Synthesis of pyridine acrylates and acrylamides and their corresponding pyridinium ions as versatile cross-linkers for tunable hydrogels

Article abstract of DOI:10.1055/s-0033-1338614

A small library of cross-linkers for hydrogels was synthesized. The cross-linkers consisted of 2,6- and 3,5-diacylpyridine or 2,4,6-triacylpyridine as the core unit, which were tethered via ethylene glycol, amino ethanol, and 1,n-diamine spacers to terminal acrylate or acrylamide moieties. Esterification and amide formation of the terminal acryl units were found to be dependent on the ratio of NH/O in the spacer, the constitution pattern of the pyridine ring, and the total number of acryl groups. Thus, esters generally gave higher yields than amides decreasing with increasing number of NH in the spacer and with increasing number of acryl units. In the case of 3,5-diacylpyridine derivatives, these trends were less prominent as compared to the 2,6-diacylpyridine series, indicating that steric hindrance and unfavorable hydrogen bonding interaction of the spacers might influence the observed reactivity differences. The 3,5-diacylpyridines were converted to the N-methylpyridinium salts and selected members of both neutral and cationic 3,5-diacylpyridinium derivatives were submitted to hydrogelations with synthetic polymer poly(1-glycidylpiperazine) via aza-Michael addition and thiolated natural hyaluronan via thio-Michael reaction, respectively. Rheological properties of the resulting hydrogels were studied, revealing that both spacer type as well as charge affected elastic moduli and degree of swelling. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1338614

PAPER
▌1243
pSapyenr thesis of Pyridine Acrylates and Acrylamides and Their Corresponding
Pyridinium Ions as Versatile Cross-Linkers for Tunable Hydrogels
Pyridine-Acryl Cross-Linkers for Hydrogels
a
b,c
d,e
a
b,c
Monika Bach, Günter E. M. Tovar,* Heike Boehm,* Sabine Laschat*a
d,e
d,e
b,c,f
a
Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
Fax +49(711)68564285; E-mail: sabine.laschat@oc.uni-stuttgart.de
b
Department of New Materials and Biosystems, Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569 Stuttgart, Germany
c
Department of Biophysical Chemistry, University of Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany
E-mail: heike.boehm@is.mpg.de
d
Institut für Grenzflächenverfahrenstechnik und Plasmatechnologie IGVP, Universität Stuttgart, Nobelstr. 12, 70569 Stuttgart, Germany
Fax +49(711)9704200; E-mail: guenter.tovar@igvp.uni-stuttgart.de
e
Fraunhofer Institut für Grenzflächen- und Bioverfahrenstechnik IGB, Nobelstr. 12, 70569 Stuttgart, Germany
f
CSF Biomaterials and Cellular Biophysics, Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569 Stuttgart, Germany
Received: 19.12.2013; Accepted after revision: 05.02.2014
In memoriam Alan R. Katritzky
ing cationic (or anionic) comonomers, charged cross-link-
ers have rarely been applied for such purpose. Pyridine
and pyridinium ions have been extensively used as sub-
Abstract: A small library of cross-linkers for hydrogels was syn-
thesized. The cross-linkers consisted of 2,6- and 3,5-diacylpyridine
or 2,4,6-triacylpyridine as the core unit, which were tethered via
2
ethylene glycol, amino ethanol, and 1,n-diamine spacers to terminal units of polyelectrolytes and polymeric hydrogels because
acrylate or acrylamide moieties. Esterification and amide formation they can be easily combined with biopolymers such as
of the terminal acryl units were found to be dependent on the ratio
of NH/O in the spacer, the constitution pattern of the pyridine ring,
and the total number of acryl groups. Thus, esters generally gave
higher yields than amides decreasing with increasing number of NH
in the spacer and with increasing number of acryl units. In the case
of 3,5-diacylpyridine derivatives, these trends were less prominent
peptides, enzymes, or even whole organisms leading to
promising applications such as decontamination of chem-
3
ical and biological warfare agents or polymeric supports
4
for microbial biofilms. Furthermore, pyridine and pyri-
dinium ions have been successfully used as electron-
as compared to the 2,6-diacylpyridine series, indicating that steric transporting layers in organic light emitting devices
5–7
hindrance and unfavorable hydrogen bonding interaction of the (OLEDs). In most cases the pyridinium unit is either
4
–8
spacers might influence the observed reactivity differences. The
,5-diacylpyridines were converted to the N-methylpyridinium salts
part of the polymer backbone (main chain polymer) or
3
3
incorporated in the side chain of homopolymers or block
and selected members of both neutral and cationic 3,5-diacylpyri-
dinium derivatives were submitted to hydrogelations with synthetic
polymer poly(1-glycidylpiperazine) via aza-Michael addition and
thiolated natural hyaluronan via thio-Michael reaction, respective-
copolymers resulting in a relatively high concentration of
pyridine units within the polymer. Copolymers in which
the heteroaromatic cross-linkers are present in relatively
9
ly. Rheological properties of the resulting hydrogels were studied, low concentrations are only rarely explored. We have re-
revealing that both spacer type as well as charge affected elastic cently prepared pyridine-derived cross-linkers, which
moduli and degree of swelling.
were tethered via ester or amide spacers to acryl units and
Key words: acylation, alkylation, biomimetic synthesis, poly- used them for the preparation of novel biocompatible hy-
anions, pyridines, cross-linkers, hydrogels
drogels derived from synthetic poly(1-glycidylpipera-
zine) and thiolated hyaluronan. It turned out that both
the type of connecting unit (ester versus amide) between
1
0
11
Synthetic and hybrid hydrogels are promising materials pyridine and spacer as well as the presence or absence of
for tissue engineering. In contrast to their animal derived a charge on the pyridine had some influence on the gela-
counterparts such as collagen, they possess a well-defined tion rate, solubility of the hydrogels and the rheological
molecular structure, their properties can be adjusted in a properties. However, in order to perform more detailed
reproducible way, for example, by using cross-linkers; studies to understand structure–property relationships and
1
they are cost-effective and provide no infection risk.
to get access to hydrogels suitable for biological studies,
However, a major disadvantage is the low tendency for we required a dedicated library of cross-linkers 1 and 2
cell attachment, which can be circumvented by modifica- with different spacers varying in length and solubility, and
tion (with small RGD peptides) or introduction of charged 2,6- or 3,5-diacylpyridines as the central heterocyclic unit
moieties. While several research groups have studied as shown in Figure 1. In order to increase the number of
charge effects on the properties of the hydrogels employ- cross-linkable groups, 2,4,6-triacylpyridines were studied
as well. Esters and amides were chosen as connecting
units between spacers and polymerizable terminal groups
to take advantage of commercially available acrylate
building blocks and easily accessible synthetic groups.
SYNTHESIS 2014, 46, 1243–1253
Advanced online publication: 06.03.2014
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
DOI: 10.1055/s-0033-1338614; Art ID: SS-2013-Z0825-OP
Georg Thieme Verlag Stuttgart · New York
©

1244
M. Mateescu et al.
PAPER
Furthermore, esters and amides should provide sufficient
solubility of the target molecules in the gelation solvent,
which is anticipated to be advantageous for the hydroge-
O
O
OH
OH
O
O
N
H
1
2–15
lation by hetero-Michael reactions.
discussed below.
The results are
3
a
3b
O
O
1
spacer
spacer
spacer
spacer
O
N
OH
O
O
O
O
O
O
O
3
g
O
H2N spacer
c–f
NH2
spacer
2
N
R
4
c
d
e
f
(CH2)2
(CH2)3
(CH2)4
Figure 1 Graphical representation of cross-linkers 1 and 2
Boc2O
–10 °C, 2 h
r.t., 2 h
CH2Cl2
In order to attach polymerizable groups to the pyridine
core, the following building blocks were selected
O
2
H2N spacer
NHBoc
(
Scheme 1): the ester-based commercially available com-
pounds 2-hydroxyethyl acrylate (3a), N-hydroxyethyl
acrylamide (3b), and 3-(acryloyloxy)-2-hydroxypropyl
methacrylate (AHM) (3g), which enabled an increase of
cross-linkable groups through its branched structure. Fur-
thermore, the amide-based compounds 3c–f were chosen,
incorporating longer alkyl and the water soluble triethyl-
ene glycol spacers, respectively. The amine hydrochlo-
rides 3c–f were prepared as shown in Scheme 1. Boc-
protection of the diamines 4c–f followed by acylation
with acryloyl chloride were carried out according to a
modified procedure by Feast and Hamachi to give the N-
tert-butoxycarbonyl-N′-acryloyl-1,2-diaminoalkanes 6c–
5c–f
O
Et3N, CH2Cl2
–10 °C, 1 h,
r.t., 1 h
Cl
O
N
H
spacer
c–f
NHBoc
6
product, yield (%)
HCl (g)
5
6
3
r.t., 3 h
16,17
f.
The protected amines 6c–f were deprotected with
CH Cl2
2
dry gaseous hydrogen chloride to assure that the final ami-
no hydrochlorides 3c–f did not contain any water prior to
acylation.17
c
d
e
f
89
90
93
84
90
63
82
95
quant.
quant.
79
O
N
spacer
c–f
3
NH Cl
As shown in Scheme 2, the pyridine dicarboxylic acid 7
was activated with thionyl chloride and reacted with the
acrylic building blocks 3a–c,g via two different methods.
The reactions with the well soluble acrylates 3a,b,g were
H
95
3
Scheme 1 Synthesis of compounds 3
carried out in CH Cl2 by employing stoichiometric
2
1
8,19
amounts of Et N for a faster conversion (method A).
3
The crude products could be purified by column chroma- easily separated by filtration, the obtained yield of 8c
tography on silica gel and the desired acryl cross-linkers (21%) was low, presumably due to the increased forma-
8a,b,g were obtained in yields of 21–93%. In contrast, for tion of hydrogen bonds with higher occurrence of amines
cross-linker 8c method A turned out to be not suitable be- and amides in 3 and thus decreased accessibility of the
cause Et NHCl, which is formed during the reaction, has acid chlorides towards OH or NH nucleophiles. The em-
3
2
similar solubility to 8c and could not be removed via col- ployed amine 3c could usually be recovered after the re-
umn chromatography (CH Cl –MeOH) but only by exten- actions.
2
2
sive repetitive recrystallization (CH Cl –acetone) with
loss of material (5–10%). Furthermore, the amine 3c was
2
2
In general, similar trends were found upon conversion of
pyridine-2,4,6-tricarboxylic acid (9) to the corresponding
acrylic derivatives 10a–c,g (Scheme 3) although direct
comparison with 8a–c,g revealed decreased yields for the
tris-functionalized pyridines 10a–c,g, presumably due to
competition between the different arms. A decrease of
yields was also observed upon increasing the number of
only partially soluble in CH Cl , leading to decreased re-
2
2
activity towards the acid chloride of 7 but increased the
tendency towards polymerization. Therefore, alternative
reaction conditions were examined: NaH in DMF was
found to be suitable (method B). Although NaCl could be
Synthesis 2014, 46, 1243–1253
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Pyridine-Acryl Cross-Linkers for Hydrogels
1245
CO2H
HO2C
N
CO2H
HO2C
N
CO2H
7
9
1) SOCl2, 85 °C, 3 h
2) 3, method A or B
1
) SOCl2, 85 °C, 3 h
2
) 3, method A, B or C
O
O
O
Z
Z
Z
O
Y
N
Y
Y
O
O
O
O
Z
Z
Z
Y
Y
N
Y
O
O
8
8
8
a
O
O
O
93% (A)
b
NH 49% (A)
Z
Y
c
NH NH 21% (B)
1
0a
O
O
O
66% (A)
1
1
0b
NH 22% (A)
O
O
0c NH NH 12% (B)
O
O
O
O
N
O
O
O
O
O
O
O
O
O
O
O
O
8g 71% (A)
O
O
Scheme 2 Synthesis of pyridine acrylates and acrylamides 8. Re-
O
O
O
agents and conditions: Method A: 3, Et N, CH Cl , 0 °C to r.t., 2 h.
3
2
2
O
N
O
Method B: 3, NaH, DMF, 0 °C to r.t., 16 h.
O
O
O
O
O
polymerizable groups, for example, two in 8a (93%) ver-
sus four in ester 8g (71%).
1
0g 33% (C)
We tried to address the low yields by using different reac- Scheme 3 Synthesis of pyridine acrylates and acrylamides 10. Re-
N, CH Cl , 0 °C to r.t., 2 h.
Method B: 3, NaH, DMF, 0 °C to r.t., 16 h. Method C: 3, Py, CH Cl ,
tion conditions for selected compounds (Table S1, Sup- agents and conditions: Method A: 3, Et
3
2
2
porting Information).2 The use of DCC led to slower
conversions (low solubility of the acids 7 and 9 in organic
solvents) with even lower yields. Furthermore, pyridine or
0,21
2
2
0
°C to r.t., 2 h.
2,6-lutidine were used as bases but caused the formation 2,6- and 2,4,6-substituted pyridines 8a–c and 10a–c sug-
of more by-products and were only advantageous in case gesting that hydrogen bonds are less prominent.
of the more bulky acrylate AHM 3g (method C). All
While acrylate 8a was a stable solid, compounds 8g,
0a,g, and 12a,g were colorless, temperature-sensitive
changes in conditions led to more extensive workups and
purifications compared to the route via SOCl and Et N
1
2
3
oils. To avoid radical polymerizations (especially in the
case of AHM 3g derivatives) and thus increase the stabil-
ity, 1 wt% of 4-methoxyphenol (MEHQ) was added as in-
hibitor to the purified products and the compounds were
stored under dry nitrogen or in solution below 4 °C. With
the acrylamide building blocks 3b–f more stable products
(
method A) or NaH and DMF (method B). In addition,
both series 8a–c and 10a–c showed decreased yields with
increasing number of NH groups again suggesting that hy-
drogen bonds between different arms interfere with effi-
cient acylation.
Next, pyridine-3,5-dicarboxylic acid (11) was converted were obtained as solids (8b,c, 10b,c, 12b–e) or as a vis-
via the corresponding acid chloride to the 3,5-disubstitut- cous oil (12f), which all showed a better water solubility
ed derivatives 12a–f and 12g (Scheme 4) bearing two and than the acrylates.
four acrylic units, respectively, via methods A, B, and C
in 40–72% yield. In the case of 12c–e longer alkyl spacers
While 2,6-disubstituted pyridines 8a–c,g could not be N-
methylated with MeI in MeCN in agreement with litera-
were introduced as well as triethylene glycol in 12f for im-
proved water solubility. Although compounds 12a–c also
showed decreased yields with increasing amount of NH
groups in the spacer, the effect was not as pronounced in
this 3,5-disubstituted pyridine series as compared to the
22,23
ture precedence
(see Supporting Information for addi-
tional experiments), the corresponding 3,5-disubstituted
pyridines 12a–g were converted to the pyridinium salts
13a–g with MeI in either MeCN (method D) or DMF
©
Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 1243–1253

1246
M. Mateescu et al.
PAPER
(
(
method E) at room temperature without any event Purification of the ionic compounds was possible by flash
Scheme 4).
chromatography (13a), extraction with water (13b), or
washing with acetonitrile (13c), or diethyl ether (13d–g).
It was observed that 13g was especially sensitive against
silica gel. Thus, it could only be roughly purified, despite
HO2C
CO2H
the fact that its R of 0.15 (hexanes–EtOAc, 4: 1) should
f
N
permit flash chromatography. In general, the N-methylat-
ed products 13a–g showed a higher stability against radi-
cal polymerizations than their neutral precursors 12a–g
and were better soluble in water, which facilitated the sub-
sequent hydrogelations.
11
1
2
) SOCl2, 85 °C, 3 h
) 3, method A or B
O
Y
O
Z
O
O
spacer
Z
spacer
Y
Preliminary cross-linking experiments revealed that
acrylic esters either did not form hydrogels with the poly-
mers or the hydrogels were too unstable as compared to
the corresponding acrylamides in agreement with earlier
N
1
2a–f
Y
spacer
O
Z
1
0,11
observations.
Therefore, from the synthesized library
1
1
1
1
1
1
2a
2b
2c
2d
2e
2f
O
63% (A)
55% (A)
47% (B)
40% (B)
51% (B)
46% (B)
cross-linkers 12d–f and 13d–f were chosen for the hydro-
gelation experiments.
O
HN
HN
HN
HN
NH
In order to test the reactivity of the different cross-linkers
12d–f and 13d–f, they were reacted with the side-chain
NH
amino-functionalized polymer poly(1-glycidylpipera-
NH
O
zine) (15), as described previously for cross-linkers 12c
1
0
and 13c (Scheme 5).
O
NH
HN
3
steps
8%
O
O
O
O
N
3
n
n
O
O
ref. 10
O
O
OH
O
O
NH
O
O
1
4
⋅2 HCl
O
O
1
5
N
12g 72% (C)
O
O
NaOH
MeI, r.t., 16 h
method D or E
2
H O–EtOH
O
O
O
O
1
2, 13
Y
spacer
Z
I
Z
spacer
Y
N
O
N
O
n
Me
O
N
1
3a–g
Y
spacer
Z
16
O
O
Scheme 5 Reaction of cross-linkers 12d–f and 13d–f with poly(1-
glycidylpiperazine) (15)
1
1
1
1
1
1
1
3a
3b
3c
3d
3e
3f
O
91%
92%
89%
(D)
(D)
(E)
HN
HN
HN
HN
NH
Via the aza-Michael reaction, the multifunctional polymer
15 and the bifunctional cross-linkers 12d–f and 13d–f
quant. (E)
quant. (E)
quant. (D)
NH
form polymer networks 16, which are highly hydrophilic
due to the polyelectrolyte used. The cross-linking reaction
was performed in water–ethanol (1:1 v/v) because of the
limited solubility of most cross-linkers in pure water. A
ratio of cross-linker-bound acrylamide groups and poly-
mer-bound amino groups of 0.8 was maintained for all gel
experiments. Corresponding gelling times are given in the
Supporting Information. The hydrogels synthesized with
the acrylamide-based cross-linkers 12d–f and 13d–f are
hydrolytically stable and were characterized by shear rhe-
ology (Figure 2) and by the determination of their equilib-
NH
O
O
O
NH
HN
O
3g
O
80%
(D)
O
Scheme 4 Synthesis of pyridine acrylates and acrylamides 12. Re-
agents and conditions: Method A: 3, Et N, CH Cl , 0 °C to r.t., 2 h.
Method B: 3, NaH, DMF, 0 °C to r.t., 16 h. Method D: MeI, MeCN,
3
2
2
r.t., 16 h. Method E: MeI, DMF, r.t., 16 h.
Synthesis 2014, 46, 1243–1253
© Georg Thieme Verlag Stuttgart · New York

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Pyridine-Acryl Cross-Linkers for Hydrogels
1247
rium degree of swelling (EDS) (Figure 3). Since the The G′-values are all in the same order of magnitude
synthetic hydrogels described here show a linear visco- around 70 kPa for the neutral and 50 kPa for the charged
elastic response only at deformations below 5%, the deter- cross-linkers. A tendency for a decrease of the elastic
mination of their E-moduli is not possible by linear moduli with the spacer length can be observed. We corre-
regression of the linear part of their stress-strain curves. late this observation with the presumably increasing mesh
Therefore, their elastic shear moduli G′ were determined size of the gels. Upon comparison of pairs of neutral and
by shear rheology. The E-moduli and G-moduli are con- charged cross-linker the neutral one always displayed a
nected by the Poisson’s ratio, the G-moduli usually being higher elastic shear modulus compared to the charged
equal to one third of the E-moduli for isotropic incom- one. A typical example is 12d versus 13d. A possible ex-
pressible hydrogels.24
planation for this effect is that the charge of the cross-link-
er also leads to higher EDS due to higher hydration,
thereby altering the mechanical properties of the gels.
We observed that the EDS is similar for all gels tested and
is between 400 and 550% for the neutral cross-linkers and
between 600 and 750% for the charged cross-linkers. That
means for pairs of neutral and charged cross-linkers the
charged ones always display the higher EDS as compared
to the corresponding neutral counterparts. The charge of
the cross-linker leads to a greater water uptake of the gels
due to its interaction with the dipolar water molecules. It
should be noted that 12f-based gels exhibit a greater EDS
than the other gels because of the increased hydrophilicity
of the spacers.
Following our previously described procedures,11 stable
hydrogels 17 were obtained from thiolated hyaluronan
Figure 2 Elastic shear moduli G′ of the hydrogels 16 formed by
polymer 15 and the respective cross-linkers. The gels were swollen to
equilibrium in PBS before measurements.
(HA-SH) and cross-linkers 12 and 13 (Figure 4). To ana-
lyze the influence of spacer length cross-linkers with
varying alkyl spacers 12d–f and 13d–f were first em-
ployed in the same thio-Michael addition with HA-SH
(degree of thiolation: 40%).
In contrast to the synthetic hydrogels described above, the
HA-SA-based hydrogels exhibit a linear viscoelastic re-
sponse up to 10% deformation (Table S2, Supporting In-
formation), enabling the determination of their E-moduli
by compression testing.
While 12d and 13d lead to stable hydrogels with E-mod-
uli of 1.6 kPa and 1.1 kPa, respectively, the poor solubility
of 12e and 13e in water or mixtures of ethanol and water
prevented the formation of HA hydrogels. In contrast the
excellent solubility of 12f and 13f led to stable HA hydro-
gels with E-moduli of 1.1 kPa and 1.3 kPa (Figure 5). All
four hydrogels withstood repetitive compressive stress of
Figure 3 Equilibrium degrees of swelling EDS in PBS of the hydro-
gels 16 formed by polymer 15 and the respective cross-linkers
O
O
S
O
HN
OH
O
O
Na
OH
O
NH
O
O
O
O
O
O
HO
O
HO
O
O
O
OH
HO
HO
OH
NH
OH
NH
17
n
Figure 4 Statistically thiolated hyaluronan (40%) attached to a cross-linker via thio-Michael addition
Georg Thieme Verlag Stuttgart · New York
©
Synthesis 2014, 46, 1243–1253

1248
M. Mateescu et al.
PAPER
up to 15% and showed a large linear elastic range. Inter- The 3,5-diacylpyridines were N-methylated and selected
estingly, here we observe that as linker length of the cross- members of both neutral and cationic 3,5-diacylpyridines
linkers increases, the E-moduli of hydrogels cross-linked were converted to hydrogels with neutral poly(1-glycidyl-
with neutral or charged linker, respectively, display simi- piperazine) and polyanionic hyaluronan. Rheological
lar values, indicating that the positive charge on the cross- studies revealed that elastic shear moduli G′ decreased for
linker only partially compensates the negative charge on neutral polymers with increasing water solubility of the
the polyanionic hydrogel.
spacers, while E-moduli of softer polyanionic hydrogels
changed only little. Swelling ratios increased with water
solubility and presence of cationic cross-linkers in both
neutral and polyanionic polymers.
All chemicals for cross-linker synthesis were purchased from
Sigma-Aldrich or Alfa-Aesar. Acryloyl chloride, CH Cl , and Et N
2
2
3
were freshly distilled before use. Pyridine-2,4,6-tricarboxylic acid
and pyridine-3,5-dicarboxylic acid were prepared by oxidation of
2
,4,6-lutidine and 3,5-lutidine, respectively, according to litera-
2
5
ture. Melting points were measured on a Stuart SMP10 instru-
ment. H and C NMR spectra were recorded on with Bruker
Avance 300 and Avance 500 spectrometers at 300 or 500 MHz ( H)
and 75 or 125 MHz ( C). Standard abbreviations were used to de-
note the multiplicities of the peaks. NMR assignments were based
on COSY, HMBC, and HSQC spectra. IR spectra were recorded on
Bruker MKII Golden Gate Single Reflection Diamant spectrometer
by ATR method. Mass spectra were recorded on a Bruker mi-
1
13
1
1
3
Figure 5 E-Moduli of the hydrogels 17 formed with the respective
cross-linkers
crOTOF-Q spectrometer under positive electrospray ionization
+
(
ESI ) conditions. The assignments for the mass signals are given
+
+
for [M] or adducts, [M] being the molecular mass of the positively
charged molecule without anion. Elemental analyses were carried
out with Carlo Erba Strumentazione Elemental Analyzer model
The EDS values of the gels (Figure 6) range between 200
and 250% and again are slightly higher for positively
charged cross-linkers 13d,f compared to the neutral cross- 1106. Column chromatography was performed using silica gel 60
linkers 12d,f.
(Fluka, mesh 40–63 μm) with hexanes (bp 30–75 °C). Reactions
were performed using standard Schlenk type conditions under inert
atmosphere.
Bis[1-(acryloyloxy)-3-(methacryloyloxy)propan-2-yl]pyridine-
2
,6-dicarboxylate (8g); Typical Procedure
To a solution of 3-(acryloyloxy)-2-hydroxypropyl methacrylate
3g; AHM, 3.94 g, 8.98 mmol) and Et N (2.6 mL, 1.9 g,
(
3
1
8.9 mmol) in anhydrous CH Cl (20 mL) was slowly added a so-
2 2
lution of pyridine-2,6-dicarbonyl dichloride (1.83 g, 8.98 mmol) in
anhydrous CH Cl (20 mL) at 0 °C. The slightly turbid mixture was
2
2
stirred at r.t. for 16 h, filtered through Celite, and concentrated in
vacuo under addition of MEHQ as polymerization inhibitor. The
crude product was purified by flash chromatography (hexanes–
EtOAc, 3:1 → 2:1) to yield 8g (3.56 g, 6.36 mmol, 71%) as a clear
colorless oil; R = 0.21 (hexanes–EtOAc, 2:1).
f
FT-IR (ATR): 2931 (w), 2184 (w), 1962 (w), 1720 (s), 1293 (m),
–
1
1
236 (m), 1162 cm (s).
1
Figure 6 Equilibrium degrees of swelling of the hydrogels 17 with
different cross-linkers
H NMR (300 MHz, CDCl ): δ = 1.93 (s, 6 H, CH ), 4.41–4.69 (m,
3
3
8
H, OCH ), 5.59–5.62 (m, 2 H, cis-3′-H), 5.65–5.73 (m, 2 H,
2
OCH), 5.85–5.91 (m, 2 H, cis-3′′-H), 6.08–6.21 (m, 4 H, trans-3′-H,
2
8
′′-H), 6.40–6.49 (m, 2 H, trans-3′′-H), 7.99–8.04 (m, 1 H, 4-H),
.23–8.26 (m, 2 H, 3-H, 5-H).
In conclusion, we have synthesized three series of neutral
cross-linkers containing 2,6-, 3,5-diacylpyridine, and
13
C NMR (75 MHz, CDCl ): δ = 18.2 (CH ), 62.4 (OCH ), 71.0
3
3
2
2,4,6-triacylpyridine as the central cores, which were teth-
(
1
(
OCH), 126.6 (C-3′), 127.6 (C-2′′), 128.2 (C-3, C-5), 131.9 (C-3′′),
35.6 (C-2′), 138.3 (C-4), 148.1 (C-2, C-6), 163.3 (Ar-C=O), 165.6
C-1′′), 166.7 (C-1′).
ered to terminal acrylates or acrylamides via ethylene gly-
col, amino ethanol, and 1,n-diamine spacers. Acrylates
were generally obtained in higher yields than acrylamides.
In addition, yields were affected not only by the position
of the side chains at the pyridine core but also by the num-
ber of polymerizable groups and the number of NH
groups in the spacer suggesting that a combination of ste-
ric hindrance and unfavorable aggregation due to hydro-
gen bonding might lead to reduced yields.
+
+
MS (ESI): m/z = 582.2 [M + Na] , 560.2 [M + H] , 197.1 [C H O
4
10
13
+
(AHM − OH)] .
HRMS (ESI): m/z calcd for C H NO : 582.1582; found:
5
27
29
12
+
82.1586 [M + Na] .
Bis[2-(acryloyloxy)ethyl]pyridine-2,6-dicarboxylate (8a)
An analogous procedure as described for 8g was used. The product
was purified by washing with H O and by flash chromatography
2
Synthesis 2014, 46, 1243–1253
© Georg Thieme Verlag Stuttgart · New York

PAPER
CH Cl –EtOAc, 5:1; R = 0.28); yield: 3.03 g (93%); colorless sol-
Pyridine-Acryl Cross-Linkers for Hydrogels
1249
(
Tris(2-acrylamidoethyl)pyridine-2,4,6-tricarboxylate (10b)
An analogous procedure as described for 8g was used. The product
was purified by flash chromatography (EtOAc–acetone, 5:1;
Rf = 0.08); yield: 993 mg (22%); colorless solid; mp 135 °C.
2
2
f
id; mp 77 °C.
FT-IR (ATR): 3088 (w), 2958 (w), 1730 (s), 1715 (s), 1400 (m),
–
1
1
289 (m), 1240 (s), 1178 cm (s).
FT-IR (ATR): 3300 (w), 3056 (w), 1750 (m), 1728 (m), 1654 (s),
1
H NMR (300 MHz, CDCl ): δ = 4.54–4.57 (m, 4 H, 1′-H), 4.67–
3
–1
1
542 (m), 1210 (s), 1170 (m), 1025 cm (m).
3
2
4
.71 (m, 4 H, 2′-H), 5.87 (dd, J = 10.4 Hz, J = 1.5 Hz, 2 H, trans-
3
3
1H NMR (500 MHz, DMSO-d
4.43–4.46 (m, 6 H, 1′-H), 5.61 [dd, J = 10.1 Hz, J = 2.1 Hz, 2 H,
CHH=CH), 6.15 (dd, J = 17.3 Hz, J = 10.4 Hz, 2 H, CH=CH ),
): δ = 3.57–3.62 (m, 6 H, 2′-H),
6
2
3
2
3
2
6
.45 (dd, J = 17.3 Hz, J = 1.5 Hz, 2 H, cis-CHH=CH), 8.05 (dd,
J = 8.3 Hz, J = 7.5 Hz, 1 H, 4-H), 8.28–8.31 (m, 2 H, 3-H, 5-H).
3
3
3
2
trans-CHH=CH (C-2, C-6)], 5.63 [dd, J = 10.1 Hz, J = 2.2 Hz, 1
3
2
H, trans-CHH=CH (C-4)], 6.10 [dd, J = 17.1 Hz, J = 2.2 Hz, 2 H,
cis-CHH=CH (C-2, C-6)], 6.11 (dd, J = 17.1 Hz, J = 2.2 Hz, 1 H,
cis-CHH=CH (C-4)], 6.23 [dd, J = 17.1 Hz, J = 10.1 Hz, 2 H,
CH=CH (C-2, C-6)], 6.23 (dd, J = 17.1 Hz, J = 10.1 Hz, 1 H,
13
C NMR (75 MHz, CDCl ): δ = 62.1 (C-2′), 63.7 (C-1′), 127.9
3
3
2
(
(
CH=CH ), 128.2 (C-3, C-5), 131.5 (CH =CH), 138.4 (C-4), 148.2
C-2, C-6), 164.2 (vinyl-C=O), 165.9 (Ar-C=O).
2
2
3
3
3
3
2
+
+
3
MS (ESI): m/z = 386.1 [M + Na] , 364.1 [M + H] , 349.2, 301.1,
CH=CH (C-4)], 8.43 [t, J = 5.7 Hz, 2 H, NH (C-2, C-6)], 8.46 [t,
2
3
209.1, 149.0, 99.0.
J = 5.8 Hz, 1 H, NH (C-4)], 8.60 (s, 2 H, 3-H, 5-H).
1
3
HRMS (ESI): m/z calcd for C H NO : 386.0846; found: 386.0854
C NMR (125 MHz, DMSO-d ): δ = 37.5 [C-2′ (C-4)], 37.6 [C-2′
1
7
17
8
6
+
[M + Na] .
(C-2, C-6)], 64.5 [C-1′ (C-2, C-6)], 65.0 [C-1′ (C-4)], 125.5
[
1
CH =CH (C-2, C-6)], 125.6 [CH =CH (C-4)], 126.6 (C-3, C-5),
31.4 (CH=CH ), 139.9 (C-4), 149.0 (C-2, C-6), 163.2 [vinyl-C=O
2
Anal. Calcd for C H NO (+ 0.03 equiv MEHQ): C, 56.32; H,
2
2
17
17
8
4.74; N, 3.81. Found: C, 56.37; H, 4.77; N, 3.75.
(C-2, C-6)], 163.3 [vinyl-C=O (C-4)], 165.0 [Ar-C=O (C-2, C-6)],
Bis(2-acrylamidoethyl)pyridine-2,6-dicarboxylate (8b)
165.1 [Ar-C=O (C-4)].
An analogous procedure as described for 8g was used. The product
was purified by flash chromatography [EtOAc–acetone, 5:1
+
+
MS (ESI): m/z = 525.2 [M + Na] , 503.2 [M + H] , 428.1, 406.1,
31.1.
3
(
Rf = 0.17) → 4:1 → 3:1]; yield: 1.59 g (49%); colorless solid;
HRMS (ESI): m/z calcd for C H N O : 525.1592; found:
mp 154 °C.
23 26
4
9
+
5
25.1606 [M + Na] .
FT-IR (ATR): 3258 (s), 3062 (w), 2963 (w), 2925 (w), 2877 (w),
–
1
1
740 (m), 1551 (s), 1242 cm (s).
Tris[1-(acryloyloxy)-3-(methacryloyloxy)propan-2-yl]pyri-
dine-2,4,6-tricarboxylate (10g)
An analogous procedure as described for 8g was used. The product
was purified by flash chromatography [hexanes–EtOAc, 3:1 → 2:1
1
3
H NMR (300 MHz, DMSO-d ): δ = 3.54 (q, J = 5.6 Hz, 4 H, 2′-
6
3
3
H), 4.39 (t, J = 5.6 Hz, 4 H, 1′-H), 5.60 (dd, J = 9.9 Hz,
2
2
3
3
J = 2.4 Hz, 2 H, trans-CHH=CH), 6.09 (dd, J = 17.1 Hz,
H, cis-CHH=CH), 6.22 (dd, J = 17.1 Hz,
J = 9.9 Hz, 2 H, CH=CH ), 8.21 (dd, J = 9.2 Hz, J = 5.9 Hz, 1 H,
3
(R = 0.24)]; yield: 2.37 g (33%); colorless oil.
f
J = 2.4 Hz,
2
3
3
FT-IR (ATR): 2960 (w), 1718 (s), 1635 (m), 1235 (m), 1155 (s),
2
3
–1
4
-H), 8.27–8.3 (m, 2 H, 3-H, 5-H), 8.38 (t, J = 5.5 Hz, 2 H, NH).
1064 (m), 946 (m), 808 cm (m).
13
1
C NMR (125 MHz, DMSO-d ): δ = 37.7 (C-2′), 64.1 (C-1′), 125.5
H NMR (300 MHz, CDCl ): δ = 1.94 (s, 9 H, CH ), 4.38–4.66 (m,
6
3
3
(
(
CH =CH), 128.2 (C-3, C-5), 131.4 (CH=CH ), 139.1 (C-4), 147.8
C-2, C-6), 163.9 (vinyl-C=O), 165.0 (Ar-C=O).
12 H, OCH ), 5.59–5.64 (m, 3 H, cis-3′-H), 5.65–5.74 (m, 3 H,
OCH), 5.82–5.92 (m, 3 H, cis-3′′-H), 6.08–6.21 (m, 6 H, trans-3′-H,
2
2
2
+
+
2′′-H), 6.41–6.49 (m, 3 H, trans-3′′-H), 8.70 (s, 2 H, 3-H, 5-H).
MS (ESI): m/z = 384.1 [M + Na] , 362.1 [M + H] , 287.1, 98.1.
13
C NMR (75 MHz, CDCl ): δ = 17.3 (CH ), 61.4 (OCH ), 70.4
3
3
2
HRMS (ESI): m/z calcd for C H N O : 384.1166; found:
17
19
3
6
+
(OCH), 125.3 (C-3′), 125.7 (C-2′′), 126.8 (C-3, C-5), 131.0 (C-3′′),
34.7 (C-2′), 138.8 (C-4), 148.4 (C-2, C-6), 161.7 (4-C=O), 164.6
384.1165 [M + Na] .
1
Anal. Calcd for C H N O : C, 56.51; H, 5.30; N, 11.63. Found: C,
17
19
3
6
(2-C=O, 6-C=O), 165.7 (C-1′′), 166.4 (C-1′).
56.29; H, 5.35; N, 11.59.
+
+
MS (ESI): m/z = 822.2 [M + Na] , 626.2 [M − AHM + H + Na] ,
5
96.2, 419.6, 237.1, 197.1.
Tris[2-(acryloyloxy)ethyl]pyridine-2,4,6-tricarboxylate (10a)
An analogous procedure as described for 8g was used. The product
was purified by flash chromatography [hexanes–EtOAc, 2:1 → 1:1
HRMS (ESI): m/z calcd for C H NO : 822.2216; found:
8
38
41
18
+
22.2208 [M + Na] .
(
Rf = 0.24)]; yield: 3.00 g (66%); colorless oil.
–1
Bis[1-(acryloyloxy)-3-(methacryloyloxy)propan-2-yl]pyridine-
3,5-dicarboxylate (12g)
FT-IR (ATR): 2985 (w), 1719 (s), 1408 (m), 1227 (s), 1183 (s) cm .
1
H NMR (300 MHz, CDCl ): δ = 4.55–4.58 (m, 6 H, 2′-H), 4.68–
An analogous procedure as described for 8g was used. The product
was purified by flash chromatography (hexanes–EtOAc, 4:1;
Rf = 0.15); yield: 3.62 g (72%); colorless oil.
3
3
2
4
.74 (m, 6 H, 1′-H), 5.88 [dd, J = 10.4 Hz, J = 1.5 Hz, 2 H, trans-
3
2
CHH=CH (C-2, C-6)], 5.90 [dd, J = 10.4 Hz, J = 1.4 Hz, 1 H,
trans-CHH=CH (C-4)], 6.16 [dd, J = 17.3 Hz, J = 10.4 Hz, 2 H,
CH=CH (C-2, C-6)], 6.17 [dd, J = 17.3 Hz, J = 10.4 Hz, 1 H,
CH=CH2 (C-4)], 6.45 [dd, J = 17.3 Hz, J = 1.5 Hz, 2 H, cis-
CHH=CH (C-2, C-6)], 6.47 [dd, J = 17.3 Hz, J = 1.4 Hz, 1 H, cis-
CHH=CH (C-4)], 8.78 (s, 3-H, 5-H).
3
3
FT-IR (ATR): 2960 (w), 2196 (w), 1966 (w), 1636 (w), 1453 (w),
3
3
2
409 (w), 1294 (m), 1234 (m), 1159 (s), 1102 (m), 809 (w), 745 cm–
1
3
2
1
(
w).
1H NMR (300 MHz, CDCl
.63 (m, 8 H, OCH ), 5.60–5.64 (m, 2 H, cis-3′-H), 5.66–6.73 (m, 2
3
2
): δ = 1.93–1.95 (m, 6 H, CH ), 4.41–
3
3
4
13
2
C NMR (75 MHz, CDCl ): δ = 61.8 [C-2′ (C-4)], 62.0 [C-2′ (C-2,
3
H, OCH), 5.88–5.94 (2 H, cis-3′′-H), 6.10–6.21 (4 H, trans-3′-H,
C-6)], 64.0 [C-1′ (C-2, C-6)], 64.2 [C-1′ (C-4)], 127.4 (C-3, C-5),
27.7 [CH=CH2 (C-4)], 127.8 [CH=CH2 (C-2, C-6)], 131.6
CH =CH (C-2, C-6)], 131.8 [CH =CH (C-4)], 139.9 (C-4), 149.3
2
9
′′-H), 6.41–6.5 (2 H, trans-3′′-H), 8.83–8.85 (m, 1 H, 4-H), 9.36–
.37 (m, 2 H, 3-H, 5-H).
1
[
(
1
2
2
13C NMR (75 MHz, CDCl
(OCH), 125.6 (C-3′), 126.6 (C-2′′), 127.4 (C-3, C-5), 132.1 (C-3′′),
): δ = 18.2 (CH ), 62.4 (OCH ), 70.8
3 3 2
C-2, C-6), 163.3 [vinyl-C=O (C-4)], 163.6 [vinyl-C=O (C-2, C-6)],
65.7 [Ar-C=O (C-4)], 168.8 [Ar-C=O (C-2, C-6)].
1
35.5 (C-2′), 138.4 (C-4), 154.5 (C-2, C-6), 163.4 (aryl-C=O),
+
+
MS (ESI): m/z = 528.1 [M + Na] , 506.1 [M + H] , 444.1, 400.1.
1
65.6 (C-1′′), 166.8 (C-1′).
HRMS (ESI): m/z calcd for C H NO : 528.1112; found:
5
23
23
12
+
28.1106 [M + Na] .
©
Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 1243–1253

1
250
M. Mateescu et al.
PAPER
6.09 [dd, J = 17.1 Hz, J = 2.5 Hz, 1 H, cis-CHH=CH (C-4)], 6.10
+
+
3
2
MS (ESI): m/z = 582.2 [M + Na] , 560.2 [M + H] , 451.2, 334.1,
37.1.
3
2
2
[dd, J = 17.1 Hz, J = 2.3 Hz, 2 H, cis-CHH=CH (C-2, C-6)], 6.22
3
3
[
dd, J = 17.1 Hz, J = 9.9 Hz, 2 H, CH=CH (C-2, C-6)], 6.26 [dd,
2
HRMS (ESI): m/z calcd for C H NO 560.1763; found: 560.1764
2
7
29
12
3
3
+
J = 17.1 Hz, J = 10.0 Hz, 1 H, CH=CH
J = 5.4 Hz, 1 H, 2′-NH (C-4)], 8.52 [t, J = 5.7 Hz, 2 H, 2′-NH (C-
2, C-6)], 8.60 (s, 2 H, 3-H, 5-H), 9.26 [t, J = 5.1 Hz, 1 H, 1′-NH (C-
4)], 9.68 [t, J = 5.9 Hz, 2 H, 1′-NH (C-2, C-6)].
2
(C-4)], 8.31 [t,
[M + Na] .
3
3
3
3
5
N ,N -Bis(2-acrylamidoethyl)pyridine-3,5-dicarboxamide
12c); Typical Procedure
A suspension of N-(2-aminoethyl)acrylamide hydrochloride (3c;
.27 g, 54.9 mmol) and NaH (2.89 g, 121 mmol) in anhydrous DMF
120 mL) was stirred for 10 min at r.t. To this mixture was added a
3
(
13
C NMR (75 MHz, DMSO-d ): δ = 37.90 (C-2′), 38.50 (C-1′),
6
8
(
1
21.70 (CH=CH ), 125.10 (CH =CH), 131.70 (C-3, C-5), 144.50
2
2
(
C-4), 149.50 (C-2, C-6), 162.90 [vinyl-C=O (C-2, C-6)], 163.60
solution of pyridine-3,5-dicarbonyl dichloride (5.60 g, 27.4 mmol)
in anhydrous DMF (25 mL) dropwise at 0 °C. The mixture was
stirred at r.t. for 16 h, filtered through silica gel, and concentrated in
vacuo. To the brown residue was added MeOH (30 mL) and the re-
sulting suspension filtered again. The colorless residue consisted of
the pure product. The filtrate could be recrystallized further to give
an additional amount of the product. Product 12c (4.92 g,
[vinyl-C=O (C-4)], 164.80 [aryl-C=O (C-4)), 165.00 (aryl-C=O (C-
2, C-6)].
+
+
MS (ESI): m/z = 522.2 [M + Na] , 500.2 [M + H] , 102.1.
HRMS (ESI): m/z calcd for C H N O : 522.2072; found:
23
29
7
6
+
5
22.2069 [M + Na] .
3
5
12.9 mmol, 47%) was obtained as a colorless solid; mp 295 °C
N ,N -Bis(3-acrylamidopropyl)pyridine-3,5-dicarboxamide
(12d)
(dec.); R = 0.16 (CH Cl –MeOH, 10:1).
f
2
2
An analogous procedure as described for 12c was used. The product
was purified by flash chromatography (CH Cl –MeOH, 10:1;
FT-IR (ATR): 3258 (w), 2945 (w), 1638 (m), 1533 (s), 1236 (m),
–
1
2
2
671 cm (m).
Rf = 0.20); yield: 1.39 g (40%); colorless solid; mp 199 °C.
1
H NMR (500 MHz, D O): δ = 3.55–3.58 (m, 4 H, 2′-H), 3.62–3.65
2
FT-IR (ATR): 3291 (m), 3247 (m), 3069 (w), 3026 (w), 2961 (w),
3
2
(
m, 4 H, 1′-H), 5.75 (dd, J = 10.1 Hz, J = 1.3 Hz, 2 H, trans-
2
924 (w), 2867 (w), 1635 (s), 1537 (s), 1407 (m), 1359 (m), 1328
3
2
CHH=CH), 6.16 (dd, J = 17.1 Hz, J = 1.3 Hz,
2
H, cis-
–1
(m), 1299 (m), 1241 (m), 1130 (m), 951 (m), 707 (s), 685 cm (s).
3
3
CHH=CH), 6.26 (dd, J = 17.1 Hz, J = 10.1 Hz, 2 H, CH=CH ),
2
1
3
4
4
H NMR (500 MHz, DMSO-d ): δ = 1.73 (quint, J = 7.0 Hz, 4 H,
9.14 (t, J = 1.9 Hz, 1 H, 4-H), 9.28 (d, J = 1.9 Hz, 2 H, 2-H, 6-H).
6
3
3
2
′-H), 3.21 (q, J = 6.6 Hz, 4 H, 3′-H), 3.32 (q, J = 6.6 Hz, 4 H, 1′-
13
C NMR (125 MHz, D O + TFA): δ = 38.4 (C-2′), 39.8 (C-1′),
3
2
2
H), 5.58 (dd, J = 10.2 Hz, J = 2.2 Hz, 2 H, trans-CHH=CH), 6.02
1
27.4 (CH=CH ), 129.9 (CH =CH), 133.5 (C-3, C-5), 142.5 (C-4),
3
2
2
2
(
dd, J = 17.1 Hz, J = 2.2 Hz, 2 H, cis-CHH=CH), 6.22 (dd,
143.3 (C-2, C-6), 163.9 (vinyl-C=O), 170.0 (aryl-C=O).
3
3
3
J = 17.1 Hz, J = 10.2 Hz, 2 H, CH=CH ), 8.15 (t, J = 5.2 Hz, 2 H,
2
+
+
4
3
MS (ESI): m/z = 382.2 [M + Na] , 360.2 [M + H] .
3′-NH), 8.58 (t, J = 2.1 Hz, 1 H, 4-H), 8.81 (t, J = 5.5 Hz, 2 H, 1′-
NH), 9.09 (d, J = 2.1 Hz, 2 H, 2-H, 6-H).
4
HRMS (ESI): m/z calcd for C H N O : 382.1486; found:
1
7
21
5
4
+
13
382.1473 [M + Na] .
C NMR (125 MHz, DMSO-d ): δ = 28.9 (C-2′), 36.4 (C-1′), 37.2
6
(
(
C-3′), 124.9 (CH =CH), 129.6 (C-3, C-5), 131.7 (CH=CH ), 133.7
C-4), 150.1 (C-2, C-6), 164.2 (aryl-C=O), 164.6 (vinyl-C=O).
2
2
Anal. Calcd for C H N O : C, 56.82; H, 5.89; N, 19.49. Found: C,
17
21
5
4
56.30; H, 5.93; N, 19.33.
+
+
Spectral data were in accordance with the literature data.11
MS (ESI): m/z = 410.2 [M + Na] , 388.2 [M + H] .
HRMS (ESI): m/z calcd for C H N O : 410.1799; found:
19
25
5
4
2
6
N ,N -Bis(2-acrylamidoethyl)pyridine-2,6-dicarboxamide (8c)
An analogous procedure as described for 12c was used. The product
was purified by recrystallization from EtOH; yield: 678 mg (21%);
colorless solid; mp 280 °C (dec.); R = 0.52 (CH Cl –MeOH, 10:1).
+
4
10.1802 [M + Na] .
Anal. Calcd for C H N O : C, 58.90; H, 6.50; N, 18.08. Found: C,
19 25
5
4
59.01; H, 6.58; N, 17.86.
f
2
2
3
5
FT-IR (ATR): 3257 (m), 3075 (m), 2940 (w), 1651 (s), 1542 (s),
N ,N -Bis(4-acrylamidobutyl)pyridine-3,5-dicarboxamide (12e)
An analogous procedure as described for 12c was used. The product
–
1
1404 (m), 1253 (m), 949 (m), 808 (m), 714 (m), 681 cm (m).
1
was purified by flash chromatography (CH
R = 0.14); yield: 1.90 g (51%); colorless solid; mp 191 °C.
f
2 2
Cl –MeOH, 10:1;
H NMR (300 MHz, DMSO-d ): δ = 3.41–3.52 (m, 8 H, CH ), 5.61
6
2
3
2
(
dd, J = 9.8 Hz, J = 2.5 Hz, 2 H, trans-CHH=CH), 6.10 (dd,
3
2
3
J = 17.1 Hz, J = 2.5 Hz, 2 H, cis-CHH=CH), 6.23 (dd, J = 17.1
FT-IR (ATR): 3298 (m), 3061 (w), 2954 (w), 2870 (w), 1654 (m),
1625 (s), 1530 (s), 1476 (m), 1318 (m), 1304 (m), 1287 (m), 1233
3
Hz, J = 9.8 Hz, 2 H, CH=CH ), 8.14–8.23 (m, 3 H, 3-H, 4-H, 5-H),
8
2
3
3
–1
.40 (t, J = 5.6 Hz, 2 H, 2′-NH), 9.47 (t, J = 5.6 Hz, 2 H, 1′-NH).
(m), 982 (m), 954 (m), 656 cm (s).
1
3
1
C NMR (125 MHz, DMSO-d ): δ = 37.4 (C-2′), 38.1 (C-1′), 123.2
H NMR (500 MHz, DMSO-d ): δ = 1.47–1.58 (m, 8 H, 2′-H, 3′-H),
6
6
3
3
(
(
CH=CH ), 124.4 (CH =CH), 130.7 (C-3, C-5), 138.6 (C-4), 147.6
C-2, C-6), 162.4 (vinyl-C=O), 164.3 (aryl-C=O).
3.16 (q, J = 6.4 Hz, 4 H, 4′-H), 3.30 (q, J = 6.4 Hz, 4 H, 1′-H),
2
2
3
2
5.56 (dd, J = 10.1 Hz, J = 2.2 Hz, 2 H, trans-CHH=CH), 6.06 (dd,
3
3
2
+
+
J = 17.1 Hz, J = 2.2 Hz,
2
H, cis-CHH=CH), 6.20 (dd,
MS (ESI): m/z = 382.2 [M + Na] , 360.2 [M + H] .
3
3
J = 17.1 Hz, J = 10.1 Hz, 2 H, CH=CH ), 8.10 (t, J = 5.3 Hz, 2H,
2
HRMS (ESI): m/z calcd for C H N O : 382.1486; found:
3
4
3
17
21
5
4
4′-NH), 8.57 (t, J = 2.1 Hz, 1 H, 4-H), 8.80 (t, J = 5.5 Hz, 2 H, 1′-
NH), 9.08 (t, J = 2.1 Hz, 2 H, 2-H, 6-H).
+
82.1492 [M + Na] .
4
1
3
2
4
6
C NMR (125 MHz, DMSO-d ): δ = 26.5 (C-2′), 26.6 (C-3′), 38.2
6
N ,N ,N -Tris(2-acrylamidoethyl)pyridine-2,4,6-tricarbox-
amide (10c)
(C-1′), 39.0 (C-4′), 124.9 (CH =CH), 129.8 (C-3, C-5), 131.8
(CH=CH ), 133.8 (C-4), 150.2 (C-2, C-6), 164.2 (aryl-C=O), 164.5
(vinyl-C=O).
2
An analogous procedure as described for 12c was used. The product
was purified by flash chromatography (CH Cl –MeOH, 10:1;
2
2
2
Rf = 0.29); yield: 538 mg (12%); colorless solid; mp 270 °C (dec.).
+
+
MS (ESI): m/z = 438.2 [M + Na] , 416.2 [M + H] , 393.3, 284.1.
FT-IR (ATR): 2922 (s), 2852 (s), 1711 (m), 1623 (s), 1537 (s), 1247
HRMS (ESI): m/z calcd for C H N O : 438.2112; found:
–
1
21 29
5
4
(s), 765 (m), 685 cm (s).
+
4
38.2134 [M + Na] .
1
H NMR (300 MHz, DMSO-d ): δ = 3.34–3.51 (m, 12 H, CH ),
6
2
Anal. Calcd for C H N O : C, 60.71; H, 7.04; N, 16.86. Found: C,
3
2
21 29
5
4
5
5
.59 [dd, J = 9.9 Hz, J = 2.3 Hz, 2 H, trans-CHH=CH (C-2, C-6)],
60.76; H, 7.40; N, 15.08.
3
2
.60 [dd, J = 10.0 Hz, J = 2.5 Hz, 1 H, trans-CHH=CH (C-4)],
Synthesis 2014, 46, 1243–1253
© Georg Thieme Verlag Stuttgart · New York

PAPER
Pyridine-Acryl Cross-Linkers for Hydrogels
1251
3
5
3
3
N ,N -Bis{2-[2-(2-acrylamidoethoxy)ethoxy]ethyl}pyridine-3,5-
dicarboxamide (12f)
8.13 (t, J = 5.7 Hz, 2 H, 4′-NH), 9.13 (t, J = 5.6 Hz, 2 H, 1′-NH),
4
4
9.24 (t, J = 1.3 Hz, 1 H, 4-H), 9.49 (d, J = 1.3 Hz, 2 H, 2-H, 6-H).
An analogous procedure as described for 12c was used. Purified by
flash chromatography (CH Cl –MeOH 10:1; R = 0.35); yield: 2.21
g (46%); pale yellow oil.
13
C NMR (125 MHz, DMSO-d ): δ = 26.2 (C-2′), 26.5 (C-3′), 38.1
C-1′), 39.4 (C-4′), 48.5 (CH ), 124.9 (CH =CH), 131.8 (CH=CH ),
33.2 (C-3, C-5), 140.6 (C-4), 146.8 (C-2, C-6), 160.9 (aryl-C=O),
64.5 (vinyl-C=O).
6
2
2
f
(
1
1
3
2
2
FT-IR (ATR): 3289 (w), 3076 (w), 2870 (w), 1652 (s), 1627 (s),
543 (s), 1291 (m), 1245 (m), 1102 (m), 705 cm (w).
–
1
1
+
+
MS (ESI): m/z = 430.3 [M] , 416.2 [M − CH + H] , 393.3, 325.3,
3
1
3
H NMR (300 MHz, DMSO-d ): δ = 3.27 (q, J = 5.7 Hz, 4 H, 6′-
H), 3.43–3.48 (m, 8 H, 1′-H, 5′-H), 3.52–3.58 (m, 12 H, 2′-H, 3′-H,
′-H), 5.56 (dd, J = 10.0 Hz, J = 2.3 Hz, 2 H, trans-CHH=CH),
.07 (dd, J = 17.1 Hz, J = 2.3 Hz, 2 H, cis-CHH=CH), 6.24 (dd,
J = 17.1 Hz, J = 10.0 Hz, 2 H, CH=CH ), 8.17 (t, J = 5.2 Hz, 2 H,
′-NH), 8.60 (t, J = 2.1 Hz, 1 H, 4-H), 8.88 (t, J = 5.4 Hz, 2 H, 1′-
NH), 9.10 (d, J = 2.1 Hz, 2 H, 2-H, 6-H).
6
311.3.
3
2
HRMS (ESI): m/z calcd for C22
430.2459 [M] .
H
32
N
5
O
4
: 430.2449; found:
4
+
3
2
6
3
3
3
2
3
,5-Bis({2-[2-(2-acrylamidoethoxy)ethoxy]ethyl}carbamoyl)-1-
4
3
6
methylpyridin-1-ium Iodide (13f)
4
An analogous procedure as described for 13d was used. Solvent:
13
DMF. Purified by washing with Et O; yield: 349 mg (quant); orange
oil; R = 0.15 (CH Cl –MeOH, 10:1).
C NMR (75 MHz, DMSO-d ): δ = 38.5 (C-6′), 39.3 (C-1′), 68.7
C-5′), 69.0 (C-2′), 69.5 (C-3′, C-4′), 125.0 (CH =CH), 129.5 (C-3,
2
6
(
f
2
2
2
C-5), 131.6 (CH=CH ), 133.9 (C-4), 150.3 (C-2, C-6), 164.4 (aryl-
C=O), 164.6 (vinyl-C=O).
2
FT-IR (ATR): 3261 (w), 3065 (w), 2868 (w), 1655 (s), 1624 (m),
539 (s), 1288 (m), 1238 (m), 1095 (s), 986 (m), 960 (m), 806 (w),
669 cm (m).
1
–
1
+
+
MS (ESI): m/z = 558.3 [M + Na] , 536.3 [M + H] , 439.2 [M −
+
H C=CHCONHC H + 2 H] , 377.2, 342.2, 290.6, 280.1 [M −
1
3
2
2
4
H NMR (500 MHz, D O): δ = 3.46 (t, J = 5.4 Hz, 4 H, 6′-H), 3.67
2
+
H C=CHCONH − H C=CHCONH(C H O) C H ] , 227.0, 218.1,
3
3
2
2
2
4
2
2
4
(t, J = 5.4 Hz, 4 H, 5′-H), 3.69 (t, J = 5.2 Hz, 4 H, 1′-H), 3.71–3.74
(m, 8 H, 3′-H, 4′-H), 3.79 (t, J = 5.2 Hz, 4 H, 2′-H), 4.55 (s, 3 H,
CH ), 5.75 (dd, J = 10.1 Hz, J = 1.5 Hz, 2 H, trans-CHH=CH),
+
1
85.1, 98.1 [H C=CHCONHC H ] .
3
2
2
4
3
2
HRMS (ESI): m/z calcd for C H N O : 558.2534; found:
3
25
37
5
8
3
2
+
6.17 (dd, J = 17.2 Hz, J = 1.5 Hz, 2 H, cis-CHH=CH), 6.25 (dd,
558.2530 [M + Na] .
3
3
4
J = 17.2 Hz, J = 10.1 Hz, 2 H, CH=CH ), 9.23 (t, J = 1.6 Hz, 1 H,
2
4
3
,5-Bis[(3-acrylamidopropyl)carbamoyl]-1-methylpyridin-1-
4-H), 9.42 (d, J = 1.6 Hz, 2 H, 3-H, 5-H).
13C NMR (125 MHz, D
O): δ = 38.9 (C-6′), 39.9 (C-1′), 49.1 (CH
8.5 (C-5′), 68.7 (C-2′), 69.4 (C-3′, C-4′), 127.3 (CH =CH), 129.8
ium Iodide (13d); Typical Procedure
3
5
2
),
3
N ,N -Bis(3-acrylamidopropyl)pyridine-3,5-dicarboxamide (12d;
00 mg, 515 μmol) was treated with MeI (0.14 mL, 2.07 mmol) in
6
2
2
(
(
CH=CH ), 134.2 (C-3, C-5), 141.5 (C-4), 146.8 (C-2, C-6), 163.1
vinyl-C=O), 168.5 (aryl-C=O).
2
DMF (5 mL) at r.t. for 16 h. The mixture was evaporated and the
residue washed with Et O (10 mL) to give 13d (273 mg, 515 μmol,
2
quant) as an orange oil; R = 0.13 (CH Cl –MeOH, 5:1).
MS (ESI): m/z = 550.3 [M]+, 453.2, 391.2, 350.2 [M
−
f
2
2
+
H C=CHCONH(C H O) C H NH + H] , 344.2, 322.2, 297.6,
2
2
4
2
2
4
FT-IR (ATR): 3256 (m), 3061 (w), 2931 (w), 1651 (s), 1543 (s),
436 (m), 1407 (m), 1386 (m), 1285 (s), 1237 (s), 1096 (m), 984
2
H
80.1,
251.1
[M
−
H C=CHCONHC H
−
2
2
4
1
(
+
m), 959 (m), 806 (w), 664 (s), 606 cm– (m).
1
2
C=CHCONH(C
2
H
4
O)
2
C
2
H
4
] , 232.1, 218.1, 189.1, 166.1, 122.1,
98.1.
1
3
H NMR (500 MHz, DMSO-d ): δ = 1.76 (quint, J = 7.0 Hz, 4 H,
6
3
3
HRMS (ESI): m/z calcd for C26
550.2875 [M] .
H
40
N
5
O
8
: 550.2871; found:
2
′-H), 3.24 (q, J = 6.5 Hz, 4 H, 3′-H), 3.38 (q, J = 6.5 Hz, 4 H, 1′-
+
3
2
H), 4.45 (s, 3 H, CH ), 5.60 (dd, J = 10.2 Hz, J = 2.1 Hz, 2 H,
trans-CHH=CH), 6.09 (dd, J = 17.1 Hz, J = 2.1 Hz, 2 H, cis-
CHH=CH), 6.24 (dd, J = 17.1 Hz, J = 10.1 Hz, 2 H, CH=CH ),
8
3
3
2
3
,5-Bis({[1-(acryloyloxy)-3-(methacryloyloxy)propan-2-
3
3
2
yl]oxy}carbonyl)-1-methylpyridin-1-ium Iodide (13g)
3
3
.17 (t, J = 5.3 Hz, 2 H, 3′-NH), 9.14 (t, J = 5.4 Hz, 2 H, 1′-NH),
.25 (t, J = 1.4 Hz, 1 H, 4-H), 9.49 (d, J = 1.4 Hz, 2 H, 2-H, 6-H).
An analogous procedure as described for 13d was used. Solvent:
4
4
9
MeCN. Purified by washing with Et O; yield: 289 mg (80%); or-
2
13
ange oil; R = 0.56 (CH Cl –MeOH, 20:1, partial decomposition on
C NMR (125 MHz, DMSO-d ): δ = 28.7 (C-2′), 36.4 (C-1′), 37.7
f
2
2
6
silica gel).
FT-IR (ATR): 3323 (w), 2959 (w), 2556 (w), 1968 (w), 1716 (s),
634 (w), 1408 (m), 1294 (m), 1247 (s), 1156 (s), 1066 (m), 1032
(
C-3′), 48.5 (CH ), 125.1 (CH =CH), 131.7 (CH=CH ), 133.1 (C-3,
3
2
2
C-5), 140.6 (C-4), 146.9 (C-2, C-6), 161.0 (aryl-C=O), 164.7 (vi-
nyl-C=O).
1
–
1
+
(m), 955 (m), 810 (m), 742 (w), 658 (m), 634 (m), 538 cm (m).
MS (ESI +): m/z = 402.2 [M] ; (ESI −): m/z = 446.2, 436.2, 418.2,
–
4
00.2, 126.9 [I] .
1
H NMR (300 MHz, CDCl ): δ = 1.92–1.94 (m, 6 H, CH ), 4.24–
3
3
4
.65 (m, 8 H, OCH ), 4.84–4.85 (m, 3 H, CH N), 5.60–5.65 (m, 2
HRMS (ESI +): m/z calcd for C H N O : 402.2136; found:
2
3
2
0
28
5
4
+
–
H, cis-3′-H), 5.67–5.72 (m, 2 H, OCH), 5.86–5.93 (m, 2 H, cis-3′′-
H), 6.10–6.19 (m, 4 H, trans-3′-H, 2′′-H), 6.40–6.48 (m, 2 H, trans-
4
02.2137 [M] ; (ESI –): m/z calcd for I 126.9039; found: 126.9025
–
[I] .
3
′′-H), 9.22–9.24 (m, 1 H, 4-H), 9.83–9.95 (m, 2 H, 2-H, 6-H).
3
,5-Bis[(4-acrylamidobutyl)carbamoyl]-1-methylpyridin-1-ium
13
C NMR (125 MHz, CDCl ): δ = 18.4 (CH C), 51.2 (CH N), 62.1
3
3
3
Iodide (13e)
(OCH ), 68.3 (OCH), 126.4 (C-3′), 127.2 (C-2′′), 127.8 (C-3, C-5),
2
An analogous procedure as described for 13d was used. Solvent:
DMF. Purified by washing with Et O; yield: 287 mg (quant); orange
solid; mp 69 °C; R = 0.17 (CH Cl –MeOH, 5:1).
1
31.8 (C-3′′), 132.5 (C-2, C-6), 135.3 (C-2′), 135.5 (C-4), 159.8 (ar-
2
yl-C=O), 166.0 (C-1′′), 166.8 (C-1′).
f
2
2
+
MS (ESI): m/z = 574.2 [M] , 538.2, 392.1.
FT-IR (ATR): 3276 (m), 3067 (m), 2932 (m), 2867 (w), 1644 (s),
616 (s), 1535 (s), 1432 (m), 1405 (m), 1308 (m), 1290 (m), 1224
1
HRMS (ESI): m/z calcd for C H NO : 574.1919; found:
574.1916 [M] .
28
32
12
–
1
+
(m), 1192 (m), 978 (m), 960 (m), 805 (w), 674 cm (m).
1
H NMR (500 MHz, DMSO-d ): δ = 1.49–1.61 (m, 8 H, 2′-H, 3′-H),
6
Hydrogel Formation with Poly(1-glycidylpiperazine)
3
3
3
.18 (q, J = 6.4 Hz, 4 H, 4′-H), 3.36 (q, J = 6.4 Hz, 4 H, 1′-H),
Hydrogel Preparation: The following is a typical experiment lead-
ing to a hydrogel formed from PEPP 15 and cross-linker 8c with a
ratio of cross-linker acrylamide groups and polymer bound amino
groups of 0.8. The preparation of all the other gels was performed
3
2
4
.44 (s, 3 H, CH ), 5.58 (dd, J = 10.2 Hz, J = 2.2 Hz, 2 H, trans-
3
3
2
CHH=CH), 6.07 (dd, J = 17.1 Hz, J = 2.2 Hz,
2
H, cis-
3
3
CHH=CH), 6.22 (dd, J = 17.1 Hz, J = 10.2 Hz, 2 H, CH=CH ),
2
©
Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 1243–1253

1252
M. Mateescu et al.
PAPER
accordingly. For the preparation of a polymer solution of PEPP 15, Acknowledgment
the polymer was dissolved in H O–EtOH (1:1) to yield a total poly-
mer concentration of 50% (w/w). The pH value was adjusted to 9.0
using a 4 M aq NaOH–EtOH mixture (1:1). The resulting solution
2
Generous financial support by the Ministerium für Wissenschaft,
Forschung und Kunst des Landes Baden-Württemberg via the pro-
gram ‘Molekulare Bionik’ (SynElast), the Landesstiftung Baden-
Württemberg, the Max-Planck Society, the Forschungsfonds der
Universität Stuttgart, and the European Commission (ERASMUS
fellowship for H. Messenger) is gratefully acknowledged.
was diluted with H O–EtOH (1:1) until a polymer concentration of
2
20% (w/w) was reached. For the preparation of the cross-linker so-
lution of 6c, the cross-linker 2c (62 mg, 345 μmol of acrylamide
groups, 0.8 equiv) was dissolved in H O–EtOH (442 mg, 1:1). To
2
this solution, 465 mg of the above prepared polymer solution was
added, containing polymer 15 (93 mg, 432 μmol of amino groups,
Supporting Information for this article is available online at
1
equiv). The total solid content of the mixture was 16% (w/w). The
http://www.thieme-connect.com/ejournals/toc/synthesis.SnoIufproi
m
tgioSrantnugIifoop
r
itmnatr
mixture was left under gentle agitation at r.t. After curing the gel for
3
×
days, the gel was washed with H O–EtOH (1:1, 3 × mL), H O (3
2 2
mL), each washing step taking 24 h. Finally, the gel was swollen
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the mass of the swollen hydrogel and the dry mass of the hydrogel.
The masses of the swollen hydrogels were determined by weighing
the hydrogels, which were obtained following the preparation pro-
cedure described later. The dry masses of the hydrogels were ob-
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