Redox active donor-substituted punicin derivatives

Article abstract of DOI:10.1039/b822353j

The redox active plant material punicin from Punica granatum, 2-hydroxy-1-(pyridinium-1-yl)-5-olate, and some derivatives were modified by substitution with 2-oxochromen-4-olate moieties to give donor-substituted molecules which form distinct types of atropisomeric mesomeric betaines and tetrapolar substances. Oxidation to new negatively-charged solvatochromic quinones was achieved on treatment of these betaines with CAN, taking advantage of the stabilizing properties of the electron-donating 2-oxochromen-4-olate partial structure. The quinones can be employed in model redox reactions. Cyclovoltametric studies have been performed.

Full text of DOI:10.1039/b822353j

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Redox active donor-substituted punicin derivatives
Marcel Albrecht,a Oliver Schneiderb and Andreas Schmidt*a
Received 12th December 2008, Accepted 14th January 2009
First published as an Advance Article on the web 25th February 2009
DOI: 10.1039/b822353j
The redox active plant material punicin from Punica granatum, 2-hydroxy-1-(pyridinium-1-yl)-5-olate,
and some derivatives were modified by substitution with 2-oxochromen-4-olate moieties to give
donor-substituted molecules which form distinct types of atropisomeric mesomeric betaines and
tetrapolar substances. Oxidation to new negatively-charged solvatochromic quinones was achieved on
treatment of these betaines with CAN, taking advantage of the stabilizing properties of the
electron-donating 2-oxochromen-4-olate partial structure. The quinones can be employed in model
redox reactions. Cyclovoltametric studies have been performed.
Introduction
For decades, hydroquinone and p-benzoquinone have been playing
important roles in numerous areas of science and technol-
ogy. Currently, interest is inter alia paid to hydrogen storage,1
electrocatalysis,2 sensors,3 biochemical effects,4 ligands for mag-
netic complexes,5 pharmaceuticals,6 and catalysis7 involving these
compounds which are consequently still the targets of organic
synthesis.8 The interconversion of p-benzoquinone and hydro-
quinone by two single electron transitions is moreover a common
redox reaction in chemistry (Scheme 1),9 and both in organic
synthesis10 and in biological processes11 this redox pair is of great
relevance. Thus, quinones play important biological roles, inter alia
as electron acceptors in electron transport chains such as those in
aerobic respiration, and photosystems I and II of photosynthesis.
Scheme 2
The 4,4¢-bipyridine derivative of 1, dipunicin 5, is also able
to form a stable radical, 6, which can be characterized by ESR
spectroscopy and solid-state ENDOR spectroscopy (Scheme 3).16
Scheme 1
The alkaloid 1 (“punicin”) is an interesting redox active
compound from the leaves of Punica granatum L.12 We recently
found that this material is a representative of the class of
heterocyclic mesomeric betaines, depending on the reaction or
measurement conditions.13 Thus, aqueous solutions of 1 consist
of equilibria of the two mesomeric betaines 2A and 2B (Scheme 2)
which are orange-yellow in color. Interestingly, the betaine 2A
belongs to the class of conjugated mesomeric betaines (CMB),
whereas its tautomer 2B is a member of the class of cross-
conjugated mesomeric betaines (CCMB).14 At higher pH values,
deprotonation to the monoanionic species 3 and finally pericyclic
ring cleavage to 4 occurs. These processes are reversible on addition
of acids. Moreover, punicin forms radical anions as well as
semiquinones when stabilized by polymer backbones (ESR).15
Scheme 3
The betaine/monoanion radical pair 5/6 can be used in coupled
photocatalytic electron transfers with EDTA as sacrificial donor
and molecular oxygen as electron acceptor (Scheme 4).16
Quinones derived from punicin have never been described
before. Pyridinium-substituted quinones seemingly are only sta-
ble when they are substituted with additional olate groups,17
amine groups,18 or chlorine.19 Exceptions are DMAP-substituted
quinones.20 To enlarge the redox capabilities and potential ap-
plications of this interesting class of compounds we envisaged
aClausthal University of Technology, Institute of Organic Chemistry,
Leibnizstrasse 6, Clausthal-Zellerfeld, Germany. E-mail: schmidt@ioc.tu-
clausthal.de; Fax: +49-(0)5323-722858; Tel: +49-(0)5323-723861
bInstitute of Metallurgy, Robert-Koch-Strasse 42, D-38678, Clausthal-
Zellerfeld, Germany
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Results and discussion
Syntheses and characterizations
Applying the method of Zhang,22 we first prepared the mesomeric
betaines 7a–7f in reasonable yields (Scheme 5). p-Benzoquinone,
4-hydroxycoumarin and the corresponding pyridines reacted in
aqueous acetone at room temperature over a period of 24 h to the
target compounds. Most of the resulting compounds (except for
7e) were obtained as analytically pure solids after recrystallization
from methanol.
Scheme 4
the synthesis of other donor-substituted punicin derivatives and
chose the (E)-3-oxoprop-1-en-1-olate group I (Fig. 1) as an anionic
partial structure of the target betaines. The central carbon atom is
a starred position as shown in the isoconjugate equivalent II, i.e. an
active position of the highest occupied molecular orbital (HOMO)
III. Joining the cationic partial structure through this position
results in a conjugated heterocyclic mesomeric betaine (cf. IV).21
We therefore expected a considerable stabilization of the system.
Connection of the unstarred positions (Fig. 1) resulted in cross-
conjugated heterocyclic mesomeric betaines which are well-known
in heterocyclic chemistry.21 A literature research revealed that the
2-oxochromen-4-olate moiety V could be a suitable substituent for
these purposes.
Scheme 5
Due to the restricted rotation of the pyridine substituents
and the 2-oxochromen-4-olate moieties all compounds formed
atropisomers. This becomes apparent by different shifts of the
1
a/a¢ and b/b¢ protons in the H NMR spectra. Consequently,
3-methylpyridine and isoquinoline as starting materials yielded
two pairs of diastereomers, which could not be separated by
column chromatography. The ratio of the two isomers can be
determined by analysis of the 1H NMR spectra. The more
deshielded resonance frequency of the methyl group at d 2.50 ppm
was attributed to 7c (Scheme 6), because of the shielding effect
of the p-electron system of the 2-oxo-chromen-4-olate ring on the
methyl group of 7c¢, which consequently appears at d 2.23 ppm.
Thus the ratio of the two isomers was determined to be 7c/7c¢ =
1.35 : 1. In the isoquinoline derivative the molecular ratio of the
two isomers 7d/7d¢ was determined to be 2.5 : 1 by comparing the
signals of 5-H of the chromen-olate ring at d 7.95 ppm (7d) and
7.75 ppm (7d¢) in the 1H NMR and H,H-COSY spectra.
Deprotonations to anionic tripoles 8 is observed on treatment
of 7 with bases. As examples, solutions of 100 mg of 7a and 7b in
20 mL of methanol were titrated with a sodium hydroxide solution
(0.01 M in methanol), respectively. Addition of one equivalent of
base, resulting in pH 8, caused a change of colour from yellow to
red due to the formation of the trianionic species 8A/8B from
the betaine 7 (Scheme 7). Additional base results in a brown
colour (pH 12.5), presumably due to the formation of a dianionic
species. Unfortunately no pKa values of these compounds could
be determined from the titration curves due to the formation of
different deprotonated species in equilibrium.
Fig. 1 Architecture of heterocyclic mesomeric betaines.
We wish to report here our results on the synthesis and examina-
tion of 2-oxochromen-4-olate-substituted punicins. We studied the
competition of protonation sites, the stabilizing properties of this
substituent on pyridinium-substituted quinones, and their redox
properties.
Next we were interested in the synthesis of new electron
donor substituted punicin derivatives bearing two chromene
structure elements, 9a–9d (Scheme 8). Quinone 8 was reacted
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solvents, except for DMSO and DMF. Similar to 7a–7f, these
compounds are formed as two different atropisomers 9 and 9¢.
Consequently, the chromene protons split into a double set of
signals in the NMR spectra, whereas the pyridine protons give
only one set of signals. In the NMR spectra, the OH groups
cannot be detected so we were not able to differentiate the types of
conjugation. The negative charge located in the 2-oxo-chromen-
4-olate moiety in para-position and meta-position with respect to
the pyridinium ring resulted in a conjugated mesomeric betaine
(VI) and a cross-conjugated mesomeric betaine (VII), respectively,
as evidenced by an inspection of starred and unstarred positions
of the isoconjugate equivalent. This finding is similar to the
original plant material punicin.
Taking advantage of the stabilizing properties of the donor-
substituents, the mesomeric betaines 7 can indeed easily be
converted into the corresponding quinone-betaines 10 by reaction
with cerium(IV) ammonium nitrate (CAN) in aqueous acetonitrile
at room temperature (Scheme 9).
Scheme 6
Scheme 7
Scheme 9
The quinones 10 possess a push-pull chromophor and are
intensely dark blue in colour in the solid state and soluble
in water, methanol and other organic solvents. They display
a strong negative solvatochromism. In nonpolar solvents such
as dichloromethane they give blue solutions, whereas in polar
solvents (water) a red colour was observed. UV measurements
were carried out in different solvents with a constant concentration
of quinone 10a (0.4 mmol L-1). The results are shown in Table 1
and Fig. 2, where the characteristic absorption bands are plotted
versus the ET values of solvent polarities.23 As shown, with
N
increasing solvent polarity, the UV absorption maxima shift to
shorter wavelengths.
Using these quinones as starting materials new tetrapolar
betaines bearing two chromenes and two pyridinium rings can be
synthesized by additional reaction of 10a with 4-hydroxycoumarin
Table 1
N
Solvent
ET
Wavelength/nm
Ethyl acetate
Dichloromethane
Acetone
Acetonitrile
2-Propanol
Ethanol
0.228
0.309
0.355
0.460
0.540
0.654
0.762
1.000
600
598
584
580
546
538
532
524
Scheme 8
Methanol
Water
with the corresponding pyridines in acetic acid to obtain
the target compounds which are sparsely soluble in most organic
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can also form atropisomers caused by the restricted rotation
of the pyridine- and 2-oxochromen-4-olate structure elements.
These two isomeric forms are responsible for the occurrence of
a double set of signals in the ratio of 3 : 1. Unfortunately no more
precise information about the major product can be gained by
spectroscopic methods, as the atropisomers of 11a are hygroscopic
and display no OH protons in the 1H spectra.
Redox properties
To examine the synthetic potential of these quinones, compound
10a was employed in model oxidation reactions of hydroquinones
12a–c to the quinones 13a–c. In addition, we studied the aromati-
zation of 9,10-dihydroanthracene 14 to anthracene 15 (Scheme 11).
The quinones 13a–c were isolated in good to quantitative yields
under mild reaction conditions. Best results were achieved using
the 2,5-tert-butyl derivate 12c. To compare the oxidizing behaviour
of 10a with other widely used quinoid-type oxidizing reagents, the
aromatization of 9,10-dihydroanthracene 14 to anthracene 15 was
performed according to literature procedures.24,25 The reaction in
1,4-dioxane (reflux, 6 h) yielded the desired product 15 in 21%
yield, whereas in xylene (reflux 33 h) a yield of 44% was observed.
Under identical reaction conditions, the observed yields are lower
than for DDQ (21%/98%, respectively24) and for p-chloroanil
(44%/63%, respectively25).
Fig. 2 Effect of negative solvatochromism of 10a.
and the corresponding pyridine in aqueous acetone. The nonsym-
metric betaine 11b was prepared by two different methods. On
reaction of 10a with 2,4¢-bipyridine (method A), as well as of 10b
with pyridine (method B), 11b was formed in yields of 70% and
53%, respectively. After separation by column chromatography
a considerable amount of 7a (method A) and 7b (method B)
was found in the reaction mixture which indicates that during
the reaction the quinones 10a/10b serves as oxidizing reagents
(Scheme 10). To get more information about the regiochemistry,
the resulting compounds were analyzed by 2D NMR spectroscopy.
In the 1H spectrum of 11b four different signals of aromatic
OH protons could be observed at d 12.13, 12.09, 11.56 and
11.54 ppm in a ratio of 3 : 3 : 1 : 1, respectively, which indicates
the formation of two different isomers in a ratio of 3 : 1. In HMBC
measurements each individual OH proton shows a coupling
with two quaternary carbon signals of the ortho position at
approximately 144 ppm and 133 ppm, respectively. These signals
can be assigned to the hydroquinone carbon atoms bearing a
2-oxo-chromen-4-olate and a pyridinium moiety, respectively, so
that both set of signals can be attributed to the regioisomer in
which the pyridinium rings are attached in 2- and 5-position. As
described for compounds 7a–7f and 9a–9d, the products 11a/11b
Scheme 11
Electrochemical characterization
The open circuit potential (OCP) of a solution of 2 mmol l-1 of
compound 10a in a supporting electrolyte of 0.5 M NaCl was
close to 0.3 V vs. Ag/AgCl. Cyclic voltammograms measured in
that solution at different sweep rates are shown in Fig. 3. There
is one characteristic cathodic and a corresponding anodic peak.
The cathodic peak potential Ep,c is close to 0 V vs. Ag/AgCl,
whereas the anodic peak potential Ep,a is ~ 0.1 V. The difference
between the Ep,c and the corresponding half peak potential Ep/2 is
close to 80 mV. There is no shift in peak potentials with increasing
sweep rate. A plot of the (baseline corrected) current densities of
the cathodic peak versus the square root of the sweep rate can be
satisfactorily described by a line through the origin as shown in
Fig. 4 for concentrations of 2 mmol l-1 and ~1 mmol l-1.
Impedance spectra were recorded for several potentials between
0.3 and -0.1 V. Before each measurement the potential was held for
a while, so that electrochemical equilibrium could be established at
Scheme 10
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Fig. 5 Charge transfer resistance obtained from impedance measure-
ments for two concentrations of compound 10a.
Fig. 3 Cyclic voltammograms of compound 10a. The sweep rate was
varied between 0.005 and 0.1 V s-1.
EEC mechanism. The voltammograms in Fig. 3 clearly show the
behavior expected for a reversible redox system,27 and one obtains
a value of approximately 0.053 V vs. Ag/AgCl (0.25 V vs. NHE) for
E
1/2. Similar values are suggested by electrochemical impedance
spectroscopy (0.06 V vs. Ag/AgCl). This is more positive than
E1/2 of unsubstituted benzoquinone. Therefore compound 10a
is a stronger oxidant than benzoquinone. It has been shown
that the waves in the cyclic voltammograms of benzoquinone
apparently correspond to a 2 electron transfer process, which in
reality is composed of two consecutive electron transfers. The
redox potentials for the individual steps are so close than one
wave instead of two results in neutral solutions. This is the
reason why the peak-to-peak separation in Fig. 3 is larger than
expected for a two electron transfer reaction,27 in agreement with
findings for benzoquinone.28 From the slopes in Fig. 4 one can
estimate the diffusion coefficient of compound 10a in the NaCl
solution as ~8.4 ¥ 10-7 cm2 s-1 assuming a two electron transfer
reaction. The reaction rates are fast compared to the diffusion
rates, therefore kinetic constants are not easy to derive from cyclic
voltammetry. Those can be taken from impedance spectroscopy.
From Rct, exchange current densities as large as 1.3 mA cm-2 at
the standard potential were found, scaling nearly linearly with
the bulk concentration. The potential dependence of Rct can be
approximated by eqn (1), which does not take the two-step nature
of the redox process in consideration. It is a simplified combination
of the expressions for the exchange current density and the
Nernst equation, where a is the transfer coefficient, c0 the bulk
concentration of compound 10a, E the applied electrode potential,
E0 the standard potential, z the number of electrons transferred, F
the Faraday constant, and k a rate constant. The lines in Fig. 5 are
the fits to this equation, which matched the data reasonably well.
Fitting only worked assumingz = 2, whichagaindemonstrates that
2 electrons are involved in the cyclovoltammetric waves shown in
Fig. 3.
Fig. 4 Sweep rate dependence of cathodic peak current densities taken
from cyclic voltammograms recorded in a solution of compound 10a
for concentrations of 2 mmol l-1 (squares) and 1 mmol l-1 (circles). The
diffusion coefficient of compound 10a could be determined from linear fits
through the origin (lines) applying equation (6.2.19) from ref. 27.
the electrode surface. In the complex plane plot impedance spectra
simply consisted of a semicircular arc at high frequencies and a
linear portion at low frequencies. The diameter of the arc was
lowest around 0.06 V. The spectra could be fitted by a Randles-
type equivalent circuit26 composed of the solution resistance RS, a
constant phase element to describe the double layer capacitance of
the electrode/electrolyte interface, the charge transfer resistance
Rct and a Warburg impedance W to account for diffusion of
the oxidized and reduced species. A plot of Rct as a function of
potential is shown in Fig. 5.
The electrochemical behavior of compound 10a as shown in
Fig. 3–5 is very similar to the behavior of the parental compound
benzoquinone in unbuffered electrolyte solutions. The electro-
chemistry of this compound has been studied for a long time in
literature. Some references can be found in the works of Quan28 and
Laviron.29 It has been shown that in neutral unbuffered electrolyte
solutions as used in this study the reduction does not proceed as in
Scheme 1, which would correspond to an ECEC mechanism.28 In
that case the use of an unbuffered electrolyte would be connected
to a change in pH at the electrode surface which in turn would
change the redox potential. Instead the dianion of hydroquinone
is the reaction product, and the dianion afterwards reacts with
water to form some protonated compounds,28 corresponding to an
È
˘
zF
È
Î
˘
˚
RT 1 + exp -
E - E
(
)
0
Í
˙
Í
˙
RT
Î
˚
Rct
=
(1)
È
Í
Î
˘
˙
˚
a -1 zFE
(
)
zFE0
RT
z 2F2kc exp
+
0
RT
Thus, one can state that compound 10a is a stronger oxidant
than benzoquinone, but otherwise very similar in its behavior. It
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shows one reversible wave corresponding to two overlapping one-
electron transfers and high exchange current densities.
In summary we present donor-substituted punicin derivatives
which belong to the class of heterocyclic mesomeric betaines
possessing different types of conjugation, or which form tripolar
or tetrapolar betainic molecules. Push-pull substituted quinones
deriving from these systems are stable and can be used in
redox reactions. Thus, the betaines described here supplement the
interesting properties of the natural product punicin from Punica
granatum.
(1H, s), 9.00 (1H, s, J = 6.2 Hz), 8.56 (1H, d, J = 5.8 Hz), 8.51
(1H, t, J = 7.8 Hz), 8.06 (1H, t, J = 7.2 Hz), 7.91-7.84 (2H, m),
7.43 (1H, t, J = 7.8 Hz), 7.17 (1H, t, J = 8.1 Hz), 7.07 (1H, d,
J = 8.1 Hz), 6.99 (2H, d, J = 1.1 Hz); dC (50 MHz, DMSO-d6)
173.8, 162.3, 153.3, 150.4, 146.8, 145.8, 143.1, 131.0, 130.4, 126.3,
126.1, 124.9, 122.6, 121.9, 121.6, 120.8, 115.5, 115.1, 92.8 ppm;
nmax (KBr)/cm-1 3070, 1600, 1504, 1339, 1280, 892, 818, 759, 683;
the spectroscopic data correspond to the literature.22a
3-[3,6-Dihydroxy-2-(4-(pyridin-2-yl)pyridinium-1-yl)phenyl]-2-
oxo-2H-chromen-4-olate 7b. Method A (methanol); yield: 60%;
orange coloured solid; mp 214 ◦C (dec); Found: C, 67.57, H, 3.73,
N, 6.26 Calcd. for C25H16N2O5·H2O: C, 67.87, H, 4.10, N, 6.33%;
dH (200 MHz, DMSO-d6) 10.03 (1H, s), 9.73 (1H, s), 9.10 (1H, d,
J = 6.6 Hz), 8.84 (1 H, d, J = 4.8 Hz), 8.73 (1H, d, J = 6.6 Hz),
8.66 (1H, d, J = 6.6 Hz), 8.51 (1H, d, J = 6.6 Hz), 8.41 (1H, d,
J = 8.0 Hz), 8.07 (1H, t, J = 7.8 Hz), 7.90 (1H, d, J = 7.8 Hz),
7.64 (1H, t, J = 6.2 Hz), 7.42 (1H, t, J = 7.4 Hz), 7.17 (1H, t, J =
7.0 Hz), 7.07-6.96 (3H, m); dC (50 MHz, DMSO-d6) 173.9, 162.3,
153.3, 152.6, 150.8, 150.7, 150.4, 149.6, 147.1, 143.1, 138.2, 131.0,
130.0, 126.7, 124.9, 123.5, 122.6, 122.5, 122.2, 121.8, 121.6, 120.9,
115.6, 115.2, 93.0 ppm; nmax (KBr)/cm-1 3068, 1634, 1600, 1505,
1425, 1276, 1206, 1036, 900, 817, 762.
Experimental part
General remarks
The 1H and 13C NMR spectra were recorded on Bruker ARX-400
and DPX-200 spectrometers. Multiplicities are described by using
the following abbreviations: s = singlet, d = doublet, t = triplet,
m = multiplet, b = broad. FT-IR spectra were obtained on a
Bruker Vektor 22 in the range of 400 to 4000 cm-1 (2.5% pellets
in KBr). The ESI mass spectra were measured with an Agilent
LCMSD Series HP1100 with APIES. Samples were sprayed from
methanol at 0 V fragmentor voltage unless otherwise noted.
3-[3,6-Dihydroxy-2-(3-methylpyridinium-1-yl)phenyl]-2-oxo-
2H-chromen-4-olate 7c. Method A (methanol); yield: 75%; yel-
low solid; mp 207–209 ◦C (dec); Found: C, 66.14, H, 3.95, N, 3.64
Calcd. for C21H15NO5·H2O: C, 66.49, H, 4.52, N, 3.69%; m/z (EI)
362.1027 Calcd. For C21H15NO5 + [H+]: 362.1027; Isomer c¢ dH
(400 MHz, DMSO-d6) 10.01 (1H, s), 9.63 (1H, s), 8.81 (1H, d, J =
6.1 Hz), 8.54 (1H, s), 8.38 (1H, d, J = 6.1 Hz), 7.95 (1H, dd, J =
11.0 Hz, J = 6.2 Hz), 7.89 (1H, t, J = 6.9 Hz), 7.43 (1H, t, J = 6.9
Hz), 7.17 (1H, t, J = 6.9 Hz), 7.09 (1H, d, J = 8.2 Hz), 7.01-6.95
(2H, m), 2.23 (3H, s); Isomer c dH (400 MHz, DMSO-d6) 10.01
(1H, s), 9.69 (1H, s), 8.99 (1H, s), 8.35 (2H, d, J = 8.1 Hz), 7.89
(1H, t, J = 6.9 Hz), 7.78 (1H, dd, J = 7.9 Hz, J = 6.2 Hz), 7.43
(1H, t, J = 6.9 Hz), 7.17 (1H, t, J = 6.9 Hz), 7.09 (1H, d, J =
8.2 Hz), 7.01-6.95 (2H, m), 2.50 (3H, s); dC (100 MHz, DMSO-d6)
173.8, 173.7, 162.4, 162.3, 153.3, 150.4, 150.3, 149.8, 147.5, 146.2,
146.1, 146.0, 144.0, 143.2, 136.7, 136.5, 131.0, 131.0, 130.5, 130.4,
125.7, 125.5, 124.9, 124.9, 122.6, 122.6, 122.1, 122.1, 121.7, 120.8,
120.6, 115.6, 115.6, 115.2, 115.1, 93.0, 92.7, 17.7, 17.4 ppm; nmax
(KBr)/cm-1 3060, 1600, 1524, 1330, 1281, 808, 760.
Electrochemical measurements
100 mL of a 2 mmol L-1 solution of compound 10a in 0.5 M
aqueous NaCl solution were used for electrochemical charac-
terization by cyclic voltammetry and impedance spectroscopy.
Afterwards the solution was diluted by replacing 50 mL of the
solution by 50 mL of 0.5 M NaCl, and experiments were repeated.
Experiments were performed at 25 ◦C in a closed teflon cell
inserted in a steel cooling jacket connected to a thermostat.
The cell was deaerated by purging with Ar gas before and in
between the measurements. As working electrode a gold electrode
on a typical quartz resonator for the electrochemical quartz
crystal microbalance technique was used. The electrode area
was 0.287 cm2. Reference electrode was a silver/silver chloride
electrode (E◦ = 0.197 V vs. NHE), counter electrode a Pt-clad Nb-
mesh. Before the experiments background currents were measured
using a pure 0.5 M NaCl solution. Cyclic voltammetry was
performed using a Solartron SI1287 electrochemical interface in
a potential range of -0.7 to 0.5 V, starting at the open circuit
potential of 0.3 V with the cathodic sweep. The sweep rate was
varied between 5 mV s-1 and 100 mV s-1. Impedance measurements
were carried out at applied potentials between 0.3 and -0.1 V in
a frequency range between 10 KHz and 0.05 Hz with a Zahner
Im5d impedance measurement unit.
3-[3,6-Dihydroxy-2-(isoquinolinium-2-yl)phenyl]-2-oxo-2H-chr-
omen-4-olate 7d. Method A (methanol); yield: 70%; yellow solid;
mp 221 ◦C (dec); Found: C, 69.92, H, 3.59, N, 3.35 Calcd. for
3C24H15NO5·2H2O: C, 70.41, H, 4.02, N, 3.42%; m/z (EI) 398.1024
Calcd. for C24H15NO5 + [H+]: 398.1028; Isomer d¢ dH (400 MHz,
DMSO-d6) 10.04 (1H, s), 9.88 (1H, s), 9.54 (1H, s), 8.59 (1H, d,
J = 6.9 Hz), 8.39 (2H, m), 8.24 (1H, m), 8.17 (1H, m), 7.86 (1H,
t, J = 7.4 Hz), 7.75 (1H, d, J = 7.7 Hz), 7.34 (1H, t, J = 8.2 Hz),
7.10-6.96 (4H, m); Isomer d dH (400 MHz, DMSO-d6) 10.24 (s,
1H), 10.03 (s, 1H), 9.84 (s, 1H), 8.69 (d, J = 8.3 Hz, 1H), 8.24
(m, 3H), 8.17 (m, 1H), 8.03 (m, 1H), 7.95 (d, J = 7.9 Hz, 1H),
7.40 (t, J = 7.4 Hz, 1H), 7.19 (t, J = 7.7 Hz, 1H), 7.10-6.96 (m,
3H); dC (100 MHz, DMSO-d6) 174.2, 173.5, 162.8, 162.0, 156.5,
153.3, 153.2, 152.6, 150.5, 150.2, 143.5, 139.6, 137.4, 137.2, 136.9,
136.8, 136.6, 131.1, 131.1, 130.9, 130.7, 130.4, 130.4, 127.2, 127.0,
126.6, 126.5, 125.0, 125.8, 123.8, 123.1, 122.7, 122.5, 122.2, 122.1,
General procedure for the preparation of the betaines 7a–7f
Samples of 810 mg of 4-hydroxycoumarin (5 mmol), 1.08 g p-
benzoquinone (10 mmol) and 0.8 mL pyridine (10 mmol) were
dissolved in a mixture of acetone/water (v/v = 1 : 1) and stirred at
r.t. for 24 h. The resulting solids were purified by recrystallization
(method A) or column chromatography (method B).
3-[3,6-Dihydroxy-2-(pyridinium-1-yl)phenyl]-2-oxo-2H-chro-
men-4-olate 7a. Method A (methanol); yield: 73%; yellow solid;
mp 209–211 ◦C (dec); dH (200 MHz, DMSO-d6) 9.96 (1H, s), 9.67
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121.7, 121.6, 120.9, 120.6, 115.6, 115.5, 115.2, 93.9, 92.3 ppm; nmax
spectra could not be measured because of the insufficient solubility
of the product in organic solvents; nmax (KBr)/cm-1 1684, 1608,
1500, 1428, 1211, 1041, 761.
(KBr)/cm-1 3060, 1600, 1524, 1330, 1281, 808, 760.
3-[3,6-Dihydroxy-2-(4-(pyridin-4-yl)pyridinium-1-yl)phenyl]-2-
oxo-2H-chromen-4-olate 7e. Method B (chloroform–methanol
7 : 1); yield: 74%; red solid; mp 203–205 ◦C (dec); Found: C, 68.22,
H, 3.75, N, 6.28 Calcd. for C25H16N2O5·H2O: C, 67.87, H, 4.20, N,
6.33%; dH (200 MHz, DMSO-d6) 10.10 (1H, s), 9.72 (1H, s), 9.17
(1H, d, J = 6.6 Hz), 8.81 (2H, d, J = 6.1 Hz), 8.75 (1H, d, J = 6.6
Hz), 8.57 (1H, d, J = 6.6 Hz), 8.37 (1H, d, J = 6.6 Hz), 8.00 (2H,
d, J = 6.1 Hz), 7.91 (1H, d, J = 7.8 Hz), 7.42 (1H, t, J = 7.8 Hz),
7.17 (1H, t, J = 8.1 Hz), 7.07 (1H, d, J = 8.1 Hz), 7.02 (2H, d, J =
1.6 Hz) ppm; dC (50 MHz, DMSO-d6) 174.0, 162.4, 153.3, 152.1,
151.0, 150.9, 150.4, 147.3, 143.2, 140.5, 131.0, 130.0, 124.9, 123.4,
123.1, 122.6, 121.8, 121.7, 121.7, 121.0, 115.6, 115.2, 92.9 ppm;
nmax (KBr)/cm-1 3050, 1635, 1598, 1509, 1273, 1205, 810, 763.
3-[3,6-Dihydroxy-2-(4-hydroxy-2-oxo-2H-chromen-3-yl)-4-(4-
(pyridin-4-yl)pyridinium-1-yl)phenyl]-2-oxo-2H-chromen-4-olate
9c. Method B (chloroform–methanol 1 : 1); yield: 34%; red
brown solid; mp > 305 ◦C; m/z (EI) 585.1299 Calcd. for
C34H20N2O8 + [H+]: 585.1298; dH (400 MHz, DMSO-d6) 9.26 (2H,
s), 8.90 (2H, s), 8.70 (2H, d, J = 16 Hz), 8.10 (2H, s), 7.93 (2H,
m), 7.37 (2H, t, J = 7.5 Hz), 7.16 (1H, m), 7.02 (3H, m); The OH-
protons are not detected. dC (100 MHz, DMSO-d6) 172.5, 171.7,
161.9, 161.7, 153.4, 153.3, 151.9, 151.0, 150.6, 150.6, 146.6, 144.3,
143.2, 141.0, 132.4, 129.9, 129.8, 129.1, 124.9, 124.8, 122.7, 122.3,
122.1, 122.0, 121.8, 121.3, 115.0, 111.1, 101.7, 100.8 ppm; IR nmax
(KBr)/cm-1 2924, 1637, 1601, 1515, 1408, 1211, 1045, 815, 761.
3-[3,6-Dihydroxy-2-(4-methylpyridinium-1-yl)phenyl]-2-oxo-
2H-chromen-4-olate 7f. Method A (methanol); yield: 51%; light
brownish solid; mp 256 ◦C (dec); Found: C, 69.19, H, 3.81, N, 3.75
Calcd. for C21H15NO5: C, 69.80, H, 4.18, N, 3.88%; dH (200 MHz,
DMSO-d6) 9.95 (1H, s), 9.65 (1H, s), 8.82 (1H, d, J = 6.3 Hz),
8.39 (1H, d, J = 6.3 Hz), 7.89 (2H, m), 7.69 (1H, d, J = 6.4 Hz),
7.43 (1H, t, J = 7.8 Hz), 7.18 (1H, t, J = 8.1 Hz), 7.09 (1H, d,
J = 8.1 Hz), 6.96 (2H, d, J = 1 Hz), 2.57 (3H, s); dC (50 MHz,
DMSO-d6) 173.8, 162.2, 159.1, 153.3, 150.3, 149.1, 145.5, 143.2,
130.9, 130.0, 126.7, 126.4, 124.9, 122.5, 122.0, 121.7, 120.5, 115.6,
115.0, 93.0, 21.5 ppm; nmax (KBr)/cm-1 3060, 1640, 1598, 1506,
1350, 1276, 1204, 1032, 831, 806, 767.
3-[3,6-Dihydroxy-2-(4-hydroxy-2-oxo-2H-chromen-3-yl)-4-(4-
methylpyridinium-1-yl)phenyl]-2-oxo-2H-chromen-4-olate
9d.
Method A (methanol); yield: 43%; light brown solid; mp
248 ◦C (dec); Found: C, 64.44, H, 3.74, N, 2.47 Calcd. for
C30H19NO8·2H2O: C, 64.63, H, 4.16, N, 2.51%; dH (400 MHz,
DMSO-d6) 9.02 (4H, d, J = 6.4 Hz), 8.08 (4H, m), 7.97 (1H, dd,
J = 7.8 Hz, J = 1.2 Hz), 7.87 (1H, dd, J = 7.8 Hz, J = 1.4 Hz),
7.82 (1H, dd, J = 7.8 Hz, J = 1.4 Hz), 7.77 (1H, dd, J = 7.8 Hz,
J = 1.2 Hz), 7.52-7.40 (4H, m), 7.27 (1H, t, J = 7.6 Hz), 7.22
(3H, m), 7.18-7.09 (4H, m), 7.06 (1H, s) 7.03 (1H, s), 2.70 (3H,
s), 2.69 (3H, s); the OH-protons are not detected. dC (100 MHz,
DMSO-d6) 170.6, 165.2, 162.0, 161.6, 161.3, 160.9, 160.0, 159.7,
153.2, 153.0, 152.9, 152.8, 149.8, 149.1, 145.3, 145.2, 142.5, 141.5,
131.2, 131.1, 130.9, 130.6, 129.9, 129.0, 128.1, 128.0, 127.4, 124.5,
124.2, 124.0, 123.1, 123.0, 122.7, 122.6, 120.8, 118.9, 115.8, 115.6,
115.5, 110.7, 102.2, 99.6, 98.8, 21.7 ppm; IR nmax (KBr)/cm-1
1696, 1609, 1505, 1447, 1211, 1047, 812, 760.
General procedure for the reaction of compound 8 with pyridines
Samples of 836 mg of 8 (1.95 mmol) were dissolved in 40 mL of
acetic acid. Then, 0.78 mL of pyridine (9.75 mmol) were added
and the mixture was heated at reflux temperature for 4 h. The
resulting solid was purified by recrystallization (method A) or
column chromatography (method B).
General procedure for the synthesis of the quinones
Samples of the corresponding betaine (1.88 mmol) were suspended
in 30 mL of a mixture of acetonitril–water (v/v = 1 : 2). Then
2.13 g of cerium(IV) ammonium nitrate (3.89 mmol) were added.
The mixture was stirred at r.t. for 1 h and the aqueous phase
was extracted with four portions of 50 mL of dichloromethane.
After drying over MgSO4 the product was purified by column
chromatography (dichloromethane–methanol 30 : 1).
3-[3,6-Dihydroxy-2-(4-hydroxy-2-oxo-2H-chromen-3-yl)-4-(pyr-
idinium-1-yl)phenyl]-2-oxo-2H-chromen-4-olate 9a. M◦ethod A
(DMF–DMSO 4 : 1); yield: 60%; yellow solid; mp 270 C (dec);
Found: C, 64.88, H, 3.55, N, 3.12 Calcd. for 2C29H17NO8·3H2O:
C, 65.17, H, 3.77, N, 2.62%; dH (200 MHz, DMSO-d6) 9.21 (2H,
d, J = 6.6 Hz), 8.74 (1H, t, J = 7.4 Hz), 8.28 (2H, t, J = 6.6
Hz), 7.95–7.79 (2H, m), 7.50-7.36 (2H, m), 7.24-7.07 (5H, m)
ppm; The OH-protons are not detected. 13C- spectra could not
be measured because of the insufficient solubility of the product
in organic solvents; nmax (KBr)/cm-1 3064, 1694, 1604, 1504, 1459,
1202, 1046, 952, 766.
3-[3,6-Dioxo-2-(pyridinium-1-yl)cyclohexa-1,4-dienyl]-2-oxo-
2H-chromen-4-olate 10a. Yield: 67%; dark blue solid; mp 196–
◦
197 C (dec); m/z (EI) 346.0717 Calcd. for C20H11NO5 + [H+]:
346.0715; dH (200 MHz, DMSO-d6) 8.92 (2H, s), 8.59 (1H, t, J =
7.8 Hz), 8.09 (2H, t, J = 7.0 Hz), 7.59 (1H, dd, J = 8.0 Hz, J = 1.9
Hz), 7.44 (1H, t, J = 7.4 Hz), 7.23 (1H, d, J = 10.2 Hz), 7.13-7.06
(3H, m); dC (50 MHz, DMSO-d6) 184.8, 178.6, 172.9, 161.6, 153.8,
146.8, 144.1, 138.5, 138.4, 135.1, 131.7, 126.5, 124.9, 122.6, 121.5,
116.0, 89.5 ppm; IR nmax (KBr)/cm-1 3067, 1666, 1600, 1537, 1470,
1397, 1310, 1275, 1109, 843, 764, 687.
3-[3,6-Dihydroxy-2-(4-hydroxy-2-oxo-2H-chromen-3-yl)-4-(4-
(pyridin-2-yl)pyridinium-1-yl)phenyl)-2-oxo-2H-chromen-4-olate
9b. Method A (acetone–methanol 1 : 1); yield: 39%; orange
coloured solid; mp 245 ◦C (dec); Found: C, 67.21, H, 3.30, N,
4.68 Calcd. for 2C34H20N2O8·3H2O: C, 66.77, H, 3.79, N, 4.58%;
dH (400 MHz, DMSO-d6) 9.26 (4H, d, J = 6.6 Hz), 8.90 (6H, m),
8.51 (2H, t, J = 6.8 Hz), 8.13 (2H, t, J = 7.8 Hz), 7.92 (1H, d, J =
8.0 Hz), 7.89 (1H, d, J = 8.0 Hz), 7.83 (1H, d, J = 7.8 Hz), 7.78
(1H, d, J = 7.8 Hz), 7.69 (2H, dd, J = 6.9 Hz, J = 4.8 Hz), 7.50
(2H, t, J = 7.5 Hz), 7.42 (2H, m), 7.29-7.20 (4H, m), 7.17-7.09
(6H, m) ppm; The OH-protons could be not detected. 13C NMR
3-[3,6-Dioxo-2-(4-(pyridin-2-yl)pyridinium-1-yl)cyclohexa-
1,4-dienyl]-2-oxo-2H-chromen-4-olate 10b. Yield: 81%; dark
blue solid; mp 129–131 ◦C; m/z (EI) 423.0982 Calcd. for
C25H14N2O5 + [H+]: 423.0981; dH (200 MHz, DMSO-d6) 9.01 (2H,
d, J = 5.1 Hz), 8.83 (1H, dd, J = 4.7 Hz, J = 0.9 Hz), 8.74 (2H,
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d, J = 6.8 Hz), 8.40 (1H, d, J = 8.0 Hz), 8.08 (1H, dt, J = 7.8 Hz,
J = 1.7 Hz), 7.64 (2H, m), 7.42 (1H, t, J = 7.7 Hz), 7.25 (1H, d,
J = 10.2 Hz), 7.15-7.03 (3H, m); dC (50 MHz, DMSO-d6) 184.8,
178.7, 173.0, 161.6, 153.9, 153.5, 150.7, 149.5, 147.2, 144.0, 138.4,
138.2, 135.2, 131.7, 126.8, 124.9, 123. 7, 122.7, 122.6, 121.6, 116.0,
89.6 ppm; IR nmax (KBr)/cm-1 3040, 1666, 1634, 1599, 1533, 1462,
1395, 1308, 1277, 1110, 838, 761.
(400 MHz, DMSO-d6) 12.14 (3H, s), 12.09 (3H, s), 11.57 (1H,
s), 11.54 (1H, s), 9.31 (3H, d, J = 6.8 Hz), 9.27 (3H, d, J = 6.2
Hz), 9.21 (1H, d, J = 7.9 Hz), 9.12 (1H, d, J = 6.2 Hz), 8.91-8.82
(10H, m), 8.79 (3H, d, J = 7.2 Hz), 8.71 (3H, d, J = 6.2 Hz), 8.63
(8H, m), 8.48 (4H, m), 8.19 (4H, m), 8.13-8.08 (4H, m), 8.00-7.89
(12H, m), 7.52-7.45 (8H, m), 7.24 (8H, t, J = 7.6 Hz), 7.16 (4H, d,
J = 8.3 Hz), 7.12 (4H, d, J = 8.2 Hz); dC (100 MHz, DMSO-d6)
174.5, 174.4, 174.1, 174.0, 162.4, 162.4, 161.8, 161.7, 153.2, 153.2,
153.1, 151.7, 151.4, 150.8, 150.3, 149.6, 146.5, 146.4, 146.0, 144.4,
144.3, 144.1, 138.3, 133.7, 133.4, 133.2, 132.8, 131.7, 126.9, 126.7,
126.5, 126.3, 126.2, 124.9, 123.8, 123.8, 123.1, 122.8, 122.2, 121.8,
121.6, 121.4, 121.1, 121.0, 120.9, 115.8, 115.8, 93.8, 93.7, 93.2,
93.0 ppm; IR nmax (KBr)/cm-1 3043, 2925, 1667, 1635, 1603, 1506,
1394, 1270, 1107, 992, 759, 677.
3-[2-(3-Methylpyridinium-1-yl)-3,6-dioxocyclohexa-1,4-dienyl]-
2-oxo-2H◦-chromen-4-olate 10c. Yield: 82%; dark blue solid; mp
179–182 C; m/z (EI) 360.0876 Calcd. for C21H13NO5 + [H+]:
360.0872; dH (200 MHz, DMSO-d6) 8.90 (1H, s), 8.73 (1H, d, J =
4.7 Hz), 8.44 (1H, d, J = 8.1 Hz), 7.98 (1H, t, J = 7.2 Hz), 7.61
(1H, dd, J = 7.9 Hz, J = 1.8 Hz), 7.45 (1H, t, J = 8.2 Hz), 7.24
(1H, d, J = 10.2 Hz), 7.14-7.07 (3H, m), 2.44 (3H, s); dC (50 MHz,
DMSO-d6) 184.8, 178.7, 173.1, 161.5, 153.8, 147.2, 146.1, 144.4,
143.9, 138.5, 136.9, 135.1, 131.7, 126.0, 124.9, 122.6, 121.6, 116.0,
89.6, 17.7 ppm; IR nmax (KBr)/cm-1 3063, 2925, 1667, 1650, 1602,
1535, 1460, 1401, 1310, 1277, 1113, 842, 763, 687.
General procedure for the synthesis of quinones 13a–13c
The corresponding hydrochinone (0.55 mmol) and 189 mg
(0.55 mmol) of quinone 10a were dissolved in a mixture of
dichloromethane–methanol (v/v = 5 : 1). After stirring at r.t. for
2.5 h the solvent was evaporated and the residue was purified
by column chromatography (chloroform–methanol 25 : 1). The
quinones 13a–13c were obtained analytically pure as yellow solids.
3-[2-(Isoquinolinium-2-yl)-3,6-dioxocyclohexa-1,4-dienyl]-2-
oxo-2H-chromen-4-olate 10d. Yield: 54%; dark blue solid; mp
191–193 ◦C (dec); m/z (EI) 396.0873 Calcd. For C24H13NO5 +
[H+]: 396.0872; dH (200 MHz, DMSO-d6) 10.16 (1H, s), 8.58 (1H,
d, J = 8.3 Hz), 8.40 (2H, m), 8.27 (2H, m), 8.05 (1H, m), 7.41
(1H, t, J = 8.0 Hz), 7.28 (1H, d, J = 10.2 Hz), 7.16 (1H, d, J =
10.8 Hz), 7.08-7.03 (3H, m) ppm; 13C NMR spectra could not
be measured because of the insufficient solubility of the product
in organic solvents; IR nmax (KBr)/cm-1 3058, 1665, 1643, 1600,
1536, 1461, 1393, 1308, 1277, 1110, 839, 759.
Acknowledgements
Dr GeraldDra¨ger, University ofHannover (Germany)is gratefully
acknowledged for measuring the HRESIMS spectra.
References
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of acetone–water (v/v = 1 : 1). Then, 98 mg of 2,4¢-bipyridine
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oxo-2H-chromen-4-olate) 11a. Yield: 16%; light orange colored
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d, J = 8.1 Hz); The OH-protons are not detected. dH (200 MHz,
MeOD) 8.88 (4H, d, J = 5.8 Hz), 8.78 (4H, t, J = 4.4 Hz), 8.42
(4H, t, J = 7.9 Hz), 8.02 -7.85 (12H, m), 7.45 (4H, t, J = 7.7 Hz),
7.21 (4H, d, J = 7.4 Hz), 7.12 (4H, d, J = 8.2 Hz); dC (100 MHz,
DMSO-d6) 172.9, 172.8, 162.6, 162.5, 153.6, 148.4, 146.6, 144.1,
132.9, 130.2, 125.6, 125.5, 124.9, 124.7, 124.6, 122.5, 122.0, 115.4,
93.1, 93.0 ppm; IR nmax (KBr)/cm-1 1630, 1599, 1520, 1439, 1269,
1220, 1045, 761, 683.
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