
dendrimer-related synthesis but shows great potential as a
versatile synthetic tool for the fabrication of different well-
defined functional macromolecules.
17 M. Malkoch, K. Schleicher, E. Drockenmuller, C. J. Hawker,
T. P. Russell, P. Wu and V. V. Fokin, Macromolecules, 2005, 38,
3663–3678.
18 P. Wu, M. Malkoch, J. N. Hunt, R. Vestberg, E. Kaltgrad,
M. G. Finn, V. V. Fokin, K. B. Sharpless and C. J. Hawker,
Chem. Commun., 2005, 5775–5777.
19 R. Vestberg, M. Malkoch, M. Kade, P. Wu, V. V. Fokin,
K. B. Sharpless, E. Drockenmuller and C. J. Hawker, J. Polym.
Sci., Part A: Polym. Chem., 2007, 45, 2835–2846.
20 P. Antoni, D. Nystroem, C. J. Hawker, A. Hult and M. Malkoch,
Chem. Commun., 2007, 2249–2251.
21 A. Gress, A. Voelkel and H. Schlaad, Macromolecules, 2007, 40,
7928–7933.
22 L. M. Campos, K. L. Killops, R. Sakai, J. M. J. Paulusse,
D. Damiron, E. Drockenmuller, B. W. Messmore and
C. J. Hawker, Macromolecules, 2008, 41, 7063–7070.
23 J. W. Chan, B. Yu, C. E. Hoyle and A. B. Lowe, Chem. Commun.,
2008, 4959–4961.
24 G. Chen, S. Amajjahe and M. H. Stenzel, Chem. Commun., 2009,
1198–1200.
25 K. L. Killops, L. M. Campos and C. J. Hawker, J. Am. Chem. Soc.,
2008, 130, 5062–5064.
26 C. Diehl and H. Schlaad, Macromol. Biosci., 2009, 9, 157–161.
27 A. Dondoni, Angew. Chem., Int. Ed., 2008, 47, 8995–8997.
28 L. Nurmi, J. Lindqvist, R. Randev, J. Syrett and D. M. Haddleton,
Chem. Commun., 2009, 2727–2729.
29 C. Finzi, G. Venturini and L. Sartini, Gazz. Chim. Ital., 1930, 60,
798–811.
30 E. P. Kohler and H. Potter, J. Am. Chem. Soc., 1935, 57,
1316–1321.
31 E. Kobayashi, T. Yoshino, S. Aoshima and J. Furukawa, J. Polym.
Sci., Part A: Polym. Chem., 1995, 33, 2403–2414.
32 B. D. Fairbanks, T. F. Scott, C. J. Kloxin, K. S. Anseth and
C. N. Bowman, Macromolecules, 2009, 42, 211–217.
33 T. Ohashi, E. Kobayashi, T. Jinbo and J. Furukawa, J. Polym.
Sci., Part A: Polym. Chem., 1997, 35, 1621–1625.
34 B. Ochiai, I. Tomita and T. Endo, Polym. Bull. (Berlin), 2004, 51,
263–269.
The authors are grateful for financial support from the
Australian Research Council (ARC) in the form of a
Discovery Grant. We thank Dr S. Amajjahe for helpful
discussions.
35 J. W. Chan, H. Zhou, C. E. Hoyle and A. B. Lowe, Chem. Mater.,
2009, 21, 1579–1585.
36 J. W. Chan, C. E. Hoyle and A. B. Lowe, J. Am. Chem. Soc., 2009,
131, 5751–5753.
37 J. C. Sauer, J. Am. Chem. Soc., 1957, 79, 5314–5315.
38 N. Malik, E. G. Evagorou and R. Duncan, Anti-Cancer Drugs,
1999, 10, 767–776.
Notes and references
1 U. Boas and P. M. H. Heegaard, Chem. Soc. Rev., 2004, 33, 43–63.
2 M. J. Cloninger, Curr. Opin. Chem. Biol., 2002, 6, 742–748.
3 A. Agarwal, A. Asthana, U. Gupta and N. K. Jain, J. Pharm.
Pharmacol., 2008, 60, 671–688.
4 L. Tao, G. Chen, G. Mantovani, S. York and D. M. Haddleton,
Chem. Commun., 2006, 4949–4951.
39 B. Schechter, M. A. Rosing, M. Wilchek and R. Arnon, Cancer
Chemother. Pharmacol., 1989, 24, 161–166.
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 6291–6293 | 6293