Glycopolymers via catalytic chain transfer polymerisation (CCTP), Huisgens cycloaddition and thiol-ene double click reactions

Article abstract of DOI:10.1039/b904249k

CCTP has been used to give alkyne-functional macromonomers which are subsequently functionalised with sugar azides and thiols, using both CuAAC and thiol-ene Michael addition reactions, to yield end-functionalised glycopolymers in a convenient manner. The

Full text of DOI:10.1039/b904249k

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Glycopolymers via catalytic chain transfer polymerisation (CCTP),
Huisgens cycloaddition and thiol–ene double click reactionsw
Leena Nurmi,ab Josefina Lindqvist,b Rajan Randev,b Jay Syrettb and David M. Haddleton*b
Received (in Cambridge, UK) 3rd March 2009, Accepted 27th March 2009
First published as an Advance Article on the web 9th April 2009
DOI: 10.1039/b904249k
CCTP has been used to give alkyne-functional macromonomers
which are subsequently functionalised with sugar azides and
thiols, using both CuAAC and thiol–ene Michael addition
end-functionalised glycopolymers in
manner.
a very convenient
The ‘‘double clickable’’ alkyne-functional macromonomer 1
was prepared by CCTP of trimethylsilane-protected propargyl
methacrylate using bis(boron difluorodimethylglyoximate)
cobalt(II) (CoBF) as catalyst, followed by deprotection of the
TMS groups (Scheme 1). The vinyl end group of propargyl
methacrylate macromonomer 1 was reacted with benzyl
mercaptan via a thio-Michael addition using dimethylphenyl-
phosphine (DMPP) as catalyst. After conversion of the vinyl
groups was achieved, the benzyl end-functionalised product, 2,
was isolated by precipitation. The success of the thio-Michael
addition was confirmed by 1H NMR with the disappearance of
vinyl peaks at 5.7 ppm and 6.2 ppm and the appearance of a
terminal benzyl at 7.2–7.4 ppm, Fig. 1a and 1b. To investigate
the versatility of the thio-Michael addition to these oligomers,
reactions, to yield end-functionalised glycopolymers in
convenient manner.
a
Click chemistry as a concept of simplifying synthesis is very
useful in polymer science to produce complex macromolecular
structures, functional polymers and protein conjugates. The
Cu(I)-catalysed azide–alkyne cycloaddition (CuAAC) has
been the most widely studied and employed of the available
click reactions.1–9 Recently, there has been an increasing
interest in using the well-known addition of thiols to alkenes
as a click process, so-called thiol–ene click chemistry.10
Although thiol–ene click coupling has mainly been focused
on a radical-mediated version to non-activated alkenes, this
reaction can also proceed via Michael addition, especially
when the vinyl group is alpha to an electron withdrawing
moiety.11–16 The use of thio-Michael addition as a click
reaction was recently reported by Lowe et al. for the synthesis
of star polymers.17
the vinyl end group of
1
was also reacted with
Synthetic glycopolymers containing pendent sugar moieties
have been shown to interact multivalently with carbohydrate-
binding proteins, lectins, in a similar manner to natural
glycoproteins. These biomimetic properties have caused
significant interest in the synthesis of glycopolymers, and a
number of different strategies have been employed to obtain
the required multivalent carbohydrate ligands.18–20
In our group, we have previously combined copper(I)-
mediated living radical polymerisation (often called ATRP)
and CuAAC to produce glycopolymers by post-functionalisation
of well-defined ‘‘clickable’’ polyalkyne scaffolds with sugar
azides.21–25 An established, but dormant, method of obtaining
end-functional polymers available for click reactions is
catalytic chain transfer polymerisation (CCTP). This is an
extremely efficient process to produce vinyl terminated
methacrylic oligomers.26–31
In this present study we have used CCTP to give
alkyne-functional oligomers available for both CuAAC
and thio-Michael addition reactions. Post-functionalisation
of the oligomers with these dual click reactions results in
a Department of Biotechnology and Chemical Technology, Helsinki
University of Technology, PO. Box 6100, TKK, 02015, Finland
b Department of Chemistry, University of Warwick, Coventry,
UK CV4 7AL. E-mail: d.m.haddleton@warwick.ac.uk;
Tel: +44 (0)2476523256
w Electronic supplementary information (ESI) available: Experimental
details, characterisation and lectin recognition data. See DOI:
10.1039/b904249k
Scheme 1 Synthetic approach to end-functionalised glycopolymers.
Conditions: (a) TMS-protected propargyl methacrylate, AIBN, CoBF,
toluene, 60 1C; (b) TBAF, acetic acid, THF, ambient temperature
(AT); (c) benzyl mercaptan or mercaptoethanol, 1, DMPP, acetone,
AT; (d) sugar azide 4, 5 or 6, polymer 2 or 3, CuBr, bipy, TEA,
DMSO, 60 1C.
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Chart 1 Sugar azides employed in the study: cellobiose azide (4),
mannose azide (5) and galactose azide (6).
was reacted with cellobiose azide 4 to yield 7 (Scheme 1). The
1H NMR spectrum of 7 shows the appearance of the triazole
peak at 8.3 ppm, confirming the successful CuAAC reaction
(Fig. 1c). It can also be seen that the benzyl end group is not
affected by this reaction.
Oligomer 2 was reacted with mannose azide 5, and
hydroxyethyl-functionalised macromer 6 was reacted with
galactose azide 7, to yield end-functionalised glycopolymers
8 and 9, respectively (Scheme 1). The CuAAC reactions with
mannose azide and galactose azide required less catalyst and
shorter reaction times than for cellobiose azide. This is in
accordance with our previous results for the CuAAC reaction
using lactose azide,21 and may be explained by increased steric
demands of the disaccharide cellobiose rendering the
unreacted alkyne functionalities in the partly clicked polymer
less accessible to further functionalisation.
Although in this study we have chosen to functionalise the
oligomer end group prior to reacting the alkyne groups, it may
for some applications be preferable to change the order of the
reactions. To ensure that the vinyl end groups in the oligomer
would not be affected by the CuAAC reaction of the alkyne
side groups, unfunctionalised oligomer was also reacted with
Fig.
1
1H NMR spectra of (a)
in acetone-d6, and (c) benzyl
1 in acetone-d6, (b) benzyl
end-functionalised polymer
2
end-functionalised cellobiose-functional glycopolymer 7 in DMSO-d6.
mercaptoethanol (Scheme 1). The 1H NMR spectrum of
the purified hydroxyethyl-functional product 3 shows the
disappearance of vinyl peaks and the appearance of peaks
corresponding to the hydroxyethyl end group, indicating a
successful reaction (see ESIw). MALDI-TOF analysis confirms
the formation of the desired product (see ESIw). In addition, a
by-product resulting from the conjugation of the DMPP to the
polymer was visible in the MALDI-TOF spectrum. However,
this by-product is cationic and is therefore expected to give
disproportionately large peaks in this spectrum. It is noted
that no peaks could be detected in the aromatic region of the
1H NMR spectrum, or indeed no peaks could be detected in
the 31P NMR spectrum, from this by-product and thus the
amount is assumed to be low.
1
cellobiose azide. The click reaction could be confirmed by H
NMR by the appearance of a triazole peak at 8.3 ppm, and it
could further be seen that the vinyl end group was retained
after this reaction (see ESIw).
The potential of the end-functionalised mannose- and
galactose-functional glycopolymers 8 and 9 to be recognised by
different lectins was investigated. For the mannose-functional
glycopolymer 8, turbidimetry was used to study the rate of the
binding of the sugar epitopes to the mannose-specific lectin
concanavalin A (con A). Absorbance changes at 420 nm were
measured over time in a solution of con A and 8 in HEPES
buffer at pH 7.4. When the glycopolymer binds to the lectin,
the formation of precipitating clusters changes the absorbance
in the solution.32 It could be seen that upon mixing 8 and the
lectin con A, the absorbance initially quickly increased, then
reached a plateau, indicating that most of the polymer was
able to rapidly interact with the lectin, forming stable clusters
(see ESIw).
Glycopolymers were synthesised by reacting the alkyne
groups in the thiol-conjugated oligomers with sugar azides
4–6 (Chart 1) using CuAAC. Mannose azide and galactose
azide were synthesised as described previously,21 and
cellobiose azide was synthesised using the same
methodology, starting from the corresponding peracetylated
sugar (see ESIw). The CuAAC reactions were catalysed by
CuBr–bipyridine and performed in DMSO to ensure complete
solubility of all reactants. Benzyl-functionalised oligomer 2
For the polymer 9, the interaction with the galactose selective
lectin Ricinus communis agglutinin I (RCA I) was studied by
affinity chromatography analysed by HPLC. A solution of 9
was injected into a column packed with immobilised RCA I.
The glycopolymer is retained by the column as the sugar
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moieties are recognised by the lectin. Using a mobile phase
containing galactose as a competing ligand, the polymer is
released from the column and can be detected by UV
absorbance.33 By increasing the concentration of galactose in
the mobile phase, the amount of eluted glycopolymer increased,
indicating that the retention of the polymer in the column was
indeed due to the sugar–lectin interaction.
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In summary, we have used CCTP to synthesise polymers
containing two different clickable groups, which can be func-
tionalised using two different click reactions. The polymer end
group has been reacted using Michael addition with different
thiols to obtain end-funtionalised polymers, and glyco-
polymers have been produced by clicking sugar moieties to
the side chain of these polymers by CuAAC. The ability of the
resulting end-functionalised glycopolymers to be recognised
by lectins has been investigated. The results from this study
indicate that the mannose- and galactose-containing glyco-
polymers prepared with the method described herein are able
to function as multivalent ligands for the recognition of the
lectins con A and RCA I, respectively.
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and CuAAC is a useful synthetic approach not only to
glycopolymers but also to many different types of functional
polymers and conjugates.
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The authors acknowledge financial support from the
Wenner-Gren Foundations (Sweden, JL), the Graduate
School in Chemical Engineering (Finland, LN), the EPSRC
(UK, RR) and Lubrizol (JS). Equipment used in this research
was part funded through support from Advantage West
Midlands (AWM, Science City initiative) and part funded by
the ERDF.
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