Oxidation of cationic 1,4-dihydropyridine derivatives as model compounds for putative gene delivery agents

Article abstract of DOI:10.1016/j.tet.2009.08.012

A new synthetic approach to cationic pyridine derivatives is described here. Two different strategies for the synthesis of 1,1′-{[3,5-bis(ethoxycarbonyl)-4-phenylpyridine-2,6-diyl]dimethylene}bispyridinium salts have been developed. The key step of the fi

Full text of DOI:10.1016/j.tet.2009.08.012

Tetrahedron 65 (2009) 8344–8349
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Oxidation of cationic 1,4-dihydropyridine derivatives as model compounds for
putative gene delivery agents
Aiva Plotniece, Karlis Pajuste, Dainis Kaldre, Brigita Cekavicus, Brigita Vigante, Baiba Turovska,
*
Sergey Belyakov, Arkadij Sobolev , Gunars Duburs
Latvian Institute of Organic Synthesis, Aizkraukles 21, Riga LV-1006, Latvia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 May 2009
Received in revised form 17 July 2009
Accepted 7 August 2009
Available online 11 August 2009
A new synthetic approach to cationic pyridine derivatives is described here. Two different strategies for
the synthesis of 1,10-{[3,5-bis(ethoxycarbonyl)-4-phenylpyridine-2,6-diyl]dimethylene}bispyridinium
salts have been developed. The key step of the first strategy relies on electrochemical and chemical
oxidation of cationic 1,4-dihydropyridines; the second one involves nucleophilic substitution of pyridine
dibromo derivatives.
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Cationic 1,4-dihydropyridines and pyridines
Oxidation
Gene delivery
1. Introduction
amphiphilic 1,4-dihydropyridine derivatives were investigated and
elaborated.18–20
1,4-Dihydropyridine (1,4-DHP) derivatives are a group of com-
pounds that play an important role in synthetic, medicinal and
bioorganic chemistry.1 Derivatives of 1,4-dihydropyridines are
well-known as calcium channel modulators for the treatment of
cardiovascular disorders.2–4 It is worth underlining that, the 1,4-
DHP nucleus is a privileged structure or scaffold that can interact
when appropriately decorated with substituents, at diverse re-
ceptors and ion channels.5 1,4-DHP derivatives possess a broad
range of other biological activities, such as antioxidant,6 anti-in-
flammatory,7 radioprotective,8 anticancer,9 antidiabetic,10 immu-
nomodulatory,11 neuroprotective,12 antibacterial,13 antiviral,14 and
reversal of multidrug resistance.15 However, 1,4-DHPs with these
activities might possess also Ca2þ regulating activity, which can be
considered as a serious side effect and could cause problems in
drug development and promotion in the future. Modern strategies
for the synthesis of pharmacologically active 1,4-DHPs include
minimisation of their Ca2þ antagonistic activity.16 Investigations of
the various dihydropyridine derivatives as a novel carrier for spe-
cific delivery of drugs to the brain, where 1,4-DHP crosses blood–
brain barrier, and oxidises there to quaternary pyridinium salts
were also described.17 During the last decade new and efficient
gene delivery systems based on cationic self-assembling
The oxidation of 1,4-dihydropyridines to their corresponding
pyridine derivatives is the most typical and general reaction for this
heterocyclic system. One of the main metabolic pathways of
biologically active 1,4-dihydropyridine derivatives is the oxidation
to their corresponding pyridines, with Cytochrome P450 (CYP)21,22
as catalyst. Several groups of scientists have developed various
methods for aromatisation of 1,4-DHPs and discussed the mecha-
nism of oxidation.23,24 New aspects of aromatisation of 1,4-DHPs,
both electrochemical25,26 and chemical, have been widely studied.
First, for the chemical oxidation of 1,4-dihydropyridines, nitric
acid27 and in situ generated nitric oxide28,29 were used as powerful
oxidising agents. Numerous new inorganic reagents and pro-
cedures have been developed for this purpose, for example I2,30
solid acids including OxoneÒ, HIO3, HIO6 and polystyrenesulfonic
acid,31 different chromium compounds,32,33 KMnO4,34 Zr(NO3)4,35
Bi(NO3)3,36 Mn(OAc)3,37 Pb(OAc)4,38 RuCl3,39 Fe(ClO4)3/AcOH40 etc.
Use of microwave-assisted oxidation of 1,4-DHPs was also repor-
ted.41 Furthermore, Hantzsch 1,4-dihydropyridine is widely used as
a safe, easy to handle and environmentally benign reagent for the
reduction of organic functional groups.42
2. Results and discussion
Among the wide range of studies on aromatisation of 1,4-DHP
derivatives, there is a lack of data about oxidation of cationic 1,4-
dihydropyridines. The elaboration for the synthesis of cationic
* Corresponding author. Tel.: þ371 67014928; fax: þ371 67550338.
E-mail address: arkady@osi.lv (A. Sobolev).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.08.012

A. Plotniece et al. / Tetrahedron 65 (2009) 8344–8349
8345
pyridines from 4-phenyl substituted Hantzsch 1,4-dihydropyr-
idines is indispensable for further investigations of the possible
metabolic pathways of cationic 1,4-dihydropyridine derivatives and
clarification of putative structure–activity relationship. The syn-
thesis of first examples of cationic 1,4-dihydropyridine derivatives
with short appendages in the positions 3 and 5 has been performed.
Our synthetic plan is shown in Scheme 1.
it was previously employed for oxidation of several heterocyclic
systems.51 Some successful examples of oxidation of 4a,b are
given in Table 1.
Experimental studies of a variety of reaction conditions, for
instance, the amount of oxidising agent, the solvent, reaction
temperature and time, showed that the best found reaction con-
ditions for conversion of cationic 1,4-dihydropyridine bromide 4a
O
O
Ph
O
O
Ph
H
H
Et
Et
Br
Ph
Et
Et
O
O
O
O
(d)
O
O
H
(c)
Et
Et
Ph
O
O
O
O
N
N
Et
Et
N+
N+
H
ClO4-
ClO4-
H
O
O
N+
N+
BrH2C
N
H
CH2Br
95%
Br
63%
N
1
2
4a
4b
(a)
(f)
(h)
48%
32%
(e)
63%
63%
37%
(f)
(b)
60%
(g)
54%
O
Ph
N
O
O
Ph
N
O
Et
Et
Ph
O
O
Et
Et
O
O
O
O
(c)
(d)
Et
Et
O
O
N+
N+
N+
N+
ClO4-
ClO4-
BrH2C
N
CH2Br
Br
Br
85%
57%
3
5b
5a
Scheme 1. Two strategies for the synthesis of cationic pyridine derivatives. Reagents and conditions: (a) Br2/AcOH at 50 ꢀC, 5 h; (b) NaNO2/AcOH at 50 ꢀC, then at rt, 1.5 h; (c)
pyridine in dry acetone, at rt, 3 h; (d) 57% HClO4; hot EtOH, then at rt, 3 h; (e) anodic oxidation in MeCN/0.1 M NaClO4; (f) NBS, MeOH, at 70 ꢀC, 10 h; (g) 10% Pd/C, EtOAc/MeCN at
80 ꢀC, 60 h; (h) SeO2, AcOH at 50 ꢀC, 5 h.
The compound 1 was obtained according to the classical
method.43 Previously we have reported that the methyl groups of
4-phenyl-3,5-diethoxycarbonyl-2,6-dimethyl-1,4-dihydropyridine43
were brominated with NBS in methanol at rt to form dibromo de-
rivative 2.44 This contrasts sharply with recent report where NBS at
rt in 5 min oxidised 4-phenyl substituted Hantzsch 1,4-dihy-
dropyridines to the corresponding pyridines.45
Recently, we have reported electrochemical oxidation of cat-
ionic 1,4-DHPs.46 Now we report the first example of chemical
oxidation of cationic 1,4-DHP as model compound for putative gene
delivery agents (Scheme 1).
or perchlorate 4b to the corresponding cationic pyridine 5a or 5b
in reasonable yields were observed when NBS in MeOH was used
(entries 1 and 2). Cationic pyridines 5a and 5b were accessible in
48% and 63% isolated yields, respectively (Scheme 1). Complete
oxidation49 of neutral 4-phenyl substituted Hantzsch 1,4-DHP
with 20 wt% of 10% Pd/C in AcOH occurred at 80 ꢀC only in 2 h.
Oxidation of cationic 1,4-dihydropyridine bromide 4a under
similar conditions gave only 23% of conversion to pyridine 5a in
21 h. Full conversion of cationic 1,4-dihydropyridine perchlorate
4b into cationic pyridine 5b was also achieved with 100 wt% of
10% Pd/C (entry 4) with isolated yield of 32% (Scheme1), while in
the case of corresponding bromide 4a at the same conditions only
50% of 1,4-dihydropyridine was oxidised (entry 3). It was possible
to reach complete oxidation of 4-phenyl substituted neutral
Hantzsch 1,4-DHP with only 1 equiv of SeO2 at rt in less than
There are two strategies to these cationic pyridines:
1. Oxidation of cationic 1,4-DHPs 4a,b:
a) Direct chemical oxidation of cationic 1,4-DHPs 4a,b;
b) Direct electrochemical oxidation of cationic 1,4-DHP 4b.
2. Nucleophilic substitution of compound 3, which can be
obtained via bromination of 4-phenyl-3,5-dialkoxycarbonyl-
2,6-dimethylpyridine 1 or oxidation of the dibromo de-
rivative 2.
Table 1
Studies of oxidation of cationic 1,4-dihydropyridines 4a,b
Entry Compound Oxidant
Solvent
Time, Temp, Ratio
h
ꢀC
(DHP:Py), %a
1
2
3
4a
4b
4a
2.6 equiv NBS MeOH
2.6 equiv NBS MeOH
100 wt% of 10% EtOAc/MeCN
10
10
60
70
70
80
0:100
0:100
50:50
We have examined various oxidants for oxidation of 4-phenyl-
3,5-dialkoxycarbonyl-2,6-dimethyl-1,4-dihydropyridine cationic
derivatives 4a,b. Our initial attempts to oxidise cationic 1,4-DHP
derivative 4a with in situ generated nitric oxide47 were
unsuccessful and led only to destruction of compound 4a,
according to NMR data. Unsuccessful experiments with classical
strong oxidising reagents led us to investigate other possibilities
of oxidation of cationic 1,4-dihydropyridine derivatives with
various reagents previously used for oxidation of neutral
1,4-DHPs, such as tetrachloro-p-benzoquinone (p-chloranil),48
Pd/C
(3:1)
4
4b
100 wt% of 10% EtOAc/MeCN
60
80
0:100
Pd/C
(3:1)
5
6
4a
4b
4 equiv SeO2
4 equiv SeO2
AcOH
AcOH/EtOH
(5:1)
5
5
50
50
0:100
70:30
7
8
4a
4b
1 equiv
THF/EtOH (1:1)
6
6
60
60
75:25
95:5
p-Chloranil
1 equiv
THF/EtOH (1:1)
p-Chloranil
50
10% Pd/C,49 and SeO2. NBS was also tested as oxidising agent as
The ratio was determined by HPLC and by 1H NMR spectra.
a

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A. Plotniece et al. / Tetrahedron 65 (2009) 8344–8349
1 h.50 Oxidation of the cationic 1,4-dihydropyridine bromide 4a
under similar conditions did not occur and only with heating at
50 ꢀC for 3 h oxidation proceeded to approx. 30%. Complete con-
version of 4a into pyridine 5a was obtained with 4 equiv SeO2
(entry 5); however, the conversion of perchlorate 4b was only
partial (entry 6), most likely due to the change of solvent from
acetic acid to ethanol/acetic acid mixture to improve solubility of
4b. It has been observed previously that the use of ethanol as
a solvent was ineffective for oxidation of Hantzsch 1,4-dihy-
dropyridines using stoichiometric selenium dioxide at ambient
temperature.50 In our hands it was possible to isolate only re-
action products containing residual elemental Se. Oxidation of the
cationic 1,4-dihydropyridine salts with p-chloranil was partial and
proceeded to 25% for bromide 4a (entry 7) and only to 5% for
perchlorate 4b (entry 8). Prolongation of reaction time did not
lead to a higher yield of pyridine.
Controlled potential electrolysis of 4b was performed at 1.70 V
in deaerated MeCN. Coulonometric measurements taken during
the exhaustive electrolysis indicate that electrochemical oxidation
of 4b is a two-electron process. According to the literature,57,58
cation radicals, the products of the first electron transfer, are more
acidic, compared with the parent neutral 1,4-DHPs. Moreover, the
presence of two pyridinium cations in the 2,6-positions of the 1,4-
DHP acts in the same way46 increasing the acidity of N–H bond. Our
assumption is that the transfer of the first electron is followed by
fast elimination of N–H proton and electrochemical oxidation of 4b
proceeds as the subsequent transfer of electrons and protons, de-
scribed as ECEC (Edelectron transfer; Cdproton transfer)
mechanism.
For the compound 5b, monocrystals were obtained, which were
characterised by single-crystal X-ray analysis, confirming the NMR
data. Figure 2 illustrates the structure of cation 5b. The calculations
for the crystal structure were carried out with the complex of
programs,59,60 and general crystallographic parameters of 5b are
given in Supplementary data. For further details, see crystallo-
graphic data for 5b deposited with the Cambridge Crystallographic
Data Centre as Supplementary Publication CCDC 723087.
The second synthetic approach to these cationic pyridine de-
rivatives involves as the key step nucleophilic substitution of bro-
mine of pyridine 3. We have elaborated rather efficient oxidation of
the dibromo derivative 2 to compound 3 using sodium nitrite in
acetic acid (5 min, at 50 ꢀC) with isolated yield of 60% (Scheme 1).
At the temperatures above 50 ꢀC dibromo derivative 2 undergoes
lactonisation to give 8-phenyl-5,8-dihydro-1H,3H-difuro[3,4-
b:30,40-e]pyridine-1,7(4H)-dione.44 4-Substituted pyridine dibromo
derivative 3 was also obtained from diethyl 2,6-dimethyl-4-phenyl-
pyridine-3,5-dicarboxylate 1 via bromination reaction with bromine
in acidic media, in 54% yield (Scheme 1). However, in our studies, it
was found that 1 was not brominated with NBS or NBS/benzoyl
peroxide or 1,3-dibromo-5,5-dimethylhydantoin in methanol.
Though, 2,6-methyl groups of the corresponding 4-phenyl-3,5-
dialkoxycarbonyl-2,6-dimethyl-1,4-dihydropyridines were bro-
52
minated with NBS in methanol in good yields.44 Using HBr/H2O2
or NBS/MeOH system53 combined with UV irradiation (254 nm)
bromination of the compound 1 does not occur.
Compound 5a was obtained in the nucleophilic substitution
reaction of 4-phenyl-2,6-dibromomethyl-3,5-diethoxycarbonyl-
pyridine 3 with pyridine in dry acetone.
Diperchlorate 4b was obtained from dibromide 4a according to
a modified method previously reported by us.46 Electrochemical
oxidation of 4b was studied with cyclic voltammetry (CV) and
chronoamperometry. In aprotic solvent CV of compound 4b shows
one irreversible oxidation step (Fig. 1). The presence of two strong
electron acceptor groups in the molecule shifts the oxidation po-
tential to 1.7 V, i.e., w1 V more anodically compared to the oxida-
tion of 4-phenyl substituted Hantzsch dihydropyridines.54–56
Figure 2. ORTEP representation of structure of cation 5b.
3. Conclusions
In conclusion, the first chemical oxidation of cationic 1,4-dihy-
dropyridines has been achieved. NBS in MeOH acts as an efficient
oxidising agent for conversion of cationic 1,4-dihydropyridine to
the corresponding cationic pyridine. Two strategies for the syn-
thesis of cationic pyridine derivatives have been elaborated. Oxi-
dation of cationic 1,4-dihydropyridines with chemical or
electrochemical methods leads to cationic pyridinesdmodel
compounds for putative gene delivery agents or their metabolites.
Our results have shown, that compared with oxidation of neutral 4-
phenyl substituted Hantzsch 1,4-dihydropyridine, chemical and
electrochemical oxidation is more difficult for cationic 1,4-
Figure 1. Cyclic voltammogram of 4b (c¼5ꢁ10ꢂ4 M) recorded on a stationary Pt disk
electrode in MeCN/0.1 M NaClO4.

A. Plotniece et al. / Tetrahedron 65 (2009) 8344–8349
8347
dihydropyridines. Alternatively, aromatisation of 1,4-dihydropyr-
idines followed by nucleophilic substitution with pyridine also
gives the target cationic pyridine derivatives. Studies of the in-
fluence of the cationic substituents on oxidation of 1,4-dihy-
dropyridine ring are currently underway in our laboratory.
After being stirred at rt for 1.5 h, the resulting mixture was poured
into ice water (20 mL) and extracted with CHCl3 (3ꢁ10 mL). The
combined organic extracts were dried over MgSO4 and the solvent
was removed in vacuo. The residue was crystallised from ethanol
giving product 3 as a pale white powder (0.12 g, 60%), mp 80–81 ꢀC.
1H NMR (CDCl3, 200 MHz) was identical to that described in
Method A for product 3.
4. Experimental
4.1. General
4.3. 1,10-{[3,5-Bis(ethoxycarbonyl)-4-phenyl-1,4-
dihydropyridine-2,6-diyl]dimethylene}-bispyridinium
dibromide (4a)
All reagents were purchased from Aldrich, Acros, Fluka or Merck
and used without further purification. TLC was performed on
20ꢁ20 cm Silica gel TLC-PET F254 foils (Fluka). Cyclic voltammetry
(CV) and preparative electrolysis were performed on advanced
electrochemical system PARSTAT 2273. A three-electrode configu-
ration was used: the working electrode was a stationary Pt disk
(d¼2 mm), Pt wire served as the counter electrode and an aqueous
saturated calomel electrode (SCE) as the reference electrode. Oxi-
dation potential was measured in MeCN/0.1 M NaClO4. Potential
scan rated100 mV/s. Chronoamperometry was carried out in a di-
vided cell with Pt meshes as working and counter electrodes and
SCE as a reference electrode. The divided cell was filled with 50 mL
MeCN/0.1 M NaClO4 solution and 0.6 g of 4b was added in the an-
ode compartment. NMR spectra were recorded with a Varian
To a solution of compound 2 (0.97 g, 2 mmol) in dry acetone
(10 mL), pyridine (0.7 mL, 4 mmol) was added and the reaction
mixture was stirred at rt for 3 h. After cooling, the precipitate was
filtered off, washed with dry acetone and crystallised from ethanol,
giving compound 4a as a yellow powder (1.33 g, 84%), mp 208–
210 ꢀC. 1H NMR (CDCl3, 400 MHz):
d
1.17 (t, 6H, J¼7.0 Hz), 4.07 (q,
4H, J¼7.0 Hz), 5.04 (s,1H), 5.87 and 6.32 (AB-q, 4H, J¼13.7 Hz), 7.14–
7.27 (m, 5H), 8.13 (dd, 4H, J¼6.3 and 7.8 Hz), 8.54 (t, 2H, J¼7.8 Hz),
9.30 (d, 4H, J¼6.3 Hz), 10.83 (br s, 1H); IR (Nujol) 3573, 3383, 1681,
1628 cmꢂ1; MS (þESI) m/z (relative intensity) 484 ([Mꢂ2Br]þ, 4).
Anal. Calcd for C29H31Br2N3O4$H2O: C, 52.50; H, 5.01; N, 6.33.
Found: C, 52.52; H, 4.86; N, 6.18.
Mercury 200BB (200 MHz) or
Chemical shifts are reported in ppm relative to hexamethyldisi-
loxane ( 0.055). Multiplicities are abbreviated as: s, singlet; d,
a Varian 400-MR (400 MHz).
4.4. 1,10-{[3,5-Bis(ethoxycarbonyl)-4-phenyl-1,4-
dihydropyridine-2,6-diyl]dimethylene}-bispyridinium
diperchlorate (4b)
d
doublet; t, triplet; q, quartet; m, multiplet; br, broad. The coupling
constants are expressed in Hertz. Diffraction data for 5b were col-
lected at 173 K on a Bruker-Nonius KappaCCD diffractometer with
a low temperature device Oxford Cryostream Plus, using Mo radi-
ation. Powder X-ray diffraction study of 5b was performed on
Rigaku Ultima IV diffractometer. Mass spectral data were de-
termined on an Acquity UPLC system (Waters) connected to
a Q-TOF micro hybrid quadrupole time of flight mass spectrometer
(Micromass) operating in the ESI positive or negative ion mode on
To a solution of 4a (0.36 g, 0.56 mmol) in ethanol (17 mL),
perchloric acid (57%, 1.70 mL) was added at reflux temperature,
after which the resulting mixture was allowed to cool to rt. The
reaction mixture was stirred at rt for 3 h. After cooling, the pre-
cipitate was filtered off and crystallised from ethanol to give 4b as
a pale yellow powder (0.11 g, 95%), mp 241–242 ꢀC. 1H NMR
(DMSO-d6, 200 MHz):
d
1.08 (t, 6H, J¼7.3 Hz), 4.04 (q, 4H, J¼7.3 Hz),
an Acquity UPLC BEH C18 column (1.7
a gradient elution with acetonitrile/formic acid (0.1%) in water. The
conversions of the reactions were analysed by HPLC on an Alltima
m
m, 2.1 mmꢁ50 mm) using
5.00 (s, 1H), 5.46 and 6.05 (AB-q, 4H, J¼14.6 Hz), 7.26–7.31 (m, 5H),
8.07 (dd, 4H, J¼8.1 and 5.9 Hz), 8.56 (t, 2H, J¼8.1 Hz), 8.88 (d, 4H,
J¼5.9 Hz), 9.84 (br s, 1H); IR (Nujol) 3484, 1695, 1634 cmꢂ1; MS
(þESI) m/z (relative intensity) 484 ([Mꢂ2ClO4]þ, 3). Anal. Calcd for
C29H31Cl2N3O12$H2O: C, 49.58; H, 4.73; N, 5.98. Found: C, 49.52; H,
4.58; N, 5.91.
CN column, 4.6ꢁ150 mm, 5
mm (Alltech) using a LC-1110 pump and
a LC-1200 UV/Vis detector at 254 nm (GBC Scientific Equipment).
The eluent was acetonitrile/phosphate buffer (pH 2.2; 0.05 M) in
water (10:90 by volume) at a flow rate of 1 mL/min. Peak areas
were determined electronically with a DP-800 (GBC Scientific
Equipment). Melting points were determined on an OptiMelt (SRS
Stanford Research Systems). Elemental analyses were determined
on an EA 1106 (Carlo Erba Instruments).
4.5. 1,10-{[3,5-Bis(ethoxycarbonyl)-4-phenylpyridine-2,6-
diyl]dimethylene}bispyridinium dibromide (5a)
4.5.1. Method A: Nucleophilic substitution of 3 with pyridine. To
a solution of compound 3 (0.05 g, 0.15 mmol) in dry acetone
(10 mL), pyridine (0.024 mL, 0.3 mmol) was added after which the
resulting mixture was stirred at rt for 3 h. After cooling, the pre-
cipitate was filtered off, washed with dry acetone and crystallised
from ethanol to give 5a as a white powder (0.04 g, 57%), mp 163 ꢀC
4.2. 2,6-Dibromomethyl-4-phenyl-3,5-
diethoxycarbonylpyridine (3)
Method A: To a solution of compound 1 (0.19 g, 0.6 mmol) in
acetic acid (10 mL), bromine (0.06 mL, 1.2 mmol) was added. The
reaction mixture was stirred at 50 ꢀC for 5 h. After cooling to rt, the
resulting mixture was diluted with water (20 mL) and extracted
with CHCl3 (3ꢁ10 mL). The combined organic extracts were dried
over MgSO4 and the solvent was removed in vacuo. The residue was
crystallised from ethanol giving product 3 as a pale white powder
(decomp.). 1H NMR (DMSO-d6, 200 MHz):
d
0.80 (t, 6H, J¼6.8 Hz),
4.02 (q, 4H, J¼6.8 Hz), 6.14 (s, 4H), 7.17–7.21 (m, 2H), 7.51–7.56 (m,
3H), 8.02 (dd, 4H, J¼7.8 and 6.8 Hz), 8.60 (t, 2H, J¼7.8 Hz), 8.76 (d,
4H, J¼6.8 Hz); IR (Nujol) 1725, 1634 cmꢂ1; MS (þESI) m/z (relative
intensity) 482 ([Mꢂ2Br]þ,15). Anal. Calcd for C29H29Br2N3O4$3H2O:
C, 49.94; H, 5.06; N, 6.03. Found: C, 50.16; H, 4.56; N, 5.89.
(0.15 g, 54%), mp 80–82 ꢀC. 1H NMR (CDCl3, 200 MHz):
d 0.88 (t, 6H,
J¼7.1 Hz), 4.01 (q, 4H, J¼7.1 Hz), 4.09 (s, 4H), 7.25–7.38 (m, 5H); IR
(CHCl3) 1723, 1558 cmꢂ1. MS (þESI) m/z (relative intensity) 486
([MþH]þ, 55). Anal. Calcd for C19H19Br2NO4: C, 47.02; H, 3.95; N,
2.89. Found: C, 47.07; H, 3.85; N, 2.89.
Method B: The compound 2 (0.2 g, 0.4 mmol) was dissolved in
acetic acid (10 mL) at 50 ꢀC, after which heating was discontinued
and sodium nitrite (0.17 g, 2 mmol) was added in several portions.
4.5.2. Method B: Oxidation of 4a with NBS. To a solution of com-
pound 4a (0.25 g, 0.39 mmol) in methanol (10 mL), NBS (0.18 g,
1 mmol) was added, after which the resulting mixture was stir-
red at 70 ꢀC for 10 h. After cooling, the solvent was removed in
vacuo. The residual crude product was triturated with hexane/
ethyl acetate (1:1). The precipitate was filtered off and crystal-
lised from ethanol giving compound 5a as a white powder

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A. Plotniece et al. / Tetrahedron 65 (2009) 8344–8349
(0.12 g, 48%), mp 163 ꢀC (decomposition). 1H NMR (DMSO-d6,
200 MHz) was identical to that described for 5a obtained with
method A.
1H NMR (DMSO-d6, 200 MHz) was identical to that described for 5b
obtained with method A.
Acknowledgements
4.5.3. Method C: Oxidation of 4a with selenium dioxide. To a solu-
tion of compound 4a (0.20 g, 0.31 mmol) in acetic acid (10 mL),
SeO2 (0.14 g, 1.26 mmol) was added. The reaction mixture was
stirred and heated at 50 ꢀC for 5 h. After cooling to rt, the reaction
mixture was quenched with a saturated aqueous NaHCO3 solution
and the resulting mixture was extracted with ethyl acetate
(3ꢁ20 mL). The combined organic extracts were dried over
Na2SO4 and the solvent was removed in vacuo. The residue was
crystallised from ethanol giving a pale orange solid (0.18 g) con-
taining 5a (40% by HPLC) and residual elemental Se. 1H NMR
(DMSO-d6, 200 MHz) (major peaks) was identical to that de-
This research work was supported by VP-05–08 and ES 05–27
from Latvian Ministry of Education and Science, fellowship ‘‘For
Women in Science’’ from L’Oreal Latvia, Latvian National Commis-
sion for UNESCO and Latvian Academy of Sciences (for Aiva Plot-
niece) and European Social Foundation (for Karlis Pajuste). We are
indebted to Dr. S. Grinberga for the mass spectral analyses, to Mrs.
E. Sarule for the elemental analyses and Dr. A. Mishnev for powder
X-ray diffraction analyses.
Supplementary data
scribed for 5a obtained with method A:
d
0.81 (t, 6H, J¼6.8 Hz),
4.03 (q, 4H, J¼6.8 Hz), 6.16 (s, 4H), 7.15–7.24 (m, 2H), 7.49–7.57 (m,
3H), 8.05 (dd, 4H, J¼7.8 and 6.8 Hz), 8.57 (t, 2H, J¼7.8 Hz), 8.78 (d,
4H, J¼6.8 Hz).
Fluorescence, UV–vis, NMR spectra as well as crystallographic
data are available in supplementary data. Supplementary data in
the form of a CIF have been deposited with the Cambridge Crys-
tallographic Data Centre for 3 (CCDC 739500). Supplementary data
associated with this article can be found in the online version, at
doi:10.1016/j.tet.2009.08.012.
4.6. 1,10-{[3,5-Bis(ethoxycarbonyl)-4-phenylpyridine-2,6-
diyl]dimethylene}bispyridinium diperchlorate (5b)
4.6.1. Method A: Electrochemical oxidation of 4b. After exhaustive
electrolysis the anolyte was evaporated in vacuo and the residue
was stirred with water (10 mL) for 3 h at rt, the precipitate was
filtered off and washed with water (10 mL). The precipitate was
crystallised from methanol (70 mL) and dried in vacuo, to give
0.38 g (63%) of 5b as a white powder, mp 235–237 ꢀC; 1H NMR
References and notes
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