Aerobic oxidative aromatization of Hantzsch 1,4-dihydropyridines via an anomeric-based oxidation in the presence of Laccase enzyme/4-Phenyl urazole as a cooperative catalytic oxidation system

Article abstract of DOI:10.1007/s13738-019-01664-9

Abstract: Cooperative catalytic system of Laccase enzyme (from Trametes versicolor) and 4-phenyl urazole in phosphate buffer/acetonitrile solution at 40?°C has been applied for the biomimetic aerobic oxidative aromatization of Hantzsch 1,4-dihydropyridine

Full text of DOI:10.1007/s13738-019-01664-9

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-019-01664-9
ORIGINAL PAPER
Aerobic oxidative aromatization of Hantzsch 1,4‑dihydropyridines
via an anomeric‑based oxidation in the presence of Laccase
enzyme/4‑Phenyl urazole as a cooperative catalytic oxidation system
Donya Khaledian1 · Amin Rostami1 · Seyed Amir Zarei2 · Behnaz Mohammadi1
Received: 11 August 2018 / Accepted: 2 April 2019
© Iranian Chemical Society 2019
Abstract
Cooperative catalytic system of Laccase enzyme (from Trametes versicolor) and 4-phenyl urazole in phosphate bufer/
acetonitrile solution at 40 °C has been applied for the biomimetic aerobic oxidative aromatization of Hantzsch 1,4-dihydro-
pyridines (1,4-DHPs) with excellent yields.
Graphical abstract
R
O
EtOOC
H3C
COOEt
CH3
NH
NH
Ph
Ph
Laccase (ox)
Laccase (red)
N
2H2O
N
O
H
N
O
H3C
CH3
N
N
O2
N
EtOOC
COOEt
O
R
R = Alkyl, Aryl
Keywords Cooperative catalytic system · 1,4-Dihydropyridines · Laccase enzyme · Oxidative aromatization · 4-Phenyl
urazole · Pyridine derivatives
Introduction
Hantzsch ester 1,4-DHPs have attracted much attention due
to both their biological signifcance in the NADH redox
process and their therapeutic performance for medicating
diferent diseases, such as cardiovascular disorders, cancer,
and AIDs [1–5]. The oxidative metabolism process of 1,4-
DHP drugs such as felodipine, nifedipine, and niguldipine
in the presence of cytochrome P450 in the liver leads to
the formation of their corresponding pyridine derivatives,
in which some of these derivatives have anti-hypoxic, anti-
ischemic, insecticidal, bactericidal, and herbicidal activi-
ties [6–9]. Therefore, oxidative aromatization of 1,4-DHPs
has substantial signifcance for synthetic organic chemists.
Many methodologies and a variety of oxidizing reagents
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-019-01664-9) contains
supplementary material, which is available to authorized users.
* Amin Rostami
a.rostami@uok.ac.ir
1
Department of Chemistry, Faculty of Science, University
of Kurdistan, Sanandaj 66177-15175, Iran
2
Department of Chemistry, Faculty of Science, Sanandaj
Branch, Islamic Azad University, Sanandaj, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
have been employed for this transformation such as TPAP/
NMO [5], graphite oxide [8], MnO2/MW [9], solid acid/
NaNO2 or NaNO3/wet SiO2 [10], 1,2,4-triazolinedione [11],
CuBr2 [7], ferric chloride [12], Pd–C/AcOH [13], and TBHP
[14]. Many of these methods are efective for the prepara-
tion of Hantzsch pyridine derivatives. However, the major
disadvantages of these reagents are poor atom efciency
and formation of large amounts of toxic waste. Therefore,
there is extensive interest toward the use of molecular oxy-
gen or air as the highly atom-economical, environmentally
benign, and abundant oxidant in oxidative aromatization
of 1,4-DHPs in the presence of catalysts. In this context,
several catalysts or catalytic systems such as FeCl3/NaNO2/
TEMPO [15], Pt/o-chloranil [16], Co(OAc)2/TEMPO [17],
N-hydroxyphthalimide [18], 3,4-dihydro-2H-pyran [19], and
AuCNT/quinone [20] have been reported for the aerobic oxi-
dation of 1,4-DHPs.
4-Substituted 1,2,4-triazole-3,5-diones were introduced
as one of the efcient oxidative reagents due to its dehydro-
genating feature [30]. Zolfgol and co-workers used stoichio-
metric amounts of 4-substituted 1,3,4-triazole-3,5-diones as
efective oxidizing agents for the oxidation of 1,4-dihydro-
pyridines to the corresponding pyridine derivatives [11].
The low stability, high cost, and toxicity of these oxidative
agents can restrict their application in organic synthesis.
The best strategy to overcome these drawbacks is the in situ
preparation of these suitable oxidative reagents from their
corresponding stable, cheap, and non-toxic urazoles.
After introducing the anomeric efect, diferent investi-
gations have been illustrated that the anomeric efect can
be used not only to clarify the carbohydrate chemistry, but
also to explain diverse observations in a variety of organic
transformations such as the precedence of the gauche confor-
mation over the anti-orientation in Newman projections of
the various compounds and the stereochemical selectivity in
radical reactions. In some ways like the Cannizzaro reaction,
the generation of the molecular hydrogen through a hydride
transfer takes place that is based on the anomeric efect.
These observations could imply the role of the anomeric
efect in the mechanistic aspects of the related reactions.
According to the computational and experimental studies,
Zolfgol and co-workers have recently introduced a new term
entitled “anomeric-based oxidation (ABO)” to explain the
mechanistic route of the oxidative aromatization of some
heterocyclic molecules such as 1,4-dihydropyridines in the
presence of 1,2,4-triazolediones oxidants [31].
Although these fndings have presented interesting break-
through in the feld of aerobic oxidative aromatization of
1,4-DHPs, these procedures still sufer from some problems
such as high reaction temperatures, toxic organic solvents,
and transition metal co-catalysts [15–20]. Therefore, it is a
purpose for chemists to gain more environmentally benign
replacements for the aerobic oxidative aromatization of
1,4-DHPs.
The economical and ecological precedence of the bio-
catalysts has stimulated organic chemists to select them as
suitable and green candidates in organic synthesis [21–23].
Laccases (benzendiol; O2 oxidoreductase, E.C. 1.10.3.2.) are
highly attractive biocatalysts in oxidation of various com-
pounds using oxygen as an electron acceptor and produc-
ing water as by-product [24]. The most efcient strategy
to extend the range of laccase substrates is the simultane-
ous application of the enzyme and redox mediators [25].
In this context, the oxidative coupling of various organic
compounds such as alcohols, ethers, heterocyclic thiols,
and amines using aerial oxygen have been examined by a
laccase-mediator system [26–29]. Although these metal-
free aerobic oxidation methods under mild conditions have
been considerably developed, the applied mediators such
as TEMPO and ABTS are quite expensive and relatively
toxic [26a, b]. Hence, introducing and/or using a cooperative
catalytic system including laccase enzyme along with cheap,
available, active, and stable auxiliary agent is valuable in
organic synthesis.
In continuous of our previous work in terms of appli-
cation of laccase and laccase-mediated catalytic system in
organic reactions [32, 26b], herein the in situ aerobic gen-
eration of 4-phenyl triazolediones from 4-phenyl urazole in
the presence of catalytic amount of laccase in the oxidative
aromatization of Hantzsch 1,4-DHPs in phosphate bufer/
acetonitrile solution using molecular oxygen or air as oxi-
dant agents at 40 °C is reported (Scheme 1).
Experimental
Material and physical measurements
All substances were purchased from the Merck and Aldrich
Chemical Companies and used without purification.
Scheme 1 Aerobic oxidative
aromatization of Hantzsch
1,4-DHPs by laccase/4-phenyl
urazole as a cooperative cata-
lytic system
R
R
EtOOC
H3C
COOEt
CH3
EtOOC
H3C
COOEt
CH3
Laccase (Cat.)/4-phenyl urazol (Cat.)
solvent, O2, 40 oC
N
H
N
R = Alkyl or Aryl
1 3

Journal of the Iranian Chemical Society
Products were characterized by assessment of their physi-
cal and spectral data with those of authentic samples. NMR
spectra were recorded using Bruker NMR spectrometers (1H
NMR 400 MHz, 13C NMR 100 MHz). All melting points
were recorded by electrothermal IA9100 apparatus.
J=7.2 Hz). 13C NMR (100 MHz, CDCl3) (δ/ppm): 167.9,
155.4, 146.1, 136.5, 128.4, 128.1, 128.0, 126.9, 61.4, 22.0,
13.6.
Diethyl 2,6‑dimethyl‑4‑(p‑methylphenyl)
pyridine‑3,5‑dicarboxylate (2b) (Table 2, entry 2) [34a]
Determination of the activity of laccase from T.
versicolor
Pale yellow-colored solid, 1H NMR (400 MHz, CDCl3) (δ/
ppm): 7.20 (d, 2H, J=8.4 Hz), 7.17 (d, 2H, J=8.4 Hz), 4.06
(q, 4H, CH2, J = 7.2 Hz), 2.62 (s, 6H, CH3), 2.44 (s, 3H,
CH3), 0.98 (t, 6H, CH3, J=7.2 Hz). 13C NMR (100 MHz,
CDCl3) (δ/ppm): 168.1, 155.3, 146.4, 138.3, 133.5, 128.8,
128.0, 127.1, 61.3, 22.9, 21.3, 13.6.
The activity of commercial enzyme laccase from Tram-
etes versicolor was determined via UV–Vis spectroscopy
by monitoring the oxidation of 2,2′-azino-bis-(3-ethylb-
enzthiazoline-6-sulfonic acid) (ABTS, ε = 36,000 M/cm).
The reaction mixture contained 5 mM of ABTS (100 µL) in
acetate bufer (100 mM, pH 5.0) and laccase enzyme. The
absorbance change was observed at 420 nm after 5 min at
room temperature. One unit was defned as the amount of
enzyme that oxidizes 1 µmol of ABTS per minute. The activ-
ity of laccase enzyme batch applied in this investigation was
evaluated at 0.87 U/mg.
Diethyl 2,6‑dimethyl‑4‑(p‑methoxyphenyl)
pyridine‑3,5‑dicarboxylate (3b) (Table 2, entry 3) [34a]
Colorless solid, 1H NMR (400 MHz, CDCl3) (δ/ppm): 7.22
(d, 2H, J = 8.4 Hz), 6.92 (d, 2H, J= 8.4 Hz), 4.08 (q, 4H,
CH2, J = 7.2 Hz), 3.86 (s, 3H, OCH3), 2.61 (s, 6H, CH3),
1.01 (t, 6H, CH3, J=7.2 Hz). 13C NMR (100 MHz, CDCl3)
(δ/ppm): 168.1, 159.8, 155.2, 145.7, 129.41, 128.7, 127.2,
113.6, 61.3, 55.3, 22.9, 13.7.
General procedure of oxidative aromatization
reaction of Hantzsch ester 1,4‑DHPs 1a–12a
to their corresponding pyridine derivatives 1b–12b
by the laccase/4‑phenyl urazole catalyst
Diethyl 2,6‑dimethyl‑4‑(3,4,5‑trimethoxyphenyl)
pyridine‑3,5‑dicarboxylate (4b) (Table 2, entry 4) [34b]
1,4-DHPs (1a–12a) were synthesized according to the previ-
ously reported procedure [33] from a combination of ethyl
acetoacetate, related aldehyde and ammonium acetate.
Then, to a solution of acetonitrile (2 mL) incorporating
1 mmol of a 1,4-DHP compound was added a solution of
8 mL of phosphate bufer including laccase (200 U) and
4-phenyl urazole (0.2 mmol). The resulting mixture was
stirred at 40 °C under molecular oxygen (balloon) for the
time given (Table 2). The reaction progress was monitored
by TLC. After completion of the reaction, a solution of
NaOH 10% (5 mL) was added to the mixture and stirred
for 5 min. The products were extracted by diethyl ether
(3×15) and the obtained organic phase dried using anhy-
drous Na2SO4. The diethyl ether solvent was removed after
fltration, and the desired product recrystallized from etha-
nol. Some of the products were known and characterized by
comparison with their spectral data (1H NMR) and physical
properties (melting point) with those of authentic samples.
White-colored solid, 1H NMR (400 MHz, CDCl3) (δ/ppm):
6.55 (s, 2H), 4.11 (q, 4H, CH2, J = 7.2 Hz), 3.88 (s, 3H,
OCH3), 3.85 (s, 6H, OCH3), 2.63 (s, 6H, CH3), 1.29 (t, 6H,
CH3, J = 7.2 Hz). 13C NMR (100 MHz, CDCl3) (δ/ppm):
168.1, 155.4, 153.0, 145.8, 138.3, 132.0, 126.8, 105.5, 61.5,
61.0, 56.2, 22.9, 13.7.
Diethyl 2,6‑dimethyl‑4‑(p‑nitrophenyl)
pyridine‑3,5‑dicarboxylate (5b) (Table 2, entry 5) [15]
Pale yellow-colored solid, 1H NMR (400 MHz, CDCl3) (δ/
ppm): 8.28 (d, 2H, J=5.5 Hz), 7.48 (d, 2H, J=5.5 Hz), 4.06
(q, 4H, CH2, J=7.2 Hz), 2.66 (s, 6H, CH3), 1.00 (t, 6H, CH3,
J=7.2 Hz). 13C NMR (100 MHz, CDCl3) (δ/ppm): 167.2,
156.2, 147.8, 143.9, 143.3, 129.4, 126.2, 123.2, 61.7, 23.1,
13.7.
Diethyl 2,6‑dimethyl‑4‑(o‑fourophenyl)
Selected spectral and physical data
pyridine‑3,5‑dicarboxylate (7b) (Table 2, entry 7) [7]
Diethyl 2,6‑dimethyl‑4‑(phenyl)pyridine‑3,5‑dicarboxylate
(1b) (Table 2, entry 1) [15]
1H NMR (400 MHz, CDCl3) (δ/ppm): 7.39–7.32 (m, 1H)
7.31–7.01 (m, 3H), 4.062 (q, 4H, CH2, J = 7.2 Hz), 2.62
(s, 6H, CH3), 0.96 (t, 6H, CH3, J = 7.2 Hz). 13C NMR
(100 MHz, CDCl3) (δ/ppm): 167.6, 163.5, 161.0, 155.7,
138.6, 138.5, 129.9, 129.8, 124.1, 124.0, 115.6, 115.5,
115.3, 115.3, 61.5, 23.0, 13.6.
1
Pale yellow-colored solid, H NMR (400 MHz, CDCl3)
(δ/ppm): 7.40–7.39 (m, 3H), 7.29–7.27 (m, 2H), 4.03 (q,
4H, CH2, J=7.2 Hz), 2.64 (s, 6H, CH3), 0.95 (t, 6H, CH3,
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Journal of the Iranian Chemical Society
Diethyl 2,6‑dimethyl‑4‑(m‑chlorophenyl)
pyridine‑3,5‑dicarboxylate (8b) (Table 2, entry 8) [34c]
Diethyl 2,6‑dimethyl‑4‑n‑butylpyridine‑3,5‑dicarboxylate
(12b) (Table 2, entry 12) [34a]
1
1
Pale yellow-colored solid, H NMR (400 MHz, CDCl3)
Colorless, H NMR (400 MHz, CDCl3) (δ/ppm): 4.42 (q,
(δ/ppm): 7.42–7.31 (m, 3H), 7.16 (s, 1H), 4.09 (q, 4H,
CH2, J = 7.2 Hz), 2.64 (s, 6H, CH3), 1.46 (t, 6H, CH3,
J = 7.2 Hz). 13C NMR (100 MHz, CDCl3) (δ/ppm): 167.5,
155.7, 146.5, 138.9, 134.1, 129.4, 128.6, 128.3, 126.7,
126.5, 58.5, 23.0, 18.5, 13.7.
4H, CH2, J=7.2 Hz), 2.59–2.55 (m, 2H, CH2), 2.52 (s, 6H,
CH3), 1.64–1.54 (m, 4H, CH2), 1.41 (t, 6H, CH3), 0.95 (t,
3H, CH3, J=7.2 Hz). 13C NMR (100 MHz, CDCl3) (δ/ppm):
168.6, 155.0, 146.3, 127.2, 34.1, 33.4, 24.2, 23.0, 14.4, 14.2.
Results and discussion
Diethyl 2,6‑dimethyl‑4‑cinnamalpyridine‑3,5‑dicarboxylate
(10b) (Table 2, entry 10) [34a]
The oxidative aromatization of 1,4-DHP (1a) as a repre-
sentative reaction has been run in order to determine the
optimized reaction conditions including solvent system,
temperature, and ratio of laccase/4-phenyl urazole (Table 1).
Accordingly, mixture of phosphate bufer solution (0.1 M,
8 mL, pH 5.0) and acetonitrile (2 mL) containing 200 U of
laccase and 0.2 mmol of 4-phenyl urazole (as co-catalyst)
was prepared, and the optimal reaction conditions for oxida-
tive aromatization of 1,4-DHP (1a) under molecular oxygen
at 40 °C was investigated (Table 1, entry 2). To evaluate the
catalytic efciency of the laccase enzyme, a blank reaction
1
Pale yellow-colored solid, H NMR (400 MHz, CDCl3)
(δ/ppm): 7.45–7.31 (m, 5H), 7.13 (d, 1H, J = 10.2 Hz),
6.84 (d, 1H, J = 10.2 Hz), 4.36 (q, 4H, CH2, J = 7.2 Hz),
2.60 (s, 6H, CH3), 1.31 (t, 6H, CH3, J = 7.2 Hz). 13C NMR
(100 MHz, CDCl3) (δ/ppm): 168.3, 155.6, 142.2, 136.6,
136.1, 128.8, 126.7, 122.7, 119.8, 61.7, 23.0, 14.2.
Table 1 Optimization of the reaction conditions in the synthesis of pyridine derivative (1b)a
Ph
Ph
EtOOC
H3C
COOEt
CH3
EtOOC
H3C
COOEt
CH3
Laccase (Cat.)/4-phenyl urazol (Cat.),
solvent, oxidant, Temp (oC)
N
H
N
1a
1b
Entry
Solvent system
Temp (°)
Laccase (unit)
4-phenyl urazole
(mmol)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
MeCN/NaPBS (pH 6) (1:4)
MeCN/NaPBS (pH 5) (1:4)
MeCN/NaPBS (pH 5) (1:4)
MeCN/NaPBS (pH 5) (1:4)
MeCN (/NaPBS (pH 5) (1:4)
Water
MeCN/NaPBS (pH 5) (1:4)
MeCN/NaPBS (pH 5) (1:4)
MeCN/NaPBS (pH 5) (1:4)
MeOH/NaPBS (pH 5) (1:4)
MeCN/NaPBS (pH 5) (1:4)
MeCN/NaPBS (pH 5) (1:4)
40
40
40
40
40
40
40
40
40
40
r.t.
40
200
200
150
100
–
200
200
200
200
200
200
200
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.05
–
80
99c
70
50
–
20
50
30
–
0.2
0.2
0.2
50
50
80d
aReaction conditions: Laccase (0–200 U), 1a (1 mmol), 4-phenyl urazole (0–0.2 mmol), solvent (10 mL), molecular oxygen (balloon), 22 h
bIsolated yield
cThe bold represented the most efective reaction conditions
dAir was used as oxidant
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Journal of the Iranian Chemical Society
Table 2 Synthesis of pyridine
derivatives by aerobic oxidative
aromatization reaction from
their corresponding Hantzsch
ester 1,4-DHPs in the presence
of laccase/4-phenyl urazole
cooperative catalytic systema
Entry
1
1,4-DHP (a)
pyridine (b)
Time (h)
22
M. p.
(oC)
M. p. (oC)
[Lit.]
Yield
(%)b
60-61
60-61 [15]
99
EtOOC
COOEt
EtOOC
COOEt
H3C
N
H
CH3
CH3
H3
C
N
CH3
CH3
2
3
4
5
20
19
18
24
72-74
71-73 [34a]
100
EtOOC
COOEt
CH3
EtOOC
COOEt
CH3
H3C
N
H
H3
C
N
OCH3
OCH3
49-51
49 [9]
100
85
EtOOC
COOEt
EtOOC
COOEt
H3C
N
CH3
H3
C
N
CH3
H
OCH3
OCH3
H3CO
OCH3
H3CO
OCH3
107-108
[34b]
105-107
115-117
EtOOC
COOEt
CH3
EtOOC
COOEt
CH3
H3
C
N
H3
C
N
H
NO2
NO2
113-114
[15]
97
EtOOC
COOEt
CH3
EtOOC
COOEt
CH3
H3
C
N
H
F
H3
C
N
F
6
28
Oil
Oil [35]
95
EtOOC
COOEt
CH3
EtOOC
COOEt
CH3
H3C
N
H
H3C
N
N
N
F
F
7
8
28
24
Oil
Oil [7]
95
96
EtOOC
COOEt
EtOOC
COOEt
H3C
N
H
CH3
H3C
CH3
Cl
Cl
42-44
41-44 [34c]
71-73 [36]
EtOOC
COOEt
EtOOC
COOEt
H3C
N
H
CH3
Br
H3C
CH3
Br
9
24
25
70-72
90
95
EtOOC
COOEt
CH3
EtOOC
COOEt
CH3
H3C
N
H3C
N
H
Ph
Ph
163-165
[34a]
EtOOC
COOEt
CH3
10
164-165
EtOOC
COOEt
CH3
H3
C
N
H
H3
C
N
O
O
EtOOC
COOEt
CH3
11
12
23
21
38-39
Oil
39-41 [34d]
Oil [34a]
97
98
EtOOC
COOEt
CH3
H3
C
N
H
H3
C
N
(CH2
(CH2
)
3CH3
)3CH3
EtOOC
COOEt
CH3
EtOOC
COOEt
H3C
N
H
H3
C
N
CH3
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Journal of the Iranian Chemical Society
Table 2 (continued)
aReaction conditions: Laccase (200 U), related 1,4-DHP (1 mmol), 4-phenyl urazole (0.2 mmol), phosphate
bufer (0.1 M, 8 mL, pH 5)/acetonitrile (2 mL), molecular oxygen (balloon), 40 °C
bIsolated yield
Ph
EtOOC
H3C
COOEt
CH3
O
N
NH
1b
Ph
Laccase (ox)
Laccase (red)
N
Ph
N
2H2O
NH
O
O
O
HN
N
H
H3C
N
CH3
Ph
O2
N
O
O
EtOOC
COOEt
N
N
Ph
EtOOC
H
H
N
..
Ph
EtOOC
1a
Scheme 2 Suggested anomeric-based oxidation mechanism for oxidative aromatization of 1,4-DHP (1a) to pyridine derivative (1b) catalyzed by
laccase/4-phenyl urazole system
was performed; even after 22 h, no progress was observed
(Table 1, entry 5). Also, to elucidate the cooperative contri-
bution of laccase enzyme and 4-phenyl urazole, a reaction
was run in the presence of each of them individually, and the
reactions showed no progress after 22 h (Table 1, entries 5
and 9). Using air, instead of oxygen gas, as oxidant resulted
in 80% yield of the expected pyridine (Table 1, entry 12).
After determining the optimized reaction conditions
for the 1,4-DHP (1a), the oxidative aromatization of some
Hantzsch ester 1,4-DHPs with diferent alkyl, aryl, and het-
rocyclic substitutions at C-4 in a phosphate bufer/acetoni-
trile mixture was successfully performed using molecular
oxygen as oxidant agent in the presence of laccase cata-
lyst and 4-phenyl urazole at 40 °C with excellent yields
(Table 2). The results of Table 2 display that the electronic
structure of R substituent at C-4 position could afect the rate
of oxidative aromatization of the investigated 1,4-DHPs. It
seems that the rate of oxidative aromatization of 1,4-DHPs
incorporating electron-withdrawing aromatic substituents is
relatively less than electron-rich groups (Table 2, entries
1–9). The reaction worked out well for both the 1,4-DHPs
(10a) and (12a), with an aliphatic n-butyl and cinnamyl
substituent groups, respectively (Table 2, Entries 10 and
12). According to earlier study, the oxidative reaction of
the 1,4-DHPs (10a) and (12a) could produce a mixture of
4-substituted and un-substituted pyridines in the presence of
peroxydisulfate-Co(II) [34], respectively. Comparison of our
fndings with the fndings of Ref. [34] could imply that the
experimental conditions of our work are mild.
A plausible mechanism for the oxidative aromatization
reaction of the 1,4-DHP (1a) to its corresponding pyridine
derivative (1b) in the presence of the cooperative catalytic
system laccase/4-phenyl urazole is shown in Scheme 2, in
which the aromatization process could be defned as ano-
meric-based oxidation mechanism [31b, 37]. Herein, the
lone pair of the nitrogen atom in 1,4-DHP (1a) may begin
the aromatization process, which help to form pyridinium
cation intermediate, followed by hydride elimination at C-4
using 4-phenyl urazole. Proton removing from pyridinium
cation by 4-phenyl urazole anion completes the aromatiza-
tion process and leads to the formation of the product (1b)
and 4-phenyl-1,2,4-triazole-3,5-dione.
Beside this mechanism, another plausible mechanism
(Scheme 3) for the conversion of 1,4-DHP (1a) to its cor-
responding pyridine derivative (1b) could be defned as
oxidation via hydrogen abstraction–addition based on the
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Journal of the Iranian Chemical Society
O
Scheme 3 Suggested hydrogen
abstraction–addition mechanism
for oxidative aromatization
Ph
NH
NH
EtOOC
H3C
COOEt
CH3
Ph
Laccase (ox)
Laccase (red)
N
of 1,4-DHP (1a) to pyridine
derivative (1b) catalyzed by
laccase/4-phenyl urazole system
2H2O
O
N
1b
O
O2
H
N
N
Ph
N
N
EtOOC
H
O
Ph
1a
COOEt
Table 3 Comparison of laccase enzyme/4-phenyl urazole cooperative catalytic system with other reagent or catalysts for oxidation of diethyl
2,6-dimethyl-4-phenyl-1,4-dihydropyridine-3,5-dicarboxylate to diethyl 2,6-dimethyl-4-phenyl-3,5-pyridinedicarboxylate
Entry Reagent or catalyst
Reaction conditions
Time
Isolated
Ref.
yield (%)
1
2
3
MnO2
CH2Cl2, MW 100 °C
CH2Cl2, rt
1 min
25 min 88
30 min 97
99
[9]
[11]
[15]
4-Phenyl urazole (1.1 equiv)
FeCl3 (0.05 mmol)/NaNO2 (0.05 mmol)/TEMPO (0.05 mmol) CH3CN, CH3COOH, O2, rt
4
5
6
7
NHPI (10 mol%)/Co(OAC)2 (0.5 mol%)
NHPI (20 mol%)
3,4-Dihydro-2H-pyran (1.5 equiv)
AuCNT (10 mol%)
CH3CN, O2, Refux
CH3CN, O2, Refux
AcOH, 80 °C
4 h
4 h
8 h
–
99
99
95
–
[17]
[18]
[19]
[20]
CHCl3/H2O
8
9
10
AuCNT (10 mol%)/2,5-dichloro-1,4-benzoquinone (20 mol%)
Laccase (168 U)/ABTS (10 mol%)
Laccase (200 U)/4-phenyl urazole (0.2 mmol)
CHCl3/H2O
Acetate bufer/MeOH, air, 50 °C 9 h
NaPBS/CH3CN, O2, 40 °C 22 h
36 h
78
80
99
[20]
[24]
This work
catalytic role of laccase and the possibility of the oxidation
conversion of 4-phenyl urazole to 4-phenyl-1,2,4-triazole-
3,5-dione [11, 32d]. It is supposed that the reaction begins
with the oxidation conversion of 4-phenyl urazole to 4-phe-
nyl-1,2,4-triazole-3,5-dione by laccase that is accompanied
by the reduction of molecular oxygen to water. Then, the
oxidative coupling reaction of 1,4-DHP (1a) is led to the
formation of the corresponding pyridine derivative (1b) that
is motivated by the reduction conversion of 4-phenyl-1,2,4-
triazole-3,5-dione to 4-phenyl urazole.
catalysts or reagent [9, 15, 17, 20], special eforts for the
preparation of catalyst [20] toxic and volatile solvents [9,
11, 15, 17, 19], elevated reaction temperatures [9, 17–19],
and low yield [24].
Conclusion
In summary, the simple, mild, and efcient aerobic oxida-
tive aromatization reaction of Hantzsch ester 1,4-DHPs to
their corresponding pyridine derivatives in phosphate bufer
(pH 5)/acetonitrile solution has been reported by laccase/4-
phenyl urazole as a cooperative catalytic system in high to
excellent yields of products in the presence of oxygen gas as
oxidant agent at 40 °C. This non-toxic cooperative catalytic
system may be considered as environmentally benign one
without any by-product and a suitable candidate for synthe-
sizing of sensitive drugs.
To illustrate the efciency of laccase enzyme/4-phenyl
urazole as a cooperative catalytic system, comparison of
some of the best and well-known reported catalytic systems
with the obtained results of our research for oxidative aro-
matization of diethyl 2,6-dimethyl-4-phenyl-1,4-dihydro-
pyridine-3,5-dicarboxylate are summarized in Table 3. As
shown in Table 3, the previously reported procedures sufer
from one or more disadvantages such as the use of stoichio-
metric oxidant [11, 15], toxic and expensive transition metal
1 3

Journal of the Iranian Chemical Society
22. H.T. Abdel-Mohsen, K. Sudheendran, J. Conrad, U. Beifuss,
Green Chem. 15, 1490 (2013)
Acknowledgements We are grateful to the Research Council of Kurd-
istan University for the fnancial support of this work.
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Augustine, J. Yoon, S. Ghosh, Acc. Chem. Res. 40, 581 (2007)
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