Orthogonally "clickable" biodegradable dendrons

Article abstract of DOI:10.1021/ma200593r

A new class of biodegradable polyester dendrons which are orthogonally functionalizable have been synthesized and their multifunctionality has been demonstrated. The surface of these dendrons were appended with alkyne groups that can undergo efficient Huisgen type "click" cycloaddition reaction, while their focal point consists of a reactive maleimide group that can undergo facile conjugation with thiol containing molecules. Multiarm poly(ethylene glycol) polymer with a maleimide-based reactive focal point was synthesized using such a dendron. Solubilization of a hydrophobic dye, BODIPY-SH, in aqueous solution via conjugation to these reactive multiarm PEGs demonstrate possible applications of such dendrons to design water-soluble scaffolds for drug delivery.

Full text of DOI:10.1021/ma200593r

ARTICLE
pubs.acs.org/Macromolecules
Orthogonally “Clickable” Biodegradable Dendrons
Meliha Merve Kose, Sebla Onbulak, Idil Ipek Yilmaz, and Amitav Sanyal*
Department of Chemistry, Bogazici University, Bebek 34342 Istanbul, Turkey
S Supporting Information
b
ABSTRACT: A new class of biodegradable polyester dendrons
which are orthogonally functionalizable have been synthesized
and their multifunctionality has been demonstrated. The sur-
face of these dendrons were appended with alkyne groups that
can undergo efficient Huisgen type “click” cycloaddition reac-
tion, while their focal point consists of a reactive maleimide
group that can undergo facile conjugation with thiol containing
molecules. Multiarm poly(ethylene glycol) polymer with a
maleimide-based reactive focal point was synthesized using
such a dendron. Solubilization of a hydrophobic dye, BODI-
PY-SH, in aqueous solution via conjugation to these reactive
multiarm PEGs demonstrate possible applications of such
dendrons to design water-soluble scaffolds for drug delivery.
’ INTRODUCTION
circulation time of drug molecules compared to the linear
polymers of the same molecular weight.21,22
Dendrimers are a unique class of globular macromolecules
that are synthesized by stepwise organic reactions.1ꢀ6 Their
method of synthesis makes dendrimers monodisperse, which is
a desirable but still hard to achieve property for macromolecules.
The fact that they are monodisperse makes them promising
scaffolds for drug delivery.7ꢀ10 Another advantage that stems
from the dendrimers architecture is the multivalency provided by
the large number of functional groups at the periphery of the
dendritic structure. It is possible to supplement the multivalent
nature of dendritic structures with multifunctionality by incor-
poration of orthogonally functionalizable reactive groups.11ꢀ17
Dendrons serve as the building blocks or components that are
stitched together during a “convergent” approach toward den-
drimer synthesis. These building blocks once primarily used for
the aforementioned purpose are increasingly becoming common
building blocks for synthesis of novel macromolecular architec-
tures such as dendronized polymers, and dendron-linear polymer
conjugates. In recent years, dendrons have been widely used to
obtain multivalent initiators bearing cores for synthesis of star
polymers.18,19 As an alternative, near monodisperse polymers
such as poly(ethylene glycol)s have been conjugated to dendrons
to provide dendron-polymer hybrids that possess the well-
defined structure and monodisperse attributes of dendrimers.20
Devising these kinds of systems on macromolecular scaffolds is
very desirable, especially in areas of polymer conjugated drug
delivery. Attachment of PEG chains to drug molecules has been
studied extensively both as dendritic structures and as linear
scaffolds to increase bioavailability. Interestingly, the branch-
ed structure of polymeric scaffolds was found to increase the
Recent years have focused on extending the multivalent nature
of polymeric structures to simultaneously allow incorporation of
multifunctionality. Successful strategies have been demonstrated
by Hawker and co-workers for functionalization of polymers
using the copper catalyzed Huisgen type 1,3-dipolar cycloaddi-
tion with other chemistries such as esterification and
amidation.23 An elegant study of simultaneous orthogonal
functionalization of polymers with thiol and amine group
containing molecules was reported by Thayumanavan and
co-workers.24 Recently Weck et al. demonstrated orthogonal
multifunctionalization of polymers by using copper catalyzed
1,3-dipolar cycloaddition, hydrazone formation and thiol
conjugation.25,26 Furthermore, such orthogonal transforma-
tions have recently been utilized for efficient synthesis of
macromolecules such as dendrimers, star polymers and het-
erograft copolymer synthesis.27ꢀ31
Alternative “click” reactions that have emerged as valuable
tools in macromolecular engineering have been the DielsꢀAlder
cycloaddition and thiol-maleimide conjugation reaction. The
incorporation of maleimide unit into macromolecular constructs
allow facile attachment of a variety of molecules since the
maleimide moiety participates efficiently in Michael addition
reactions with thiols and DielsꢀAlder cycloaddition reactions
with electron rich diene containing molecules.32ꢀ38
Received: March 15, 2011
Published: March 31, 2011
r
2011 American Chemical Society
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Scheme 1. Orthogonally Functionalizable Dendron as a
Scaffold for Core Reactive Multiarm Polymers
Figure 1. Chemical structures of the orthogonally functionalizable
dendrons.
anhydride with (1:1) mixture of pyridine and water (4.0 mL) for 12 h.
Reaction mixture was extracted with 1 M NaHSO4 (3 ꢁ 20 mL), 10%
Na2CO3 (3 ꢁ 20 mL) and then with brine (1 ꢁ 20 mL) combined
organic layers were dried over anhydrous Na2SO4. The residue was
concentrated in vacuo. Crude product was purified by column chroma-
tography and then the isolated product was dissolved in MeOH (30 mL)
and to this solution Dowex Hþ resin was added with a tip of spatula. The
resulting mixture was stirred at ambient temperature until the consump-
tion of the starting material was observed via TLC. The resin was then
filtered off and washed with MeOH. The filtrate was concentrated in
vacuo to give a white solid, 1a (1.78 g, 99% yield). 1a (0.4 g, 1.1 mmol)
was then added to a solution of DMAP (0.07 g, 0.6 mmol), pyridine
(0.8 mL) and compound C (0.60 g, 3.3 mmol) in dry CH2Cl2 (5 mL).
The mixture was then stirred at room temperature for 12 h. Excess
anhydride was quenched with water (2.0 mL) for 3 h. Reaction mixture
was diluted with 50 mL CH2Cl2 and extracted with 1 M NaHSO4
(3 ꢁ 20 mL), 10% Na2CO3 (3 ꢁ 20 mL) and then with brine (1 ꢁ
20 mL) combined organic layers were dried over anhydrous Na2SO4.
The residue was concentrated in vacuo. Crude product was purified by
column chromatography to give 0.52 g of 1b as a colorless viscous liquid
(95% yield).
Herein we report synthesis of orthogonally functionalizable
and biodegradable bis-MPA based dendrons that are furnished
with alkyne moieties at the periphery and a maleimide group at
the focal point. In order to provide examples demonstrating one-
pot orthogonal functionalization property of these novel den-
drons, a series of reactions with benzyl azide and undecene thiol
was made. Furthermore, to demonstrate the utility of their
multifunctionality, the periphery of these dendrons were mod-
ified with PEG chains via copper catalyzed Huisgen click reaction
and the focal point was employed to attach BODIPY thiol, a
hydrophobic dye via efficient Michael addition (Scheme 1).
’ EXPERIMENTAL SECTION
Materials and Characterization. Chemicals were purchased
1H NMR (CDCl3, δ, ppm): 6.49 (s, 2H, CHdCH), 5.24 (s, 2H, CH
bridgehead protons), 4.28 (dd, 4H, J = 18.8, 11.1 Hz, CH2 ester
protons), 4.07 (t, 2H, J = 6.2 Hz, OCH2), 3.55 (t, 2H, J = 6.8 Hz,
NCH2), 2.82 (s, 2H, bridge protons), 2.57(m, 4H, CH2CH2C ꢂ CH),
2.45 (m, 4H, CH2CH2CtCH), 1.96 (t, 2H, J = 2.54 Hz, C ꢂ CH), 1.91
(tt, 2H, J = 6.8, 6.4 Hz, NCH2CH2CH2O), 1.27 (s, 3H, CCH3). 13C
NMR (CDCl3, δ, ppm): 176.0, 172.4, 171.1, 136.4, 82.3, 80.9, 69.2, 65.4,
61.9, 47.3, 46.2, 35.2, 33.1, 26.6, 17.8, 14.2. FTIR (cmꢀ1): 1732.0,
1695.1.
from commercial resources and were used as received unless otherwise
stated. Characterization of compounds were carried out with 1H and 13
C
solution NMR spectroscopy (Varian 400 MHz), Fourier transform
infrared (FTIR) spectra were recorded on a Nicolet 360 spectrometer.
Gel Permeation Chromatography (GPC) data was acquired with
Viscotek GPCmax VE-2001 analysis system. PLgel (length/ID 300
mm ꢁ 7.5 mm, 5 μm particle size) Mixed-C column was calibrated
with polystyrene standards, using refractive index detector. THF was
used as eluent at a flow rate of 1 mL/min at 30 °C.
Synthesis of 1b. Compounds A and B were prepared according to
previously reported literature procedures.39,40 To a solution of A (2.37 g,
10.50 mmol) in dry CH2Cl2 (9 mL), B (5.20 g, 15.7 mmol), DMAP
(0.52 g, 4.20 mmol) and pyridine (3.2 mL) was added. The mixture was
stirred at ambient temperature for 12 h followed by quenching of excess
Synthesis of 2b. Compound 1a (1.1 g, 3.2 mmol) was added to a
solution of DMAP (0.30 g, 2.4 mmol), pyridine (2.0 mL) and B (3.20 g,
9.7 mmol) in dry CH2Cl2 (5 mL). The mixture was then stirred at
room temperature for 12 h. Excess anhydride was quenched with (1:1)
mixture of pyridine and water (4.5 mL) for 12 h. Reaction mixture was
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Figure 2. Synthesis of bifunctional dendrons.
diluted with 50 mL CH2Cl2 and extracted with 1 M NaHSO4 (3 ꢁ
20 mL), 10% Na2CO3 (3 ꢁ 20 mL) and then with brine (1 ꢁ 20 mL)
combined organic layers were dried over anhydrous Na2SO4. The
residue was concentrated in vacuo. Crude product was purified by
column chromatography. The isolated product was then dissolved in
MeOH (10 mL) and to this solution Dowex Hþ resin was added with
the tip of a spatula. The resulting mixture was stirred at ambient
temperature until the consumption of was observed via TLC. The resin
was then filtered off and washed with MeOH. The filtrate was
concentrated in vacuo to give a white solid, 2a, (0.86 g, 98% yield). 2a
(0.14 g, 0.20 mmol) was then added to a solution of DMAP (0.03 g, 0.20
mmol), pyridine (0.4 mL) and C (0.26 g, 1.20 mmol) in dry CH2Cl2
(3 mL). The mixture was then stirred at room temperature for 12 h.
Excess anhydride was quenched with water (2.0 mL) for 3 h. Reaction
mixture was diluted with 20 mL CH2Cl2 and extracted with 1 M
NaHSO4 (3 ꢁ 20 mL), 10% Na2CO3 (3 ꢁ 20 mL) and then with
brine (1 ꢁ 20 mL) combined organic layers were dried over anhydrous
Na2SO4. The residue was concentrated in vacuo. Crude product was
purified by column chromatography to give 0.20 g of 2b as a colorless
viscous liquid (91% yield).
1H NMR (CDCl3, δ, ppm): 6.50 (s, 2H, CHdCH), 5.24 (s, 2H, CH
bridgehead protons), 4.27 (s, 4H), 4.24 (d, 4H, J = 11.3 Hz, CH2 ester
protons), 4.21 (d, 4H, J = 11.2 Hz, CH2 ester protons), 4.04 (t, 2H, J =
6.2 Hz, OCH2), 3.56 (t, 2H, J = 6.8 Hz, NCH2), 2.84 (s, 2H, bridge
protons), 2.56ꢀ2.53 (m, 8H, CH2CH2CtCH), 2.49ꢀ2.44 (m, 8H,
CH2CH2CtCH), 1.96 (t, 4H, J = 2.6 Hz, CtCH), 1.92 (tt, 2H, J = 6.8,
6.2 Hz, NCH2CH2CH2O), 1.27 (s, 3H, CCH3), 1.23 (s, 6H, CCH3).
13C NMR (CDCl3, δ, ppm): 176.1, 171.9, 171.8, 171.1, 136.4, 82.3, 80.9,
69.2, 65.6, 65.3, 61.9, 47.4, 46.6, 46.3, 35.2, 33.1, 26.5, 17.8, 17.5, 14.2.
FTIR (cmꢀ1): 1732.0, 1697.8.
Synthesis of 3b. Compound 2b (1.1 g, 3.2 mmol) was added to a
solution of DMAP (0.30 g, 2.4 mmol), pyridine (2.0 mL) and B (3.20 g, 9.7
mmol) in dry CH2Cl2 (5 mL). The mixture was then stirred at room
temperature for 12 h. Excess anhydride was quenched with (1:1) mixture of
pyridine and water (4.5 mL) for 12 h. Reaction mixture was diluted with
50 mL CH2Cl2 and extracted with 1 M NaHSO4 (3 ꢁ 20 mL), 10%
Na2CO3 (3 ꢁ 20 mL) and then with brine (1 ꢁ 20 mL) combined organic
layers were dried over anhydrous Na2SO4. The residue was concentrated in
vacuo. Crude product was purified by column chromatography and then the
isolated product was dissolved in MeOH (8 mL) and to this solution
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the reaction was monitored by TLC until consumption of compound 1b
is observed. The mixture was then concentrated in vacuo to give 1 (0.12
g, 98%) as a pale yellow viscous liquid.
1H NMR (CDCl3, δ, ppm): 6.68 (s, 2H, CHdCH), 4.29 (d, 2H, J =
11.2 Hz, CH2 ester protons), 4.24 (d, 2H, J = 11.2 Hz, CH2 ester
protons), 4.08 (t, 2H, J = 6.2 Hz, OCH2), 3.59 (t, 2H, J = 6.8 Hz, NCH2),
2.56ꢀ2.53 (m, 4H, CH2CH2CtCH), 2.48ꢀ2.44 (m, 4H,
CH2CH2CtCH), 1.99ꢀ1.90 (m, 4H, CtCH and NCH2CH2CH2O),
1.26 (s, 3H, CCH3). 13C NMR (CDCl3, δ, ppm): 172.5, 171.2, 170.5,
134.2, 82.3, 69.2, 65.5, 62.2,46.4, 34.6, 33.2, 27.6, 17.8, 14.3. FTIR
(cmꢀ1): 1732.5, 1701.8. Anal. Found: C, 61.2; H, 5.9; N, 3.7. Calcd for
C22H25NO8: C, 61.25; H, 5.8; N, 3.25.
Synthesis of 2. Compound 2b (0.25 g, 0.28 mmol) was dissolved in
dry toluene (15 mL) and the mixture was heated to reflux. Progress of
the reaction was monitored by TLC until consumption of compound 2b
is observed. The mixture was then concentrated in vacuo to give 2 (0.20
g, 91%) as a pale yellow viscous liquid.
1H NMR (CDCl3, δ, ppm): 6.70 (s, 2H, CHdCH), 4.26 (s, 4H, CH2
ester protons), 4.25 (d, 4H, J = 11.2 Hz, CH2 ester protons), 4.21 (d, 4H,
J = 11.2 Hz, CH2 ester protons), 4.07 (t, 2H, J = 6.4 Hz, OCH2), 3.61 (t,
2H, J = 6.4 Hz, NCH2), 2.56ꢀ2.53 (m, 8H, CH2CH2CtCH),
2.48ꢀ2.44 (m, 8H, CH2CH2CtCH), 1.98ꢀ1.92 (m, 6H, CtCH
and NCH2CH2CH2O), 1.27 (s, 3H, CCH3), 1.24 (s, 6H, CCH3). 13
C
NMR (CDCl3, δ, ppm): 171.6, 170.8, 170.3, 133.9, 82.1, 69.1, 65.3, 65.0,
61.9, 46.3, 46.0, 34.0, 32.8, 27.2, 17.5, 17.3, 13.9. FTIR (cmꢀ1): 1732.2,
1704.7. Anal. Found: C, 61.15; H, 6.0; N, 2.0. Calcd for C42H49NO16: C,
60.8; H, 5.85; N, 1.7.
Synthesis of 3. Compound 3b (0.07 g, 0.042 mmol) was dissolved
in dry toluene (10 mL) and the mixture was heated to reflux. Progress of
the reaction was monitored by TLC until consumption of compound 3b
is observed. The mixture was then concentrated in vacuo to give 3 (0.06
g, 91%) as a pale yellow viscous liquid.
1H NMR (CDCl3, δ, ppm) 6.71 (s, 2H, CHdCH), 4.30ꢀ4.19 (m,
28H, CH2 ester protons), 4.09 (t, 2H, J = 5.6 Hz, OCH2), 3.61 (t, 2H, J =
6.0 Hz, NCH2), 2.56ꢀ2.52 (m, 16H, CH2CH2CtCH), 2.50ꢀ2.44 (m,
16H, CH2CH2CtCH), 1.97 (bs, 8H, CtCH), 1.84 (bm, 2H,
NCH2CH2CH2O), 1.30 (s, 3H, CCH3), 1.23 (s, 18H, CCH3). 13C
NMR (CDCl3, δ, ppm): 171.9, 171.8, 171.4, 171.1, 170.6, 134.2, 82.3,
69.3, 66.2, 65.3, 65.2, 62.4, 46.7, 46.6, 46.3, 34.4, 33.1, 27.5, 17.8, 17.52,
17.48, 14.3, FTIR (cmꢀ1): 1731.1, 1707.4. Anal. Found: C, 61.2; H, 5.9;
N, 1.0. Calcd for C82H97NO32: C, 61.2; H, 6.1; N, 0.9.
Synthesis of 4 According to Route 1. Compound 2 (0.05 g,
0.061 mmol) was dissolved in 3 mL of THF and benzyl azide (0.04 g,
0.27 mmol), CuIBr (0.003 g, 0.02 mmol), and PMDETA (5.01 μL, 0.02
mmol) were then added to the solution and the mixture was stirred at
ambient temperature for 24 h. The mixture was then diluted with
CH2Cl2 (10 mL) and extracted with water (3 ꢁ 10 mL) combined
organic layers were dried over anhydrous Na2SO4. The residue was
concentrated in vacuo. Crude product was purified by column chroma-
tography to give 0.07 g of 4a (88% yield). 0.04 g of the isolated
compound (0.03 mmol) was dissolved in 1 mL THF along with
undecene thiol (0.007 g, 0.04 mmol) and triethylamine (0.3 mL) and
was stirred at room temperature for 24 h. The mixture was then diluted
with CH2Cl2 (10 mL) and extracted with water (2 ꢁ 10 mL), combined
organic layers were dried over anhydrous Na2SO4 and the residue was
concentrated in vacuo. The crude product was purified by column
chromatography to give 0.03 g of 4 (71% yield).
Figure 3. Model reactions to demonstrate the orthogonal reactivity of
the dendrons.
Dowex Hþ resin was added with a tip of spatula. The resulting mixture was
stirred at ambient temperature until the consumption of the starting
material was observed via TLC. The resin was then filtered off and washed
with MeOH. The filtrate was concentrated in vacuo to give 3a as a white
solid (0.17 g, 98% yield). 3a (0.09 g, 0.1 mmol) was then added to a solution
of DMAP (0.02 g, 0.2 mmol), pyridine (0.3 mL) and compound C (0.24 g,
1.4 mmol) in dry CH2Cl2 (5 mL). The mixture was then stirred at room
temperature for 12 h. Excess anhydride was quenched with water (3.0 mL)
for 12 h. Reaction mixture was diluted with 30 mL CH2Cl2 and extracted
with 1 M NaHSO4 (3 ꢁ 20 mL), 10% Na2CO3 (3 ꢁ 20 mL) and then with
brine (1 ꢁ 20 mL) combined organic layers were dried over anhydrous
Na2SO4. The residue was concentrated in vacuo. Crude product was
purified by column chromatography to give 0.15 g of 3b as a colorless
viscous liquid (98% yield).
1H NMR (CDCl3, δ, ppm) 6.50 (s, 2H, CHdCH), 5.23 (s, 2H, CH
bridgehead protons), 4.30ꢀ4.19 (m, 28H, CH2 ester protons), 4.05 (t,
2H, J = 6.0 Hz, OCH2), 3.56 (t, 2H, J = 6.6 Hz, NCH2), 2.84 (s, 2H,
bridge protons), 2.56ꢀ2.53 (m, 16H, CH2CH2CtCH), 2.48ꢀ2.44 (m,
16H, CH2CH2CtCH), 1.97 (t, 8H, J = 2.4 Hz, C ꢂ CH), 1.93 (tt, 2H,
J = 6.6, 6.0 Hz, NCH2CH2CH2O), 1.30 (s, 3H, CCH3), 1.24 (s, 6H,
CCH3), 1.23 (s, 12H, CCH3). 13C NMR (CDCl3, δ, ppm): 176.1, 171.8,
171.7, 171.4, 171.0, 136.5, 82.3, 80.9, 69.3, 66.1, 65.3, 65.2, 62.1, 47.4,
46.7, 46.6, 46.3, 35.2, 33.1, 26.6, 17.7, 17.5, 17.4, 14.2. FTIR (cmꢀ1):
1731.6, 1699.0
1H NMR (CDCl3, δ, ppm) 7.31ꢀ7.18 (m, 24H, aromatic and triazole
CHdCH), 5.75 (tt, 1H, J = 17.0, 6.6 Hz, CH2 = CH), 5.41 (s, 8H,
benzylic protons), 4.98ꢀ4.84 (m, 2H, CH2dCH), 4.16 (s, 4H, CH2
ester protons), 4.08ꢀ3.98 (m, 10H, CH2 ester protons and OCH2), 3.51
(t, 2H, J = 6.9 Hz, NCH2), 3.08 (dd, 1H, J = 18.6, 9.0 Hz, CH2ꢀCHꢀS),
2.90 (t, 8H, J = 7.3 Hz, CH2CH2C2H3N3), 2.83ꢀ2.76 (m, 1H,
CHꢀSꢀCH2), 2.64 (m, 10H, CH2CH2C2H3N3 and CH2ꢀS), 2.42
Synthesis of 1. Compound 1b (0.14 g, 0.28 mmol) was dissolved in
dry toluene (15 mL) and the mixture was heated to reflux. Progress of
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Figure 4. Orthogonal functionalization of dendron 2. Inset: Aqueous solution of BODIPY-SH (on the left) and aqueous solution of polymer conjugated
BODIPY-SH 6 (on the right).
(dd, 1H, J = 18.6, 15.1 Hz, CH2ꢀCHꢀS), 2.00ꢀ1.95 (m, 2H,
NCH2CH2CH2O), 1.86 (m, 2H, CH2dCHꢀCH2), 1.31ꢀ1.18 (m,
18H, CH2 protons of undecene thiol and CCH3), 1.06 (s, 6H, CCH3).
13C NMR (CDCl3, δ, ppm): 176.6, 174.7, 172.0, 171.95, 171.91, 146.5,
139.2, 134.9, 129.0, 128.6, 128.0, 121.2, 114.1, 65.6, 65.1, 62.2, 53.9, 46.5,
46.3, 39.1, 36.1, 35.4, 33.8, 33.2, 31.8, 29.38, 29.36, 29.1, 29.05, 28.95,
28.86, 28.7, 26.6, 20.9, 17.6, 17.5. ESI-TOF: m/z calcd for [M þ H]þ,
1542.7130; found, 1542.7126.
Synthesis of 4 According to Route 2. Compound 2 (0.03 g,
0.036 mmol) was dissolved in 3 mL of THF, and benzyl azide (0.02 g,
0.16 mmol), triethyl amine (5.00 μL, 0.036 mmol), and undecene thiol
(0.01 g, 0.06 mmol) were then added to the solution and the mixture was
stirred at ambient temperature for 24 h. Without any isolation, CuIBr
(0.002 g, 0.01 mmol), and PMDETA (3.01 μL, 0.01 mmol) was added to
the reaction mixture solution and the mixture was stirred at ambient
temperature for another 24 h. The mixture was then diluted with
CH2Cl2 (10 mL) and extracted with water (3 ꢁ 10 mL) combined
organic layers were dried over anhydrous Na2SO4. The residue was
concentrated in vacuo. Crude product was purified by column chroma-
tography to give 0.02 g of 4 (43% yield).
triethyl amine (5.00 μL, 0.036 mmol) and undecene thiol (0.03 g, 0.14
mmol) in 3 mL of THF was added to the reaction mixture and stirred at
ambient temperature for 24 h. The mixture was then diluted with
CH2Cl2 (10 mL) and extracted with water (3 ꢁ 10 mL) combined
organic layers were dried over anhydrous Na2SO4. The residue was
concentrated in vacuo. Crude product was purified by column chroma-
tography to give 0.04 g of 4 (76% yield).
Synthesis of Multiarm PEG-BODIPY Conjugate 6. PEG750N3
was prepared according to previously reported literature procedure.34
PEG750N3 (0.166 g, 0.218 mmol) was dissolved in THF and mixed with
compound 2 (0.04 g, 0.048 mmol) in the presence of Cu(I)Br/PMDETA
catalyst system under N2 atmosphere. The mixture was then stirred at room
temperature for 24 h. Product was purified by precipitation in ether and
dialysis to give 5 (0.14 g, 69%) as a light brown viscous liquid. GPC revealed
a single monomodal peak Mn = 3445, PDI = 1.1. Compound 5 (0.008 g,
0.002 mmol) was dissolved in THF and mixed with Bodipy-SH (0.008 g,
0.02 mmol). The reaction mixture was then stirred at room temperature for
24 h. HNMR of the crude revealed complete consumption of the maleimide
functional group. Product was purified by SiO2 column chromatography
using MeOH/CH2Cl2 solution as a solvent.
Synthesis of 4 According to Route 3. Compound 2 (0.03 g,
0.036 mmol) was dissolved in 3 mL of THF, and benzyl azide (0.02 g,
0.16 mmol), CuIBr (0.002 g, 0.01 mmol), and PMDETA (3.01 μL, 0.01
mmol) were then added to the solution and the mixture was stirred
at ambient temperature for 24 h. Without any isolation, a solution of
’ RESULTS AND DISCUSSION
Three generations of biodegradable polyester dendrons that
can be functionalized at the periphery and the focal point using
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Figure 5. 1H NMR spectrum: (a) dendron 2; (b) dendron 2 þ PEG750 after click reaction; (c) dendron 2 þ PEG750 þ BODIPY dye after Michael
addition to the maleimide unit.
orthogonal chemical transformations were designed (Figure 1).
The periphery of dendrons was appended with alkyne functional
groups capable of undergoing copper catalyzed Huisgen type
[3 þ 2] cycloaddition reaction with azide containing molecules.
The focal point of the dendrons consists of electron deficient
maleimide units that can undergo either a [4 þ 2] cycloaddition
reaction with diene appended molecules or a Michael type
conjugate addition with molecules containing a thiol unit.
During the synthesis of the dendrons, maleimide unit was
protected with a furan moiety to prevent any possible side
reactions. After the alkyne moieties were attached onto the
periphery of the dendrons, the focal point was activated by
subjecting the dendrons to cycloreversion conditions at elevated
temperatures (Figure 2). This unmasking of the protected
maleimide to their reactive form results in the formation of
bifunctional dendrons.
In order to evaluate the conditions for orthogonal functiona-
lization of the bifunctional dendrons, reactions utilizing unde-
cene thiol for use in Michael addition to the maleimide unit and
benzyl azide for the click reaction were carried out (Figure 3).
First a stepwise sequence was followed: Cu(I) catalyzed Huisgen
type click reaction with benzyl azide was carried out, followed by
conjugate addition of undecene thiol to the isolated product. As
expected, both steps proceeded uneventfully in good yields. As
an alternative, one-pot simultaneous addition of both the thiol
nucleophile and the azide along with the Cu(I) catalyst was
evaluated. The desired compound was isolated in 43% yield.
Lower than expected yields could be due to deactivation of the
copper catalyst by remnant thiol nucleophiles. On the basis of
this hypothesis, sequential addition of click reagents along with
benzyl azide, followed by the addition of undecene thiol after 24
h was attempted. As expected, a much higher isolated yield (76%)
was obtained using this route.
The bifunctional nature of these dendrons was used to
synthesize a multiarm PEG polymer with a maleimide based
reactive focal point. A sequential stepwise route was chosen to
afford an all purpose multiarm water-soluble macromolecule.
The second generation dendron 2 was used for click reaction
with PEG750ꢀN3. The cycloaddition reaction was carried out at
room temperature in presence of CuIBr/PMDETA catalyst
system using THF as solvent to afford multiarm polymer 5 in
69% isolated yield (Mn = 3445, PDI = 1.1) after precipitation and
dialysis in ether (Figure 4). The reaction was very clean and no
interference with the maleimide group was observed. It must be
noted that loss of maleimide unit was observed at reaction
temperatures around 50 °C, probably due to competing cycload-
dition of the azides to the reactive alkene group of the maleimide.
1H NMR of the multiarm PEG product 5, has shown that the
peak at 6.70 ppm which corresponds to the double bond of the
maleimide was retained, thus proving that the system is indeed
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orthogonal; furthermore, complete disappearance of the triplet at
∼1.96 ppm corresponding to the proton of the terminal alkyne
moieties and formation of a new peak at 7.49 ppm arising from
the newly formed triazole ring were observed (Figure 5b).
The multiarm PEG polymer formed after the click reaction
was subjected to conjugate addition with a hydrophobic thiol
containing dye BODIPY-SH,41 at room temperature to demon-
strate efficient functionalization of the focal point. Efficient
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1
conjugation reaction was evident from H NMR spectra that
showed the complete disappearance of the maleimide peak
around 6.7 ppm, and appearance of peaks corresponding to
BODIPY skeleton, noticeably the resonances around 6.0
ppm due to the pyrrole units (Figure 5c). Unbound BODIPY
dye could be easily removed via SiO2 column chromatography. It
is clearly visible that while the nonconjugated dye is insoluble in
aqueous solution containing 10% methanol, the polymer con-
jugated dye is freely soluble (inset, Figure 4). Thus, the periphery
and the focal point of these dendrons can be independently
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’ CONCLUSION
A new class of biodegradable orthogonally functionalizable
dendrons has been synthesized. The periphery of these dendrons
can be functionalized with azide containing small molecules
and polymers using the efficient copper catalyzed Huisgen type
[3 þ 2] cyccloaddition reaction, while the focal of these den-
drons can be functionalized with the Michael addition reaction
with thiol containing molecules. Utilization of such dendrons
toward the synthesis of a water-soluble multiarm PEG polymer
with a reactive focal point was demonstrated. Thus, the obtained
reactive polymer was utilized for solubilizing a hydrophobic dye
molecule, BODIPY via thiolꢀene conjugation.
’ ASSOCIATED CONTENT
(22) Lee, C. C.; Gillies, E. R.; Fox, M. E.; Guillaudeu, S. J.; Frꢀechet,
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S
Supporting Information. Characterization of dendrons,
b
multiarm PEG, and core functionalization including NMR
spectra and GPC traces. This material is available free of charge
via the Internet at http://pubs.acs.org.
(24) Ghosh, S.; Basu, S.; Thayumanavan, S. Macromolecules 2006,
39, 5595–5597.
’ AUTHOR INFORMATION
(25) Yang, S. K.; Weck, M. Macromolecules 2008, 41, 346–351.
(26) Yang, S. K.; Weck, M. Soft Matter 2009, 5, 582–585.
(27) Killops, K. L.; Campos, L. M.; Hawker, C. J. J. Am. Chem. Soc.
2008, 130, 5062–5064.
Corresponding Author
*E-mail: amitav.sanyal@boun.edu.tr. Telephone: 90-212-359-
7613. Fax: 90-212-287-2467.
(28) Durmaz, H.; Dag, A.; Hizal, A.; Hizal, G.; Tunca, U. J. Polym.
Sci., Part A: Polym. Chem. 2008, 46, 7091–7100.
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’ ACKNOWLEDGMENT
The authors thank the Bogazici University Research Fund
(No. 5132) and Turkish Scientific and Technological Research
Council (TUBITAK 109T493) for the financial support of this
research. A.S. acknowledges the Turkish Academy of Sciences for
the TUBA-GEBIP fellowship.
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