Gelation ability of novel oxamide-based derivatives bearing a stilbene as a photo-responsive unit

Article abstract of DOI:10.1002/ejoc.200500535

Oxamide-based derivatives containing one or two oxamide moieties coupled to the 4- or 4,4′-positions of cis- and transstilbene have been synthesised. In order to modulate the solvent gelation tendencies, a series of derivatives with modified terminal functions were prepared from them. The transstilbene dioxamide-based derivatives were found to be sparingly soluble or insoluble in water and in organic solvents, whereas the trans-stilbene monooxamide-based derivatives are soluble in most organic solvents. Incorporation of a C ₁₂ alkyl chain in the structure of the latter compounds results in a decreased solubility but in an increased gelation tendency of the substances. The ethyl ester oxamide-based derivative trans-3a and the amino acid oxamide-based derivative trans-3e act as efficient gelators of various organic solvents. In contrast, cis-3a shows a poor gelation ability or none at all, owing to its good solubility. Considering the difference in gelation abilities of the compounds trans-3a and cis-3a and the photo-responsive conformational changes of the stilbene part of the molecule, a controlled gelation by light was achieved. FT-IR and ¹H NMR spectroscopic measurements support the view that hydrogen bonding between the oxamide fragments plays an important role in gel formation. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.

Full text of DOI:10.1002/ejoc.200500535

FULL PAPER
DOI: 10.1002/ejoc.200500535
Gelation Ability of Novel Oxamide-Based Derivatives Bearing a Stilbene as a
Photo-Responsive Unit
[a]
[b]
[a]
[b]
ˇ
ˇ
´
´
´
Snezana Miljanic,* Leo Frkanec, Zlatko Meic, and Mladen Zinic
Keywords: Gels / Self-assembly / Oxamides / Stilbenes
Oxamide-based derivatives containing one or two oxamide
moieties coupled to the 4- or 4,4Ј-positions of cis- and trans-
stilbene have been synthesised. In order to modulate the sol-
vent gelation tendencies, a series of derivatives with modi-
fied terminal functions were prepared from them. The trans-
stilbene dioxamide-based derivatives were found to be spar-
ingly soluble or insoluble in water and in organic solvents,
whereas the trans-stilbene monooxamide-based derivatives
are soluble in most organic solvents. Incorporation of a C12
alkyl chain in the structure of the latter compounds results in
a decreased solubility but in an increased gelation tendency
of the substances. The ethyl ester oxamide-based derivative
trans-3a and the amino acid oxamide-based derivative trans-
3e act as efficient gelators of various organic solvents. In con-
trast, cis-3a shows a poor gelation ability or none at all, ow-
ing to its good solubility. Considering the difference in gela-
tion abilities of the compounds trans-3a and cis-3a and the
photo-responsive conformational changes of the stilbene part
of the molecule, a controlled gelation by light was achieved.
1
FT-IR and H NMR spectroscopic measurements support the
view that hydrogen bonding between the oxamide fragments
plays an important role in gel formation.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Owing to its unique photochemistry[16] and molecular
structure, which is similar to that of azobenzene, stilbene is
a molecule of choice for the synthesis of gelators that can
form gels that are sensitive to light.
Introduction
Some low-molecular-weight organic molecules are cap-
able of forming thermoreversible and stable gels with
water[1] and with various organic solvents.[2] Owing to a
successful design and synthesis of these small gelling mole-
cules, the diversity of gelators has been steadily increasing.
The preparation of substances with a gelation ability that
can be affected by external stimuli such as heat, light, mag-
netic or electric field, pH, or by chemical reactions seems
to be particularly attractive.[3] Apart from temperature,
which is the most commonly used tool for stimulating a
gelation process, the development of gelling systems con-
trolled by light has attracted much attention. A promising
approach towards such smart gels is the introduction of a
photo-responsive unit into the structure of the gelator
molecule. Photoinduced changes in the structure[4–8] or ge-
ometry[9–15] of the integrated photochromic moiety result
in gel formation and dissolution, potentially reversibly. The
most frequently exploited process involving interconversion
of two different molecule forms is photoisomerisation.[9–15]
Among gelators capable of photoreversible gelation based
on the difference in the gelation ability of cis- and trans-
isomers, the majority incorporate an azobenzene unit.[9–12]
In light of the excellent gelation ability of molecules con-
taining oxamide fragments,[17–19] and considering the well-
known phenomenon of stilbene photoisomerisation[16] as
well as the gelation tendency of stilbenoid com-
pounds,[13,20,21] in this work we report the synthesis of new
stilbene oxamide-based gelators. A variety of oxamide com-
pounds bearing one or two oxamide moieties at the para
position of cis- and trans-stilbenes has been prepared and
the gelation behaviour of the synthesised substances tested
in water and in various organic solvents. The photoinduced
gelation of ethanol by the selected pair of isomers is also
investigated.
Results and Discussion
Rational Design
Compounds containing an oxamide function as a self-
complementary binding group have been found to form
thermoreversible and stable gels with water and with vari-
ous organic solvents.[17–19] It has also been demonstrated
that intermolecular lipophilic interactions between alkyl
substituents and intermolecular hydrogen bonding involv-
ing oxamide fragments stabilize the networks of a hydro-
and organogel, respectively. In addition, an irreversible
photoinduced gelation system on the principle of photo-
[a] Laboratory of Analytical Chemistry, Faculty of Science, Uni-
versity of Zagreb,
Horvatovac 102 A, 10000 Zagreb, Croatia
Fax: +385-1-460-6181
E-mail: miljanic@rudjer.irb.hr
[b] Laboratory of Supramolecular and Nucleoside Chemistry,
Rueer Bosˇkovic´ Institute,
Bijenicˇka 54, 10002 Zagreb, Croatia
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S. Miljanic´, L. Frkanec, Z. Meic´, M. Zinic´
FULL PAPER
Scheme 1. Structures of stilbene oxamide-based compounds.
chemical isomerisation of non-gelling maleic acid amide to Synthesis
gelling fumaric acid amide has been prepared,[14] thereby
indicating that incorporation of a photo-responsive unit in
the structure of an oxamide-based gelator could lead to tion of stilbenes[24] as the availability of the starting materi-
light-sensitive gels. als and simple and mild reaction conditions override draw-
The Wittig reaction is a popular method for the prepara-
In light of the well-known cis/trans photoisomerisation backs such as separation of the product from tri-
of the stilbene molecule,[16] the stilbene skeleton was chosen phenylphosphane oxide and the formation of a mixture of
to act as a photo-responsive unit, and stilbenoid com- cis and trans isomers. Consequently, the arylmethyl bromide
pounds bearing one or two oxamide moieties at the para was treated with triphenylphosphane to give the arylmethyl-
position of the stilbene molecule were synthesised triphosphonium bromide.[25] This phosphonium salt was
(Scheme 1). In order to modulate the solvent gelation tend- deprotonated to the corresponding phosphorus ylide and
encies, a series of derivatives with modified terminal func- then treated with an aryl aldehyde. One or both of the aryl
tions were prepared. Based on the strong stabilisation of gel reactants can be substituted by a nitro group depending on
aggregates observed in gelators possessing terminal sodium the final mono- or dioxamide-based compound required.
carboxylate groups,[22] the preparation of sodium salts of This preparation of nitrostilbenes was followed by their re-
the corresponding carboxylic acids was carried out, and duction to aminostilbenes by treatment with zinc and am-
owing to the good gelation ability of monoalkyloxamide monium chloride[26] or stannous chloride dehydrate.[27] The
amphiphiles,[23] some derivatives containing a long lipo- oxamide ethyl ester derivatives were obtained by reaction
philic chain, (C12) either bound directly to an oxamide frag- of the ethyl oxalyl chloride with the amino group of the
ment or as an oxyalkyl substituent on a stilbene unit, were stilbenes (Scheme 2). The resulting esters were saponified
also synthesised. Due to the fact that the stereogenic centre with sodium hydroxide to yield the neutral carboxylic forms
in the oxamide-based gelator molecule contributes to its ge- after hydrolysis. The sodium salts were prepared by treat-
lation tendency,[17–19] an amino acid (-leucine) was in- ment of the carboxylic compounds with sodium methoxide,
cluded in the structure of some derivatives.
whereas dissolution of the ethyl ester derivatives in satu-
With regard to the difference in solubility of the cis and rated ammonia/methanol solution gave the amides. Acyl
trans isomers, it was assumed that the trans derivative would chlorides were prepared in order to couple a C12 alkyl chain
act as a better gelator than the corresponding cis isomer. or -leucine to the oxamide part of the molecule.[28]
For this reason, only the preparation of trans compounds
with significant gelation properties was followed by synthe- amide derivatives, 4Ј-dodecyloxystilbene oxamide com-
sis of their cis isomers. pounds were obtained in the trans as well as in the cis form,
Apart from the trans isomers of stilbene mono- and diox-
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Gelation Ability of Stilbene-Substituted Oxamides
FULL PAPER
Scheme 2. Synthesis of stilbene oxamide-based derivatives.
except for the amino acid derivative; in this case only a trans bonyl group that should have reacted with the amino func-
isomer was synthesised. The reaction via the acyl chloride, tion of the amino acid.
which was prepared from the starting cis carboxylic acid
derivative, yielded the trans product, whereas the condensa-
tion reaction with dicyclohexylcarbodiimide (DCC) did not
Gelation Studies
give the expected compound.[29] The synthesis of the cis-
isomer failed, most likely due to the reaction conditions.
The gelation behaviour of the prepared compounds was
We assume that traces of chlorine formed during the prepa- investigated in water and in various organic solvents. The
ration of the acyl chloride induce isomerisation of the cis ethyl ester derivative trans-1a and the carboxylic derivative
isomers into the energetically more favourable trans iso- trans-1b of trans-stilbene monooxamide compounds were
mers. Upon peptide condensation, however, an adjacent found to be soluble in almost all of the organic solvents
carbonyl group could lower the reactivity of the target car- tested (Table 1). Introduction of a metal ion into the struc-
Eur. J. Org. Chem. 2006, 1323–1334
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S. Miljanic´, L. Frkanec, Z. Meic´, M. Zinic´
FULL PAPER
ture of trans-1c resulted in a poor solubility of this com- Table 2. Gelation properties of trans-stilbene dioxamide derivatives
expressed as minimal gel concentration (10–3 ) of tested com-
pound. The long-chain derivative trans-1d is insoluble in
alcohols and precipitates from other organic solvents. In the
pound.[a]
Solvent trans-2a
trans-2b
trans-2c
trans-2d
trans-2e
series of derivatives investigated only compound trans-1e,
which bears an amino acid, showed a significant gelation
Water
I
I
I
I
I
tendency. It is capable of forming a gel with methanol − a MeOH
I
I
S
I
S
I
I
I
P
I
I
SS
I
I
I
I
I
I
I
I
SS
I
I
I
I
I
I
I
I
SS
I
I
I
I
I
I
I
I
S
I
P
I
I
GP
6.1
EtOH
polar solvent − as well as with toluene and tetralin, which
DMSO
are non-polar solvents. We assume that in methanol the gel-
EtOAc
ator molecules form a gel network due to aromatic π–π
THF
stacking of trans-stilbene units and lipophilic interactions
between the amino acid parts of the molecules, whereas hy-
drogen bonding involving the oxamide fragments leads to
self-assembly of the molecules in non-polar solvents.
Weaker intermolecular interactions between the molecules
of the symmetric derivative trans-1f, which contains two
trans-stilbene units, cause the formation of a gelatinous pre-
cipitate. None of the compounds is soluble in water.
Acetone
CH2Cl2
Toluene
Tetralin
[a] Abbreviations: S, soluble; SS, slightly soluble; I, insoluble; P,
precipitate; GP, gelatinous precipitate.
A comparison of the gelation properties of the 4Ј-dode-
cyloxy-trans-stilbene oxamide derivatives (Table 3) and
those of the corresponding compounds without an alkyloxy
substituent on the stilbene part of the molecule (Table 1)
indicated that the incorporation of a C12 chain into the
structure simultaneously caused a decrease in solubility and
an increase in the gelation tendency of some substances.
The ethyl ester derivative trans-3a and amino acid derivative
trans-3e act as efficient gelators of most of the tested or-
ganic solvents. They exhibit an excellent gelation tendency
towards ethanol, which they can gelatinize at concentra-
tions as low as 0.21 wt.-% (3.4 m) and 0.45 wt.-%
(6.2 m), respectively. In general, they form thermorevers-
ible gels with a turbid appearance. The gels formed from
trans-3a are sensitive to mechanical agitation and the gel-
ator tends to crystallise after about 24 hours. On the other
hand, the gels of trans-3e are stable for weeks, thus indicat-
ing that a stereogenic centre in the gelator molecule contrib-
utes to the formation of a three-dimensional gel network.
The cis isomer of the ethyl ester derivative cis-3a cannot
Table 1. Gelation properties of trans-stilbene monooxamide deriva-
tives expressed as minimal gel concentration (10–3 ) of tested com-
pound.[a]
Solvent trans-1a trans-1b trans-1c trans-1d trans-1e trans-1f
Water
I
I
I
I
I
I
MeOH
EtOH
DMSO
EtOAc
THF
Acetone
CH2Cl2
Toluene
Tetralin
S
S
S
S
S
S
S
S
S
S
P
S
S
S
S
S
P
S
I
I
P
I
I
I
SS
I
I
I
33.4
GP
S
S
S
I
I
S
GP
P
S
P
P
P
I
S
S
I
GP
GP
21.8
P
P
110.3
123.0
I
[a] Abbreviations: S, soluble; SS, slightly soluble; I, insoluble; P,
precipitate; GP, gelatinous precipitate.
The trans-stilbene dioxamide compounds are sparingly
soluble or insoluble in water and in the organic solvents gelatinize most of the tested solvents owing to its good solu-
tested (Table 2). Apart from the ethyl ester derivative trans- bility. Although it is capable of forming a gel with ethanol
2a, which dissolves in dimethyl sulfoxide and tetra- (5.09 wt.-%; 82.4 m), its gelation ability is greatly inferior
hydrofuran, the leucine derivative trans-2e shows a gelation to that of the corresponding trans isomer.
tendency towards aromatic non-polar solvents such as
Intermolecular lipophilic interactions between long alkyl
tetralin.
chains, aromatic π–π stacking of the stilbene moieties and
Table 3. Gelation properties of 4Ј-dodecyloxy-trans-stilbene and 4Ј-dodecyloxy-cis-stilbene oxamide derivatives expressed as minimal gel
concentration (10–3 ) of tested compound.[a]
Solvent
trans-3a
trans-3b
trans-3c
trans-3d
trans-3e
trans-3f
cis-3a
cis-3b
cis-3c
cis-3d
Water
MeOH
EtOH
DMSO
EtOAc
THF
Acetone
CH2Cl2
Toluene
Tetralin
I
I
I
I
I
I
I
I
I
I
I
GP
6.2
19.0
19.4
76.9
11.4
57.0
18.8
41.9
I
I
I
I
I
S
I
P
I
I
I
S
I
I
GP
S
I
P
I
I
I
S
I
10.9
3.4
S
46.9
6.4
41.2
137.6
45.3
50.6
GP
GP
19.5
I
39.4
I
P
82.4
S
S
S
S
S
S
S
GP
GP
P
S
S
P
S
48.3
53.8
121.8
I
SS
I
I
I
SS
38.7
I
GP
I
I
I
GP
122.1
I
P
I
I
I
I
GP
21.5
42.9
[a] Abbreviations: S, soluble; SS, slightly soluble; I, insoluble; P, precipitate; GP, gelatinous precipitate.
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Gelation Ability of Stilbene-Substituted Oxamides
FULL PAPER
hydrogen bonding involving oxamide fragments are the
driving forces for the gelation of organic solvents by the
trans isomer. We assume that the bent geometry of the cis-
stilbene unit hinders an efficient unidirectional self-as-
sembly of the cis isomers in the gel network, thereby in-
creasing the solubility of the compounds.[10]
Photoinduced Gelation
In light of the different gelation abilities of trans-3a and
cis-3a as well as the presence of the photo-isomerizing stil-
bene moiety in the molecule, gel formation stimulated by
light was investigated.[30] Upon irradiation with a high-pres-
sure mercury lamp (250 nm Ͻ λ Ͻ 520 nm) at room tem-
perature, the clear solution of cis-3a in ethanol
(2.0×10–2 ) turned into an opaque gel. Prolonged irradia-
tion of the gel does not affect its structure; however, the
gelator crystallised from the system with time. Heating a gel
or a crystalline precipitate and cooling the obtained solu-
tion resulted in repeated gel formation (Scheme 3).
FT-Raman spectra indicated that a fast conversion of the
cis molecules into trans molecules occurs during irradiation,
leading to self-assembly of the trans molecules in the gel
network. The double bond stretching band at 1628 cm–1
and the aromatic ring vibration band at 1607 cm–1 of cis-3a
in solution are shifted to 1632 cm–1 and 1603 cm–1, respec-
tively (Figure 1). The position of the vibrational bands in
the spectrum after irradiation for one hour confirmed the
organisation of the formed trans molecules in the gel struc-
ture (Table 4).
Figure 1. Raman spectra before and after 60-min irradiation of a
cis-3a/ethanol solution.
Table 4. Raman bands of C=C double bond stretching and aro-
matic ring vibration in the spectra of trans-3a and cis-3a in the
solid state, solution and gel.
Sample
ν [cm–1]
˜
C=C
ArC–C
trans-3a[a]
1632
1621
1636
1628
1633
1622
1601
1607
1603
1607
1603
1607
cis-3a[a]
trans-3a/ethanol[b]
cis-3a/ethanol[b]
trans-3a/ethanol[c]
cis-3a/ethanol[c]
[a] Solid state. [b] Solution [c(trans-3a) = 0.020 , 75 °C; c(cis-3a)
= 0.020 ]. [c] Gel [c(trans-3a) = 0.020 ; c(cis-3a) = 0.100 ].
Investigation of Intermolecular Interactions by FT-IR and
1H NMR Spectroscopy
Although trans-3e acts as a versatile gelator, synthesis of
its cis isomer was not possible, thus excluding the possibility
of designing a similar photo-induced gelation system (see
above).
Apart from the results of photoisomerisation and simul-
taneous gelation with compound 3a, its spectroscopic data
Scheme 3. Photoirreversible and thermoreversible gelation of ethanol by oxamide-based stilbene compounds cis-3a and trans-3a.
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S. Miljanic´, L. Frkanec, Z. Meic´, M. Zinic´
FULL PAPER
implied that intermolecular oxamide–oxamide hydrogen with increasing temperature (Figure 3). We assume that the
bonding plays an important role in the formation of the NH protons participate in hydrogen bonding in gel aggre-
trans-3a/ethanol gel.[30] The same interactions are also ex- gates and are therefore deshielded, whereas they become
pected to be present in gels of the amino acid oxamide- more shielded upon gel dissolution. In comparison to the
based gelator trans-3e.
non-linear temperature dependence of the NH proton shifts
In the FT-IR spectrum of trans-3e/toluene gel observed for the trans-3e/toluene gel, the linear relationship
(2.0×10–2 ), N–H and amide I (C=O) vibrational bands of the temperature-induced shifts of trans-3e NH protons
are observed at 3285 and 1661 cm–1, respectively, and at- in DMSO gel allowed evaluation of the temperature coeffi-
tributed to the hydrogen-bonded oxamide units. Upon in- cients (∆δNH/∆T) of 5.9×10–3 and 7.8×10–3 ppmK–1.
creasing the temperature (25–70 °C) the gel dissolved, re- Moreover, higher temperature-induced shifts were obtained
sulting in a reduction in intensity of these bands and the in the more polar solvent, which is capable of competing
appearance of new bands at higher wavenumbers (Fig- with intermolecular oxamide hydrogen bonding. Although
ure 2). In the FT-IR spectrum taken at 70 °C the band with a downfield shift of the aromatic protons, indicating inter-
maxima at 3364 and 3385 cm–1 is assigned to the free NH molecular aromatic stacking interactions, was expected, a
groups, close to the leucine and stilbene fragments of the characteristic, temperature-dependent trend in the spectra
molecule, whereas the band at 1691 cm–1 is assigned to the of both gels was not obtained.
non-hydrogen-bonded carbonyl functionality. These results
indicate that intermolecular hydrogen bonding between the
oxamide parts of the gelator molecule plays a dominant role
in the stabilisation of the organogel network.
Figure 3. Temperature-induced chemical-shift changes of NH pro-
tons in the 1H NMR spectra of a) trans-3e/toluene gel and b) trans-
3e/DMSO gel.
Aside from the shifts of the proton signals, a temperature
increase caused an increase in peak intensity, implying a
collapse of the gel network, and, finally, dissolution of the
Figure 2. Temperature-dependent (25–70 °C) FT-IR spectra of
trans-3e in toluene: (a) NH region; (b) carbonyl region.
The same argument was supported by the temperature- fibrous aggregates. Using the internal standard 1,1,2,2-tet-
dependent 1H NMR spectroscopic data. The amide NH rachloroethane, the concentration of the dissolved gelator,
protons, stilbene-NH and leucine-NH, are shifted upfield cd, was determined at each temperature, and the gelation
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Gelation Ability of Stilbene-Substituted Oxamides
FULL PAPER
equilibrium constant [K]gel, defined as Kgel = 1/[cd], was indicating that each gel network disintegrates into aggre-
evaluated.[17] From the plot of lnKgel vs. 1/T the gelation gates of similar size. For the trans-3e/toluene and trans-3e/
enthalpy (Figure 4), ∆Hgel, was estimated according to the DMSO gels, the ∆Hgel values obtained were –61 and
equation lnKgel = (–∆Hgel/R)1/T + C. A linear dependence –57 kJmol–1, respectively.
of lnKgel on 1/T (r2 Ͼ 0.98) was obtained for both gels,
TEM Investigation
TEM images were obtained for ethanol gels formed from
trans-3a and trans-3e (Figure 5). Both gelators form a typi-
cal three-dimensional network entangled by the self-as-
sembled gel fibres. Molecules of trans-3a form rod-like
fibres with a diameter of 150 nm, whereas trans-3e mole-
cules aggregate in slightly twisted ribbons having a signifi-
cantly larger diameter in the 1500–3000 nm range. The pres-
ence of an amino acid moiety in the structure of the latter
compound implies that intermolecular interactions between
chiral units in molecules lead to the formation of large
microfibres.
Conclusions
Figure 4. Linear dependence of lnKgel vs. 1/T for trans-3e/toluene
and trans-3e/DMSO gels used for the calculation of ∆Hgel
.
The present study has demonstrated that the coupling of
a photo-responsive unit to the fragment of a gelator mole-
cule responsible for solvent gelation leads to the prepara-
tion of new gelators capable of forming smart gels. As gela-
tion ability is not fully predictable by molecular design, a
variety of oxamide-based derivatives bearing a stilbene as a
photochromic moiety have been prepared and their gelation
properties investigated. Depending only on the terminal
functional group of the oxamide fragment, the substance
either dissolved, precipitated or formed a gel with a solvent,
thus indicating that, besides the self-complementary bind-
ing groups, the substituent plays an important role in the
gel network formation. Introduction of an alkyl chain in
the structure of the stilbene part of the molecule results in
the preparation of amphiphilic molecules capable of form-
ing stable self-assemblies. Owing to the different gelation
abilities of stilbene isomers of long-chain oxamide-based
ethyl ester derivatives, gelation controlled by light as an ex-
ternal stimulus has been achieved.
Experimental Section
General Information: Melting points were determined on a Kofler
stage. 1H and 13C NMR spectra were recorded at 300 and 75 MHz,
respectively, with a Bruker Avance 300 spectrometer using tet-
ramethylsilane as internal standard. FT-IR spectra were recorded
with a Bruker Equinox 55 interferometer. A Micromass UK mass
spectrometer (model Q-TOF Micro) was used for recording mass
spectra. Thin-layer chromatography was performed on Merck 60
F254 coated silica plates, preparative thin-layer chromatography on
silica gel Merck 60 coated glass plates, and preparative column
chromatography using Merck 60 silica gel (0.063–0.200 mm). All
chemicals were commercially available and were used without fur-
ther purification. Solvents were purified and dried according to
standard procedures.
Figure 5. TEM images of ethanol gels of: a) trans-3a (0.02 , mag-
nification 25000×), b) trans-3e (0.02 , magnification 7500×), c)
trans-3e (0.02 , magnification 5000×).
(4-Nitrobenzyl)triphenylphosphonium Bromide: A mixture of 4-ni-
trobenzyl bromide (5.401 g, 25.0 mmol) and triphenylphosphane
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FULL PAPER
(7.200 g, 27.5 mmol) in toluene (25 mL) was stirred at room tem-
perature for 16 h.[25] The solvent was then removed by filtration to
give a light-yellow solid (9.924 g, 83%). 1H NMR (300 MHz,
CDCl3, 25 °C): δ = 7.87–7.73 (m, 11 H, Ar-CH), 7.59 (m, 6 H, Ar-
CH), 7.48 (d, J = 8.81 Hz, 2 H, C2-H, C6-H), 6.03 (d, J = 15.81 Hz,
2 H, CH2P) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ = 147.16
= 7.15 Hz, 3 H, CH3) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ
= 156.25 (COOEt), 148.98 (Ar-CONH), 132.41 (C4), 130.87 (C1Ј),
129.97 (C1), 123.97 (C3Ј, C5Ј), 122.98 (C4Ј), 122.92 (C2, C6),
122.54 (C2Ј, C6Ј), 121.76 (=CH), 115.22 (C3, C5), 59.05 (CH2),
9.28 (CH ) ppm. FT-IR (KBr) ν = 3374 (νN–H), 2929 (νC–H),
˜
3
1718 (νC=O, ester), 1707 (νC=O, amide I), 1587 (ArC–C), 1539
(C4), 135.69 (C1), 134.90, 134.86 (P-C1), 134.43, 134.30, 132.80, (νsN–C=O, amide II), 1509 (δNH), 1310 (νC–O), 1173 (δipArC–
132.73, 130.05, 129.88, 123.00, 122.96, 117.68, 116.54 (Ar-C), 29.78
H), 971 (δoop=CH), 823 (δoopArC–H). HRMS (ES+): calcd. for
(CH P) ppm. FT-IR (KBr) ν = 2919 cm–1, 2852, 2771 (νC–H), C18H18NO3 [MH+] 296.1287; found 296.1284.
˜
2
1597 (νasNO2), 1519 (νN=O), 1347 (νsNO2), 1111 (δipAr-CH), 865
(δoopAr-CH).
trans-Stilbene Derivative trans-1b: Compound trans-1a (0.670 g,
2.3 mmol) was suspended in a mixture of ethanol (55 mL) and
water (30 mL), 2  NaOH was added (11.50 mL, 23.0 mmol) and
the reaction mixture was refluxed for 20 h. Stirring was then
stopped and the ethanol was removed under reduced pressure. A
2  HCl solution was added to this suspension until pH 2 and the
product was then extracted with ethyl acetate. The organic phase
was separated, dried (Na2SO4) and the solvents evaporated to dry-
ness. Addition of petroleum ether to a solution of this crude residue
in dichloromethane gave trans-1b as a light-brown precipitate
(0.510 g, 83%; m.p. 163–165 °C). 1H NMR (300 MHz, DMSO,
25 °C): δ = 10.78 (s, 1 H, NH), 7.80 (d, J = 8.71 Hz, 2 H, C3-H,
C5-H), 7.59 (d, J = 8.80 Hz, 4 H, C2-H, C6-H, C2Ј-H, C6Ј-H),
7.38 (t, J = 7.49 Hz, 2 H, C3Ј-H, C5Ј-H), 7.26 (t, J = 7.29 Hz, 1
H, C4Ј-H), 7.22 (s, 2 H, =CH) ppm. 13C NMR (75 MHz, DMSO,
25 °C): δ = 162.56 (COOH), 157.29 (Ar-CONH), 137.63 (C1Ј),
137.56 (C1), 133.79 (C4), 129.17 (C3Ј, C5Ј), 128.31 (C4Ј), 128.21
(=CH), 127.99 (=CH), 127.34 (C2, C6), 126.84 (C2Ј, C6Ј), 120.84
4-Nitro-trans-stilbene: (4-Nitrobenzyl)triphenylphosphonium bro-
mide (4.783 g, 10.0 mmol), benzaldehyde (1.01 mL, 10.0 mmol) and
a few crystals of dibenzo-18-crown-6 were added to a suspension
of potassium carbonate (1.589 g, 11.5 mmol) in dichloromethane
(80 mL). The red reaction mixture was stirred under argon at room
temperature until it turned yellow. After 48 h stirring was stopped,
the precipitate was removed by filtration and the filtrate was evapo-
rated to dryness. The obtained solid was purified by column
chromatography (silica gel; CH2Cl2) resulting in a mixture of iso-
mers of 4-nitrostilbene. The isomer mixture (1.779 g, 79%) was dis-
solved in toluene (50 mL) and a catalytic amount of iodine was
added.[31] The reaction mixture was stirred at room temperature for
24 h. The solvent was evaporated to a yellow solid. Recrystalli-
sation from hot ethanol gave yellow crystals of the trans isomer
(1.654 g, 93%; m.p. 158–159 °C).[32] 1H NMR (300 MHz, CDCl3,
25 °C): δ = 8.21 (d, J = 8.85 Hz, 2 H, C3-H, C5-H), 7.62 (d, J =
8.79 Hz, 2 H, C2-H, C6-H), 7.55 (d, J = 7.00 Hz, 2 H, C2Ј-H, C6Ј-
H), 7.40 (t, J = 7.23 Hz, 2 H, C3Ј-H, C5Ј-H), 7.36–7.32 (m, 1 H,
C4Ј-H), 7.20 (dd, 2 H, =CH) ppm. 13C NMR (75 MHz, CDCl3,
25 °C): δ = 146.60 (C4), 143.70 (C1), 136.02 (C1Ј), 133.16 (C4Ј),
128.76, 128.71 (C3Ј, C5Ј), 126.88 (C2, C6), 126.71 (C2Ј, C6Ј),
(C3, C5) ppm. FT-IR (KBr) ν = 3309 (νN–H), 3250 (νCOOH),
˜
2931 (νC–H), 1764 (νC=O, carboxylic acid), 1685 (νC=O, amide
I), 1541 (νsN–C=O, amide II), 1509 (δNH), 1352 (νC–N), 1315
(νC–O), 965 (δoop=CH), 823 (δoopArC–H).
trans-Stilbene Derivative trans-1c: Compound trans-1b (0.045 g,
0.17 mmol) and a solution of sodium methoxide in methanol
[7.1 mL, 0.17 mmol, c(Na) = 0.0236 ] were stirred together for
30 min. The solvent was then evaporated to give trans-1c as a yel-
low solid (0.048 g, 99%). 1H NMR (300 MHz, DMSO, 25 °C): δ =
10.26 (s, 1 H, NH), 7.80 (d, J = 8.71 Hz, 2 H, C3-H, C5-H), 7.55
(m, 4 H, C2-H, C6-H, C2Ј-H, C6Ј-H), 7.37 (m, 2 H, C3Ј-H, C5Ј-
H), 7.26 (m, 1 H, C4Ј-H), 7.18 (m, 2 H, =CH) ppm. 13C NMR
(75 MHz, DMSO, 25 °C): δ = 165.26 (COONa), 162.84 (Ar-
CONH), 138.88 (C1Ј), 137.74 (C1), 132.39 (C4), 129.15 (C3Ј, C5Ј),
129.05 (C4Ј), 128.10 (=CH), 127.28 (=CH), 126.72 (C2, C6), 126.17
126.12 (=CH), 124.00 (C3, C5) ppm. FT-IR (KBr) ν = 1631
(νArC–C), 1589 (νasNO2), 1509 (νN=O), 1344 (νsNO2), 1189, 1109
(δipArC–H), 849 (δoopArC–H), 695 (δNO2).
˜
trans-Stilbene Derivative trans-1a: A solution of ammonium chlo-
ride (0.440 g, 8.2 mmol) in water (2 mL) was added to a solution
of 4-nitro-trans-stilbene (1.000 g, 4.4 mmol) in acetone (11 mL).[26]
After heating the mixture to boiling point the oil bath was removed
and zinc (0.880 g, 13.5 mmol) was added in small portions. After
the reaction subsided, an additional amount of zinc (0.440 g,
6.7 mmol) was added and the reaction mixture refluxed for two
hours. The precipitate was filtered off while hot and the filtrate was
evaporated until an aqueous solution remained. This was basified
with 2  NaOH until pH 10, followed by extraction of the suspen-
sion with dichloromethane. The organic phase was separated, dried
(Na2SO4) and the solvent was evaporated to give an orange solid
of 4-amino-trans-stilbene (0.653 g, 76%). The crude amine (0.615 g,
3.1 mmol) was dissolved in dichloromethane (30 mL) and triethyl-
amine (0.48 mL, 3.4 mmol) was added.[28] The mixture was cooled
(–10 °C) under argon. Ethyl oxalyl chloride (0.35 mL, 3.1 mmol)
was added to the cooled mixture and stirring was continued at
–10 °C and at room temperature for 30 min and for 20 h, respec-
tively. The reaction mixture was washed with 2  HCl, 2  NaOH
and with water. The organic phase was separated, dried (Na2SO4)
and the solvent was evaporated to dryness. Addition of petroleum
ether to a solution of the crude residue in dichloromethane gave
trans-1a as a light brown precipitate (0.701 g, 77%; m.p. 158–
160 °C). 1H NMR (300 MHz, CDCl3, 25 °C): δ = 8.91 (s, 1 H,
NH), 7.65 (d, J = 8.69 Hz, 2 H, C3-H, C5-H), 7.53 (d, J = 8.67 Hz,
(C2Ј, C6Ј), 119.73 (C3, C5) ppm. FT-IR (KBr) ν = 3334 (νN–H),
˜
1671 (νC=O, amide I), 1644 (νC=O), 1532 (νsN–C=O, amide II),
1508 (δNH), 1396 (νC–N).
trans-Stilbene Derivative trans-1d: Compound trans-1b (0.229 g,
0.9 mmol) was suspended in dichloromethane (25 mL) and thionyl
chloride (0.63 mL, 9.0 mmol) and a drop of dimethylformamide
were added. The reaction mixture was stirred at 40–50 °C for two
hours. Evaporation of the solvent resulted in a yellow solid of the
acyl chloride. This was suspended in dichloromethane (10 mL) and
cooled (–10 °C) under argon.[28] Dodecylamine (0.159 g, 0.9 mmol)
was dissolved in dichloromethane (10 mL) and triethylamine
(0.13 mL, 1.0 mmol) was added. This solution of amines was added
to the cooled suspension of the acyl chloride. The reaction mixture
was stirred for 30 min at –10 °C and then for 20 h at room tempera-
ture. Filtration resulted in isolation of trans-1d as a light-yellow
1
precipitate (0.257 g, 69%; m.p. 210–211 °C). H NMR (300 MHz,
DMSO, 80 °C): δ = 10.46 (s, 1 H, ar-NH), 8.74 (s, 1 H, dodecyl-
2 H, C2Ј-H, C6Ј-H), 7.51 (d, J = 7.10 Hz, 2 H, C2-H, C6-H), 7.36 NH), 7.82 (d, J = 7.50 Hz, 2 H, C3-H, C5-H), 7.58 (d, J = 7.36 Hz,
(t, J = 7.44 Hz, 2 H, C3Ј-H, C5Ј-H), 7.26 (t, J = 7.27 Hz, 1 H, C4Ј- 4 H, C2-H, C6-H, C2Ј-H, C6Ј-H), 7.37 (t, J = 7.03 Hz, 2 H, C3Ј-
H), 7.08 (s, 2 H, =CH), 4.43 (q, J = 7.15 Hz, 2 H, CH2), 1.44 (t, J H, C5Ј-H), 7.26 (t, J = 7.81 Hz, 1 H, C4Ј-H), 7.19 (s, 2 H, =CH),
1330
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© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 1323–1334

Gelation Ability of Stilbene-Substituted Oxamides
FULL PAPER
1.54–1.53 (m, 2 H, NHCH2), 1.26 [s, 20 H, (CH2)10], 0.86 (t, 3 H,
CH3) ppm. Owing to the poor solubility of trans-1d its 13C NMR
4-nitrobenzaldehyde (2.331 g, 15.4 mmol) and a catalytic amount
of dibenzo-18-crown-6 were added. The orange reaction mixture
was stirred at room temperature for four days until it turned yellow.
The precipitate was filtered off and the filtrate was evaporated to
spectrum was not recorded. FT-IR (KBr) ν = 3324 (νN–H), 3297
˜
(νN–H), 3250 (νCOOH), 2919, 2850 (νC–H), 1652 (νC=O, amide
I), 1587 (ArC–C), 1526 (νsN–C=O, amide II), 1507 (δNH), 1466 give a yellow solid of a mixture of isomers of 4,4Ј-dinitrostilbene
(δasCH3), 1414 (νC–N), 962 (δoop=CH), 819 (δoopArC–H). HRMS
(MALDI+): calcd. for C28H38N2O2 [M+] 434.2933; found
434.2939.
(3.121 g, 75%). Recrystallisation from hot ethanol gave the trans-
isomer as yellow crystals (2.372 g, 57%).[33] 1H NMR (300 MHz,
CDCl3, 25 °C): δ = 8.25 (d, J = 8.53 Hz, 4 H, C3-H, C5-H, C3Ј-
H, C5Ј-H), 7.67 (d, J = 8.51 Hz, 4 H, C2-H, C6-H, C2Ј-H, C6Ј-
H), 7.28 (s, 2 H, =CH) ppm. 13C NMR (75 MHz, CDCl3, 25 °C):
δ = 147.65 (C4, C4Ј), 142.47 (C1, C1Ј), 131.33 (=CH), 127.43 (C2,
trans-Stilbene Derivative trans-1e: The acyl chloride of compound
trans-1b (0.457 g, 1.6 mmol), which was prepared according to the
procedure for the synthesis of the compound trans-1d, was sus-
pended in dichloromethane (45 mL) and cooled to –10 °C under
argon.[28] A solution of -leucine methyl ester hydrochloride
(0.292 g, 1.6 mmol) and triethylamine (0.49 mL, 3.5 mmol) in
dichloromethane (10 mL) was added to this suspension and the
reaction mixture was stirred in an ice bath for 30 min and then at
room temperature for 22 h. The mixture was washed with 2  HCl,
2  NaOH and with water. The organic phase was separated, dried
(Na2SO4) and evaporated to give a solid product. A light yellow
solid of trans-1e was isolated by preparative thin-layer chromatog-
raphy (silica gel; CH2Cl2) (0.146 g, 23%; m.p. 206–208 °C). 1H
NMR (300 MHz, DMSO, 25 °C): δ = 10.71 (s, 1 H, ar-NH), 9.23
(d, J = 8.27 Hz, 1 H, Leu-NH), 7.79 (d, J = 7.50 Hz, 2 H, C3-H,
C6, C2Ј, C6Ј), 124.23 (C3, C5, C3Ј, C5Ј) ppm. FT-IR (KBr) ν =
˜
1593 (νasNO2), 1506 (νN=O), 1339 (νsNO2), 1179, 1110 (δipArC–
H), 853 (δoopArC–H), 694 (δNO2).
trans-Stilbene Derivative trans-2a: 4,4Ј-Dinitro-trans-stilbene
(2.585 g, 9.6 mmol) and stannous chloride dihydrate (21.595 g,
96.0 mmol) were suspended in ethanol (45 mL).[27] The reaction
mixture was stirred and heated to 70 °C for two hours and then
cooled to room temperature. The mixture was poured over ice and
5% aqueous sodium carbonate was added until pH 7–8. The prod-
uct was extracted with ethyl acetate and the organic phase was
washed with brine solution. The solution was treated with charcoal
and dried (Na2SO4). The solvent was removed by filtration and
C5-H), 7.53 (d, J = 8.96 Hz, 2 H, C2Ј-H, C6Ј-H), 7.52 (d, J = evaporated to give a red-brown solid of 4,4Ј-diamino-trans-stilbene
7.23 Hz, 2 H, C2-H, C6-H), 7.31 (t, J = 7.75 Hz, 2 H, C3Ј-H, C5Ј- (1.469 g, 72%). A solution of this diamine (1.400 g, 6.7 mmol) and
H), 7.20 (t, J = 7.33 Hz, 1 H, C4Ј-H), 7.15 (s, 2 H, =CH), 4.38–
triethylamine (2.05 mL, 14.7 mmol) in dichloromethane (125 mL)
4.34 (m, 1 H, CHαLeu), 3.59 (s, 3 H, OCH3), 1.84–1.78 (m, 1 H, was stirred at –10 °C under argon,[28] ethyl oxalyl chloride
CH), 1.57–1.51 (m, 2 H, CH2), 0.84 (d, J = 6.27 Hz, 3 H, CH3), (1.51 mL, 13.6 mmol) was added, and stirring was continued in an
0.81 (d, J = 6.11 Hz, 3 H, CH3) ppm. 13C NMR (75 MHz, DMSO,
25 °C): δ = 171.94 (COOMe), 160.25 (CONH-Leu), 158.01 (Ar-
CONH), 137.12 (C1Ј), 137.01 (C1), 133.40 (C4), 128.72 (C3Ј, C5Ј),
ice bath for 30 min and at room temperature for 20 h. Filtration of
the reaction mixture gave trans-2a as a yellow solid (1.732 g, 63%).
1H NMR (300 MHz, DMSO, 25 °C): δ = 10.85 (s, 2 H, NH), 7.77
127.86 (C4Ј), 127.81 (=CH), 127.54 (=CH), 126.87 (C2, C6), 126.38 (d, J = 8.68 Hz, 4 H, C3-H, C5-H, C3Ј-H, C5Ј-H), 7.58 (d, J =
(C2Ј, C6Ј), 120.53 (C3, C5), 52.13 (CαLeu), 50.78 (OCH3), 24.33 8.63 Hz, 4 H, C2-H, C6-H, C2Ј-H, C6Ј-H), 7.18 (s, 2 H, =CH),
(CH ), 22.91 (CH), 21.03 [(CH ) ]. FT-IR (KBr) ν = 3294 (νN–H), 4.31 (q, J = 7.09 Hz, 4 H, CH2), 1.32 (t, J = 7.11 Hz, 6 H, CH3)
˜
2
3 2
2957, 2927 (νC–H), 1750 (νC=O, ester), 1663 (νC=O, amide I),
1521 (νsN–C=O, amide II), 1507 (δNH), 1416 (νC–N), 1278 (νC– 155.88 (Ar-CONH), 137.30 (C4, C4Ј), 134.07 (C1, C1Ј), 127.75
ppm. 13C NMR (75 MHz, DMSO, 25 °C): δ = 161.09 (COOEt),
O), 963 (δoop=CH), 816 (δoopArC–H). HRMS (ES+): calcd. for
(=CH), 127.26 (C2, C6, C2Ј, C6Ј), 121.03 (C3, C5, C3Ј, C5Ј), 62.87
C23H27N2O4 [MH+] 395.1971; found 395.1971.
(CH ), 14.32 (CH ) ppm. FT-IR (KBr) ν = 3333 (νN–H), 2997
˜
2
3
(νC–H), 1730 (νC=O, ester), 1703 (νC=O, amide I), 1589 (ArC–
C), 1533 (νsN–C=O, amide II), 1417 (νC–N), 1365 (δsCH3), 1291
(νC–O), 1184 (δipArC–H), 822 (δoopArC–H). HRMS (ES+): calcd.
for C22H23N2O6 [MH+] 411.1556; found 411.1555.
Bis(trans-stilbene) Derivative trans-1f: 4-Amino-trans-stilbene
(0.203 g, 1.0 mmol), which was prepared according to the pro-
cedure for the synthesis of compound trans-1a, was dissolved in
toluene (20 mL) and the solution refluxed in a Dean–Stark appara-
tus for two hours. After cooling this solution to room temperature
triethylamine (0.15 mL, 1.1 mmol) was added and the reaction mix-
ture was stirred at –10 °C under argon. Ethyl oxalyl chloride
(0.044 mL, 0.5 mmol) was added to the cooled mixture and stirring
was continued for 20 h at room temperature. Filtration of the reac-
tion mixture gave trans-1f as a light-yellow solid (0.207 g, 93%).
1H NMR (300 MHz, DMSO, 25 °C): δ = 10.86 (s, 2 H, NH), 7.76
trans-Stilbene Derivative trans-2b: Compound trans-2a (1.630 g,
4.0 mmol) was suspended in a mixture of ethanol (100 mL) and
water (50 mL) and 2  NaOH was added (20.00 mL, 40.0 mmol).
The reaction mixture was refluxed for 20 h. Stirring was stopped
and ethanol was removed under reduced pressure. A 2  HCl solu-
tion was added to this water suspension until pH 1, resulting in
precipitation of trans-2b as a brown solid (1.092 g, 77%). 1H NMR
(d, J = 8.46 Hz, 4 H, C3-H, C5-H), 7.29–7.21 (m, 14 H, C2-H, C6- (300 MHz, DMSO, 25 °C): δ = 10.79 (s, 2 H, NH), 7.79 (d, J =
H, C2Ј-H, C3Ј-H, C4Ј-H, C5Ј-H, C6Ј-H), 6.62 (s, 4 H, =CH) ppm.
13C NMR (75 MHz, DMSO, 25 °C): δ = 158.95 (CONH), 137.63,
137.51, 137.43, 137.11, 133.99, 133.59, 130.23, 130.00, 129.40,
129.14, 128.92 (=CH), 128.83 (=CH), 128.38, 127.71, 127.34,
8.48 Hz, 4 H, C3-H, C5-H, C3Ј-H, C5Ј-H), 7.57 (d, J = 8.43 Hz, 4
H, C2-H, C6-H, C2Ј-H, C6Ј-H), 7.17 (s, 2 H, =CH) ppm. 13C
NMR (75 MHz, DMSO, 25 °C): δ = 161.84 (COOH), 156.81 (Ar-
CONH), 136.82 (C4, C4Ј), 133.35 (C1, C1Ј), 127.12 (=CH), 126.55
(C2, C6, C2Ј, C6Ј), 120.29 (C3, C5, C3Ј, C5Ј) ppm. FT-IR (KBr)
126.85 (C2, C6), 121.07, 120.61 (C3, C5) ppm. FT-IR (KBr) ν =
˜
3344 (νN–H), 3299 (νN–H), 2927 (νC–H), 1675 (νC=O, amide I), ν = 3333 (νN–H), 2924, 2857 (νC–H), 1673 (νC=O, carboxylic
˜
1661 (νC=O, amide I), 1585 (ArC–C), 1523 (νsN–C=O, amide II), acid), 1644 (νC=O, amide I), 1588 (ArC–C), 1533 (νsN–C=O,
1416 (νC–N), 834 (δoopArC–H). HRMS (ES+): calcd. for
amide II), 1387 (νC–N), 1315 (νC–O), 818 (δoopArC–H).
C30H24N2NaO2 [MNa+] 467.1735; found 467.1729.
trans-Stilbene Derivative trans-2c: This product was obtained by the
same procedure as trans-1c but with trans-2b (0.073 g, 0.2 mmol)
as starting material. trans-2c was isolated as a light-brown solid,
(0.082 g, 100%). Due to its poor solubility 1H and 13C NMR spec-
4,4Ј-Dinitro-trans-stilbene:
Potassium
carbonate
(2.451 g,
17.7 mmol) was suspended in dichloromethane (125 mL) and (4-
nitrobenzyl)triphenylphosphonium bromide (7.378 g, 15.4 mmol),
Eur. J. Org. Chem. 2006, 1323–1334
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1331

ˇ
S. Miljanic´, L. Frkanec, Z. Meic´, M. Zinic´
FULL PAPER
tra were not recorded. FT-IR (KBr) ν = 3333 (νN–H), 2924, 2857
(νC–H), 1673 (νC=O), 1644 (νC=O, amide I), 1588 (ArC–C), 1560
(d, J = 8.25 Hz, 2 H, C2-H, C6-H), 7.15 (d, J = 8.29 Hz, 2 H, C2Ј-
H, C6Ј-H), 6.81 (d, J = 8.15 Hz, 2 H, C3Ј-H, C5Ј-H), 6.48 (dd, 2
˜
(νsN–C=O, amide II), 1516 (δNH), 1388 (νC–N), 1315 (νC–O), 817 H, =CH), 3.92 (t, J = 6.21 Hz, 2 H, OCH2CH2), 1.70–1.65 (m, 2
(δoopArC–H).
H, OCH2CH2), 1.24 [s, 18 H, (CH2)9], 0.83 (t, 3 H, CH2CH2CH3)
ppm. 13C NMR (75 MHz, DMSO, 25 °C): δ = 162.01 (COOH),
157.84 (C4Ј), 156.74 (Ar-CONH), 136.49 (C4), 133.31 (C1), 129.73
(C2Ј, C6Ј), 129.29 (=CH), 128.85 (C1Ј), 128.75 (C2, C6), 127.70
(=CH), 119.95 (C3, C5), 114.19 (C3Ј, C5Ј), 67.32 (OCH2), 31.23
(CH2CH2CH3), 28.96, 28.94, 28.91, 28.69, 28.64, 28.61 (CH2),
25.44 (OCH2CH2), 22.03 (CH2CH2CH3), 13.87 (CH3) ppm. FT-IR
trans-Stilbene Derivative trans-2d: This product was prepared by the
same procedure as trans-1d but with trans-2b (0.108 g, 0.3 mmol) as
starting material. The precipitate was purified by suspension in 2 
HCl and in 2  NaOH using an ultrasound bath, which gave trans-
2d as a brown solid after filtration (0.060 g, 29%). 1H NMR
(300 MHz, DMSO, 25 °C): δ = 7.67 (d, J = 8.69 Hz, 4 H, C3-H,
C5-H, C3Ј-H, C5Ј-H), 7.46 (d, J = 7.96 Hz, 4 H, C2-H, C6-H, C2Ј-
H, C6Ј-H), 7.04 (s, 2 H, =CH), 1.27 [s, 44 H, (CH2)11], 0.86 (t, J =
6.75 Hz, 6 H, CH3) ppm. Owing to the poor solubility of trans-2d
(KBr) ν = 3369 (νN–H), 2920, 2851 (νC–H), 1734 (νC=O), 1685
˜
(νC=O, amide I), 1607 (ArC–C), 1541 (νsN–C=O, amide II), 1508
(δNH), 1458 (δasCH3), 1248 (νasC–O–C), 1177 (δipArC–H), 1029
(νsC–O–C), 836 (δoopArC–H).
its 13C NMR spectrum was not recorded. FT-IR (KBr) ν = 3332
˜
(νN–H), 2922, 2857 (νC–H), 1673 (νC=O, amide I), 1645 (νC=O, trans-Stilbene Derivative trans-3c: This product was obtained by the
amide I), 1588 (ArC–C), 1532 (νsN–C=O, amide II), 1387 (νC–N),
1315 (νC–O), 1193 (δipArC–H), 817 (δoopArC–H).
same procedure as trans-1c but with trans-3b (0.100 g, 0.2 mmol)
as starting material. trans-3c was isolated as a yellow solid (0.103 g,
1
99%). Due to the poor solubility of this compound its H and 13C
trans-Stilbene Derivative trans-2e: This product was prepared by the
same procedure as trans-1e but with trans-2b (0.196 g, 0.5 mmol)
as starting material. Filtration of the reaction mixture gave trans-2e
as a light-brown solid (0.112 g, 38%). 1H NMR (300 MHz, DMSO,
25 °C): δ = 10.78 (s, 2 H, Ar-NH), 9.30 (d, J = 8.22 Hz, 2 H, Leu-
NH), 7.85 (d, J = 8.46 Hz, 4 H, C3-H, C5-H, C3Ј-H, C5Ј-H), 7.58
(d, J = 8.42 Hz, 4 H, C2-H, C6-H, C2Ј-H, C6Ј-H), 7.18 (s, 2 H,
=CH), 4.45–4.40 (m, 2 H, CHαLeu), 3.65 (s, 6 H, OCH3), 1.91–1.84
(m, 2 H, CH), 1.60–1.58 (m, 4 H, CH2), 0.90 (d, J = 6.29 Hz, 6 H,
CH3), 0.88 (d, J = 5.54 Hz, 6 H, CH3) ppm. Owing to a poor
solubility of trans-2e its 13C NMR spectrum was not recorded. FT-
NMR spectra were not recorded. FT-IR (KBr) ν = 3341 (νN–H),
˜
2918, 2850 (νC–H), 1672 (νC=O, amide I), 1636 (νC=O), 1609
(ArC–C), 1582 (νasCOO–), 1540 (νsN–C=O, amide II), 1473
(δasCH3), 1419 (νsCOO–), 1257 (νasC–O–C), 1179 (δipArC–H),
1025 (νsC–O–C), 830 (δoopArC–H).
cis-Stilbene Derivative cis-3c: This product was obtained by the
same procedure as trans-1c but with cis-3b (0.060 g, 0.13 mmol) as
starting material. cis-3c was isolated as a yellow solid, (0.061 g,
99%). 1H NMR (300 MHz, DMSO, 25 °C): δ = 8.53 (s, 1 H, NH),
7.64 (d, J = 7.41 Hz, 2 H, C3-H, C5-H), 7.18–7.13 (m, 4 H, C2-H,
C6-H, C2Ј-H, C6Ј-H), 6.81 (d, J = 8.70 Hz, 2 H, C3Ј-H, C5Ј-H),
6.44 (s, 2 H, =CH), 3.92 (t, J = 6.31 Hz, 2 H, OCH2CH2), 1.68–
1.66 (m, 2 H, OCH2CH2), 1.24 [s, 18 H, (CH2)9], 0.85 (t, 3 H,
CH2CH2CH3) ppm. Due to the poor solubility of this compound
IR (KBr) ν = 3294 (νN–H), 2957, 2927, 2877 (νC–H), 1740 (νC=O,
˜
ester), 1662 (νC=O, amide I), 1589 (ArC–C), 1522 (νsN–C=O,
amide II), 1512 (δNH), 1418 (νC–N), 1321 (νC–O), 829 (δoopArC–
H). HRMS (MALDI+): calcd. for C32H40N4O8 [M+] 608.2846;
found 608.2840.
its 13C NMR spectrum was not recorded. FT-IR (KBr) ν = 3361
˜
(νN–H), 2920, 2850 (νC–H), 1669 (νC=O, amide I), 1631 (νC=O),
1606 (ArC–C), 1559 (νasCOO–), 1523 (νsN–C=O, amide II), 1508
(δNH), 1458 (δasCH3), 1388 (νsCOO–), 1253 (νasC–O–C), 1179
(δipArC–H), 1025 (νsC–O–C), 846 (δoopArC–H).
trans-Stilbene Derivative trans-3a: This substance was prepared ac-
cording to ref.[30]
cis-Stilbene Derivative cis-3a: This substance was prepared accord-
ing to ref.[30]
trans-Stilbene Derivative trans-3d: Compound trans-3a (0.063 g,
0.1 mmol) was suspended in a mixture of saturated ammonia solu-
tion in methanol (25 mL) and dichloromethane (10 mL). The sus-
pension was left to stand at 4 °C for seven days, after which time
the solvent was evaporated to dryness. Addition of petroleum ether
to a solution of the crude residue in ethyl acetate gave trans-3d
as a light-brown precipitate (0.054 g, 92%). 1H NMR (300 MHz,
DMSO, 80 °C): δ = 10.42 (s, 1 H, ar-NH), 8.09 (s, 1 H, CONH2),
7.80 (d, J = 7.60 Hz, 2 H, C3-H, C5-H), 7.72 (d, J = 7.69 Hz, 1 H,
CONH2), 7.53 (d, J = 8.98 Hz, 2 H, C2-H, C6-H), 7.46 (d, J =
8.57 Hz, 2 H, C2Ј-H, C6Ј-H), 7.07 (dd, 2 H, =CH), 6.92 (d, J =
6.27 Hz, 2 H, C3Ј-H, C5Ј-H), 3.99 (t, 2 H, OCH2CH2), 1.80–1.63
(m, 2 H, OCH2CH2), 1.27 [s, 18 H, (CH2)9], 0.87 (t, 3 H,
CH2CH2CH3) ppm. Due to the poor solubility of this compound
trans-Stilbene Derivative trans-3b: This product was obtained by the
same procedure as trans-2b but with trans-3a (0.290 g, 0.6 mmol)
as starting material. trans-3b was isolated as a light-brown solid
(0.255 g, 94%; m.p. 201–203 °C). 1H NMR (300 MHz, DMSO,
80 °C): δ = 10.75 (s, 1 H, NH), 7.77 (d, J = 8.63 Hz, 2 H, C3-H, C5-
H), 7.55 (d, J = 8.73 Hz, 2 H, C2-H, C6-H), 7.50 (d, J = 8.80 Hz, 2
H, C2Ј-H, C6Ј-H), 7.09 (dd, 2 H, =CH), 6.92 (d, J = 8.68 Hz, 2 H,
C3Ј-H, C5Ј-H), 3.98 (t, J = 6.36 Hz, 2 H, OCH2CH2), 1.73–1.66
(m, 2 H, OCH2CH2), 1.24 [s, 18 H, (CH2)9], 0.85 (t, J = 6.64 Hz,
3 H, CH2CH2CH3) ppm. 13C NMR (75 MHz, DMSO, 80 °C): δ =
162.04 (COOH), 158.31 (C4Ј), 156.66 (Ar-CONH), 136.65 (C4),
133.72 (C1), 129.57 (C1Ј), 127.0 (C2Ј, C6Ј), 127.43 (=CH), 126.61
(C2, C6), 125.42 (=CH), 120.34 (C3, C5), 114.62 (C3Ј, C5Ј), 67.43
(OCH2), 31.24 (CH2CH2CH3), 28.93, 28.70, 28.64 (CH2), 25.44
(OCH2CH2), 22.03 (CH2CH2CH3), 13.88 (CH3) ppm. FT-IR (KBr)
its 13C NMR spectrum was not recorded. FT-IR (KBr) ν = 3404
˜
(NH2), 3313 (νN–H), 2918, 2851 (νC–H), 1655 (νC=O, amide I),
1609 (ArC–C), 1529 (νsN–C=O, amide II), 1472 (δasCH3), 1397
(νC–N), 1257 (νasC–O–C), 1179, 1112 (δipArC–H), 1035 (νsC–O–
C), 831 (δoopArC–H).
ν = 3368 (νN–H), 3318 (νCOOH), 2918, 2850 (νC–H), 1683
˜
(νC=O, amide I), 1609 (ArC–C), 1540 (νsN–C=O, amide II), 1521
(δNH), 1473 (δasCH3), 1361 (νC–N), 1303 (νC–O), 1257 (νasC–O–
C), 1178 (δipArC–H), 1025 (νsC–O–C), 831 (δoopArC–H).
cis-Stilbene Derivative cis-3d: This product was obtained by the
same procedure as trans-3d but with cis-3a (0.100 g, 0.2 mmol) as
starting material. cis-3d was isolated as a light-brown solid,
(0.075 g, 83%; m.p. 179–181 °C). 1H NMR (300 MHz, DMSO,
cis-Stilbene Derivative cis-3b: This product was obtained by the
same procedure as trans-2b but with cis-3a (0.100 g, 0.2 mmol) as
starting material. cis-3b was isolated as a light-yellow solid (0.080 g,
1
89%; m.p. 114–115 °C). H NMR (300 MHz, DMSO, 25 °C): δ = 25 °C): δ = 10.60 (s, 1 H, ar-NH), 8.29 (s, 1 H, CONH2), 7.98 (s,
10.68 (s, 1 H, NH), 7.67 (d, J = 8.18 Hz, 2 H, C3-H, C5-H), 7.21 1 H, CONH2), 7.72 (d, J = 8.59 Hz, 2 H, C3-H, C5-H), 7.21 (d, J
1332
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Gelation Ability of Stilbene-Substituted Oxamides
FULL PAPER
= 8.56 Hz, 2 H, C2-H, C6-H), 7.15 (d, J = 8.63 Hz, 2 H, C2Ј-H,
a clear solution remained or a precipitate or a gel formed. The
C6Ј-H), 6.81 (d, J = 8.68 Hz, 2 H, C3Ј-H, C5Ј-H), 6.48 (dd, 2 H, minimal gel concentration was estimated by adding 100-µL por-
=CH), 3.92 (t, J = 6.45 Hz, 2 H, OCH2CH2), 1.74–1.62 (m, 2 H, tions of the solvent to the suspension until a low-viscosity solution
OCH2CH2), 1.24 [s, 18 H, (CH2)9], 0.85 (t, J = 6.15 Hz, 3 H,
CH2CH2CH3) ppm. 13C NMR (75 MHz, DMSO, 25 °C): δ =
162.03 (CONH2), 158.74 (Ar-CONH), 157.83 (C4Ј), 136.53 (C4),
133.14 (C1), 129.73 (C2Ј, C6Ј), 129.21 (=CH), 128.89 (C1Ј), 128.71
(C2, C6), 127.76 (=CH), 119.85 (C3, C5), 114.20 (C3Ј, C5Ј), 67.32
(OCH2), 31.23 (CH2CH2CH3), 28.96, 28.95, 28.91, 28.69, 28.64,
28.61 (CH2), 25.44 (OCH2CH2), 22.03 (CH2CH2CH3), 13.88 (CH3)
instead of a gel formed upon cooling the hot solution.
Photoinduced Gelation: FT-Raman spectra were recorded with a
Bruker Equinox 55 interferometer equipped with an FRA 106/S
module using a Nd-YAG laser excitation at 1064 nm. Spectra of
the sample (cis-3a, 2.0×10–2  in ethanol) in a quartz cell
(10×10 mm2) were recorded during one hour irradiation (250 Ͻ λ
Ͻ 520 nm) by an Elektrokovina high-pressure mercury lamp (type
Ballast DHM 520–250). To obtain a spectrum of trans-3a in solu-
tion (2.0×10–2  in ethanol), the cell was thermostatted at
75 1 °C by means of a Medingen Dresden thermostat (model U3).
All FT-Raman spectra were corrected by subtracting the solvent
spectrum.
ppm. FT-IR (KBr) ν = 3383 (NH ), 3292 (νN–H), 2920, 2851 (νC–
˜
2
H), 1661 (νC=O, amide I), 1605 (ArC–C), 1527 (νsN–C=O, amide
II), 1510 (δNH), 1468 (δasCH3), 1395 (νC–N), 1252 (νasC–O–C),
1179, 1114 (δipArC–H), 832 (δoopArC–H).
trans-Stilbene Derivative trans-3e: This product was obtained by the
same procedure as trans-1e but with trans-3b (0.098 g, 0.2 mmol)
as starting material. trans-3e was isolated as a yellow solid,
(0.110 g, 87%; m.p. 174–176 °C). 1H NMR (300 MHz, CDCl3,
25 °C): δ = 9.14 (s, 1 H, ar-NH), 7.78 (d, J = 8.97 Hz, 1 H, Leu-
NH), 7.55 (d, J = 8.64 Hz, 2 H, C3-H, C5-H), 7.42 (d, J = 8.61 Hz,
2 H, C2-H, C6-H), 7.36 (d, J = 8.72 Hz, 2 H, C2Ј-H, C6Ј-H), 6.92
(dd, 2 H, =CH), 6.82 (d, J = 8.73 Hz, 2 H, C3Ј-H, C5Ј-H), 4.61–
4.57 (m, 1 H, C*H), 3.90 (t, J = 6,52 Hz, 2 H, OCH2CH2), 3.71 (s,
3 H, OCH3), 1.74–1.67 (m, 2 H, OCH2CH2), 1.67–1.60 (m, 2 H,
CH2), 1.41–1.37 (m, 1 H, CH), 1.20 [s, 18 H, (CH2)9], 0.96–0.89
[m, 6 H, (CH3)2], 0.81 (t, J = 7.03 Hz, 3 H, CH2CH2CH3) ppm.
Owing to the gelation of the solvent by trans-3e the 13C NMR
Temperature-Dependent Measurements: Temperature-dependent
FT-IR spectra of the gel (trans-3e/toluene, 2.0×10–2 ) were re-
corded with an ABB Bomen MB 102 FT-IR spectrometer,
equipped with CsI optics, DTGS detector and Specac 3000 Series
high stability temperature controller with a 21-20730 heating
jacket. A sealed heatable liquid cell (Specac, type 2051, path length
0.05 mm, KBr windows) was used for handling the sample. Tem-
perature-dependent 1H NMR spectra of toluene and DMSO gels
(trans-3e, 2.0×10–2 ) was recorded on a Bruker Avance 300 spec-
trometer using tetramethylsilane and 1,1,2,2-tetrachloroethane as
internal standards.
TEM Investigation: Transmission electron micrographs were taken
on an EM-10 Zeiss transmission electron microscope from a small
amount of an unstained gel sample (trans-3a/ethanol or trans-3e/
ethanol, 2.0×10–2 ) placed on a carbon-coated grid (copper,
100 mesh).
spectrum was not recorded. FT-IR (KBr) ν = 3291 (νN–H), 2920,
˜
2849 (νC–H), 1749 (νC=O, ester), 1663 (νC=O, amide I), 1610
(ArC–C), 1521 (νsN–C=O, amide II), 1512 (δNH), 1473 (δasCH3),
1414 (νC–N), 1258 (νasC–O–C), 1177 (δipArC–H), 1019 (νsC–O–
C), 835 (δoopArC–H). HRMS (MALDI+): calcd. for C35H50N2O5
[M+] 578.3720; found 578.3718.
trans-Stilbene Derivative trans-3f: A 1  LiOH solution (0.19 mL,
0.2 mol) was added to a suspension of trans-3e (0.109 g, 0.2 mmol)
in a mixture of methanol (5 mL) and dichloromethane (10 mL).
The reaction mixture was stirred at room temperature for 20 h. The
organic solvents were removed under reduced pressure and water
(10 mL) was added to the remaining aqueous suspension. Dropwise
addition of 2  HCl until pH 1 precipitated trans-3f as a light-
Acknowledgments
This research was supported by the Croatian Ministry of Science,
Education and Sport (project nos. 0119641 and 0098053). The au-
thors thank Dr. N. Ljubesˇic´ of the Rueer Bosˇkovic´ Institute, Zag-
reb, Croatia, for the TEM experiments and Dr. M. Cindric´, PLIVA,
Zagreb, Croatia, for the MS experiments.
1
brown solid (0.070 g, 65%). H NMR (300 MHz, DMSO, 25 °C):
δ = 10.67 (s, 1 H, Ar-NH), 8.55 (d, J = 8.97 Hz, 1 H, Leu-NH),
7.83 (d, J = 8.71 Hz, 2 H, C3-H, C5-H), 7.54 (d, J = 8.75 Hz, 2 H,
C2-H, C6-H), 7.50 (d, J = 8.80 Hz, 2 H, C2Ј-H, C6Ј-H), 7.09 (dd,
2 H, =CH), 6.92 (d, J = 8.73 Hz, 2 H, C3Ј-H, C5Ј-H), 3.97 (t, J =
6.52 Hz, 2 H, OCH2CH2), 3.82–3.79 (m, 1 H, C*H), 1.73–1.66 (m,
2 H, OCH2CH2), 1.64–1.55 (m, 2 H, CH2), 1.41–1.36 (m, 1 H,
CH), 1.25 [s, 18 H, (CH2)9], 0.91–0.86 [m, 6 H, (CH3)2], 0.86 (t, J
= 7.08 Hz, 3 H, CH2CH2CH3) ppm. 13C NMR (75 MHz, DMSO,
80 °C): δ = 172.03 (COOH), 158.39 (CONH),158.19 (C4Ј), 136.34
(C4), 133.53 (C1), 129.55 (C1Ј), 127.35 (C2Ј, C6Ј), 126.16 (C2, C6),
125.43 (=CH), 120.22 (C3, C5), 114.58 (C3Ј, C5Ј), 67.44 (OCH2),
52.77 (C*), 41.56 (Leu-CH2), 30.93 (CH2CH2CH3), 28.59, 28.39,
28.30 (CH2), 25.16 (OCH2CH2), 24.39 (Leu-CH), 22.8 (Leu-CH3),
22.00 (Leu-CH3), 21.69 (CH2CH2CH3), 13.49 (CH3) ppm. FT-IR
[1] L. A. Estroff, A. D. Hamilton, Chem. Rev. 2004, 104, 1201–
1217.
[2] P. Terech, R. G. Weiss, Chem. Rev. 1997, 97, 3133–3159.
[3] B. L. Feringa, R. A. van Delden, N. Koumura, E. M. Geert-
sema, Chem. Rev. 2000, 100, 1789–1816.
[4] L. N. Lucas, J. van Esch, R. M. Kellogg, B. L. Feringa, Chem.
Commun. 2001, 759–760.
[5] J. J. D. de Jong, L. N. Lucas, R. M. Kellogg, J. H. van Esch,
B. L. Feringa, Science 2004, 304, 278–281.
[6] S. Hecht, Small 2005, 1, 26–29.
[7] S. A. Ahmed, X. Sallenave, F. Fages, G. Mieden-Gundert,
W. M . M üller, U. Müller, F. Vögtle, J. L. Pozzo, Langmuir 2002,
18, 7096–7101.
[8] M. Ayabe, T. Kishida, N. Fujita, K. Sada, S. Shinkai, Org.
Biomol. Chem. 2003, 1, 2744–2747.
(KBr) ν = 3305 (νN–H), 2920, 2850 (νC–H), 1654 (νC=O, amide
˜
[9] M. de Loos, J. van Esch, R. M. Kellogg, B. L. Feringa, Angew.
Chem. Int. Ed. 2001, 40, 613–616.
[10] K. Murata, M. Aoki, T. Nishi, A. Ikeda, S. Shinkai, J. Chem.
Soc., Chem. Commun. 1991, 1715–1718.
I), 1521 (νsN–C=O, amide II), 1512 (δNH), 1473 (δasCH3), 1419
(νC–N), 1258 (νasC–O–C), 1179 (δipArC–H), 1021 (νsC–O–C), 831
(δoopArC–H).
[11] M. Moriyama, N. Mizoshita, T. Yokota, K. Kishimoto, T.
Kato, Adv. Mater. 2003, 15, 1335–1338.
[12] N. Koumura, M. Kudo, N. Tamaoki, Langmuir 2004, 20, 9897–
9900.
Gelation Experiments: A suspension of the gelator (5 mg) in a mea-
sured volume of solvent was heated in a sealed test tube to boiling
and then cooled to room temperature. Depending on the solvent,
Eur. J. Org. Chem. 2006, 1323–1334
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
1333

ˇ
S. Miljanic´, L. Frkanec, Z. Meic´, M. Zinic´
FULL PAPER
[13]
[14]
[15]
[23] X. Luo, C. Li, Y. Liang, Chem. Commun. 2000, 2091–2092.
[24] K. B. Becker, Synthesis 1983, 5, 341–368.
[25] I. Hughes, W. P. Nolan, R. A. Raphael, J. Chem. Soc., Perkin
Trans. 1 1990, 2475–2480.
[26] J. H. Boyer, H. Alul, J. Am. Chem. Soc. 1959, 81, 2136–2137.
[27] C. Ling, M. Minato, P. M. Lahti, H. van Willigen, J. Am.
Chem. Soc. 1992, 114, 9959–9969.
[28] X. Cha, K. Ariga, M. Onda, T. Kunitake, J. Am. Chem. Soc.
1995, 117, 11833–11838.
C. Geiger, M. Stanescu, L. Chen, D. G. Whitten, Langmuir
1999, 15, 2241–2245.
ˇ
L. Frkanec, M. Jokic´, J. Makarevic´, K. Wolsperger, M. Zinic´,
J. Am. Chem. Soc. 2002, 124, 9716–9717.
J. Eastoe, M. Sánchez-Dominguez, P. Wyatt, R. K. Heenan,
Chem. Commun. 2004, 2608–2609.
[16]
[17]
H. Meier, Angew. Chem. Int. Ed. Engl. 1992, 31, 1399–1420.
J. Makarevic´, M. Jokic´, B. Peric´, V. Tomisˇic´, B. Kojic´-Prodic´,
ˇ
M. Zinic´, Chem. Eur. J. 2001, 7, 3328–3341.
[29] Z. Pravda, K. Podusˇka, K. Bláha, Collect. Czech. Chem. Com-
mun. 1964, 29, 2626–2631.
ˇ
[18] J. Makarevic´, M. Jokic´, L. Frkanec, D. Katalenic´, M. Zinic´,
Chem. Commun. 2002, 2238–2239.
[19] J. Makarevic´, M. Jokic´, Z. Raza, Z. Stefanic´, B. Kojic´-Prodic´,
ˇ
[30] S. Miljanic´, L. Frkanec, Z. Meic´, M. Zinic´, Langmuir 2005, 21,
ˇ
2754–2760.
ˇ
M. Zinic´, Chem. Eur. J. 2003, 9, 5567–5580.
[31] P. Ruggli, F. Lang, Helv. Chim. Acta 1938, 21, 38–50.
[32] M. Calvin, H. W. Alter, J. Chem. Phys. 1951, 19, 765–767.
[33] M. Calvin, R. E. Buckles, J. Am. Chem. Soc. 1940, 62, 3324–
3328.
[20] R. Wang, C. Geiger, L. Chen, B. Swanson, D. G. Whitten, J.
Am. Chem. Soc. 2000, 122, 2399–2400.
[21] D. C. Duncan, D. G. Whitten, Langmuir 2000, 16, 6445–6452.
[22] V. Caplar, M. Zinic´, J.-L. Pozzo, F. Fages, G. Mieden-Gundert,
F. Vögtle, Eur. J. Org. Chem. 2004, 19, 4048–4059.
ˇ
ˇ
Received: July 15, 2005
Published Online: December 19, 2005
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Eur. J. Org. Chem. 2006, 1323–1334