3,4-Dihydro-2H-pyran promoted aerobic oxidative aromatization of 1,3,5-trisubstituted pyrazolines and Hantzsch 1,4-dihydropyridines

Article abstract of DOI:10.1016/j.tetlet.2014.07.051

An unprecedented facile oxidation of 1,3,5-trisubstituted pyrazolines and Hantzsch 1,4-dihydropyridines (DHPs) to the corresponding pyrazoles and pyridines was observed, mediated by 3,4-dihydro-2H-pyran in air. The reaction showed excellent reactivity, functional group tolerance, and high yield without using any metal and/or halogen based oxidizing agents.

Full text of DOI:10.1016/j.tetlet.2014.07.051

Tetrahedron Letters 55 (2014) 5333–5337
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
3,4-Dihydro-2H-pyran promoted aerobic oxidative aromatization of
1,3,5-trisubstituted pyrazolines and Hantzsch 1,4-dihydropyridines
⇑
Dipanwita Banerjee, Utpal Kayal, Rajiv Karmakar, Gourhari Maiti
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 April 2014
Revised 9 July 2014
Accepted 12 July 2014
Available online 29 July 2014
An unprecedented facile oxidation of 1,3,5-trisubstituted pyrazolines and Hantzsch 1,4-dihydropyridines
(DHPs) to the corresponding pyrazoles and pyridines was observed, mediated by 3,4-dihydro-2H-pyran
in air. The reaction showed excellent reactivity, functional group tolerance, and high yield without using
any metal and/or halogen based oxidizing agents.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
1,3,5-Trisubstituted pyrazoline
Hantzsch 1,4-dihydropyridines (DHPs)
3,4-Dihydro-2H-pyran
Pyrazoles and pyridine derivatives
Oxidative aromatization
Most of the biologically active natural products and many
synthetic compounds of medicinal interest having five- and six-
membered heterocyclic ring contain one or more hetero atoms.1
Among them, 1,3,5-trisubstituted pyrazolines can readily be pre-
pared from the condensation reaction of chalcones with phen-
ylhydrazines2 and dihydropyridine derivatives can also be
synthesized by Hantzsch reaction of aldehydes, b-ketoester, and
ammonium acetate.3 A number of pyrazole derivatives (oxidized
product of pyrazolines) have been synthesized for the treatment
of CNS, metabolic, and oncological diseases etc.4 and have been
successfully commercialized as pharmaceutical drugs, such as cele-
coxib (non-steroidal anti-inflammatory drug)5 and rimonabant (an
anorectic antiobesity drug).6 Moreover, substituted pyrazoles have
been utilized as useful ligands for various cross-coupling reactions7
and some derivatives also display agrochemical properties such as
herbicidal and soil fungicidal activities and have applications as
pesticides and insecticides.8
PhI(OAc)2,10b I2-MeOH,10c I2O5/KBr,10d and poly-1,3-dichloro-
5-methyl-5(40-vinylphenyl) hydantoin.10e
Recently, other
miscellaneous oxidizing agents also have been used namely poly-
mer-supported (diacetoxyiodo)benzene (PSDIB),11a silica supported
1,3-dibromo-5,5-dimethylhydantoin (DBH)11b trichloroisocyanuric
acid,11c 4-(p-chloro)phenyl-1,2,4-triazole-3,5-dione11d N-hydrox-
yphthalimide (NHPI)–cobalt(II) acetate/oxygen,11e Dess–Martin
(DMP) hypervalent iodine11f reagent etc., (see Scheme 1).
While many of these methods are effective for the preparation
of target compounds, some of them suffer from disadvantages,
such as use of toxic metals, prolonged reaction time, poor yields,
harsh reaction conditions, use of expensive and excessive reagents,
and formation of side products. The above mentioned demerits
demand further improvement for the conversion of pyrazolines
to pyrazoles. Herein, we wish to report an environmentally friendly
and extremely facile protocol for the preparation of pyrazoles and
pyridines by the oxidation of pyrazolines and Hantzsch dihydro-
pyridines, respectively. In a preliminary experiment the oxidative
aromatization of 1,3,5-trisubstituted pyrazolines to the
corresponding pyrazoles was carried out mediated by 3,4-dihy-
dro-2H-pyran, a small organic molecule under aerobic condition.
Due to the above mentioned activities of pyrazoles, a wide
variety of oxidative methods using various reagents have been
developed for the synthesis of such type of compounds. The
oxidizing agents generally used can broadly be classified into two
categories: (a) transition metal containing oxidizing agents such
as HgO,9a AgNO3,9b KMnO4,9c Pb(OAc)4,9d MnO2,9e Co(II)/O2,9f
9j
Bi(NO3)3ꢀ5H2O,9g Pd-C/AcOH,9h Zr(NO3)4,9i and HAuCl4-O2 and
R3
R3
(b) halogen-based oxidants such as N-bromosuccinimide (NBS),10a
N
3,4-dihydro-2H-pyran
N
N
N
R2
R2
Acetonitrile, 60 °C
R1
R1
⇑
Corresponding author.
E-mail address: ghmaiti123@yahoo.co.in (G. Maiti).
Scheme 1. Oxidation of 1,3,5-trisubstituted pyrazolines to pyrazole.
http://dx.doi.org/10.1016/j.tetlet.2014.07.051
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

5334
D. Banerjee et al. / Tetrahedron Letters 55 (2014) 5333–5337
Table 1
For optimization of the reaction conditions 1,3,5-triphenyl
Optimization of reaction conditionsa
pyrazoline 1a was chosen as a model substrate. During the course
of screening a variety of reaction conditions such as the amount of
the reagent, effect of solvent, and reaction temperature were
studied and the results are summarized in Table 1.
-pyran, air
3,4-dihydro-2H
N
N
N
N
solvent,temperature
It was found from Table 1 that the reaction in acetonitrile under
aerobic conditions at 60 °C for 2 h using 1.5 equiv of 3,4-DHP affor-
ded 1,3,5-trisubstituted pyrazole 2a with best yield (entry 3,
Table 1). It has been observed that when the reaction was per-
formed in acetic acid solvent other conditions remaining identical,
the product was obtained with low yield (entries 8–9, Table 1). The
conversion even became much slower in acetic acid without using
3,4-dihydro-2H-pyran (entry 7, Table 1). The reaction also pro-
ceeded with 2,3-dihydrofuran but needed much longer reaction
time and also the yield was low. Applying the optimized reaction
conditions17 a number of substrates were subjected to the conver-
sion and the results are summarized in Table 2. The experimental
results showed that electron-donating or electron-withdrawing
functionalities on aromatic rings did not affect much with respect
2a
1a
Entry 3,4-Dihydro-2H-pyran
Solvent
Temp.
Time
(h)
Yieldb
(%)
(equiv)
(°C)
1
2
3
4
5
6
7
8
9
1.5
1.5
1.5
1.5
1.5
1.5
Ethanol
PrOH
70
80
12
12
2
12
12
12
12
12
12
10
69
98
30
34
65
20
40
45
Acetonitrile 60
THF
Toluene
DCM
HOAc
HOAc
HOAc
60
90
40
80
80
80
—
1.0
1.5
a
Reaction conditions: 1,3,5-triphenylpyrazolidine (1.0 mmol), 3,4-dihydro-2H-
pyran (1.5 mmol) solvent (4 mL).
b
Yields of pure and isolated product.
Table 2
Oxidative aromatization of 1,3,5-trisubstituted-1H-pyrazolines (1) to their corresponding pyrazoles (2)a
R3
R3
3,4-dihydro-2H-pyran (1.5 eqv.)
N
N
N
N
R1
R2
R2
°
CH3CN, 60 C
R1
1
2
Entry
1
Substrates (1)
Products (2)
Time (h)
Yieldb (%)
N
N
N
N
2
98
1a
2a
N
N
N
N
N
N
2
3
4.5
4.5
99
96
Cl
Br
F
Cl
Br
1b
2b
N
N
N
1c
2c
N
N
N
4
3.5
98
F
2d
1d
N
N
N
N
N
N
5
6
3
4
98
97
Me
Me
1e
2e
N
N
OMe
OMe
1f
2f
N
N
N
N
7
4
99
Cl
Cl
Br
Br
1g
2g

D. Banerjee et al. / Tetrahedron Letters 55 (2014) 5333–5337
5335
Table 2 (continued)
Entry
Substrates (1)
Products (2)
Time (h)
Yieldb (%)
Cl
Cl
N
N
N
N
8
6
74
Cl
Cl
Br
Br
1h
2h
N
N
N
N
9
5
6
99
83
Me
Me
Me
Me
1i
2i
Br
Br
Me
Me
N
N
N
N
10
Br
1j
Br
2j
N
N
N
N
Me
Me
11
12
3.5
2.5
95
87
MeO
MeO
1k
2k
Me
Me
N
N
N
N
Cl
Cl
1l
Me
2l
Me
N
N
N
N
Me
Me
13
14
15
3
93
1m
Cl
Cl
2m
N
N
N
N
N
N
3
94
OMe
OMe
Me
Me
1n
2n
N
N
Cl
Cl
3.5
98
97
Me
Me
2o
1o
N
N
N
N
N
N
16
17
4
6
1p
2p
Me
Me
N
N
72
2q
1q
(continued on next page)

5336
D. Banerjee et al. / Tetrahedron Letters 55 (2014) 5333–5337
Table 2 (continued)
Entry
Substrates (1)
Products (2)
Time (h)
Yieldb (%)
N
N
N
N
18
2
79
Cl
Cl
2r
1r
Cl
Cl
N
N
N
N
19
9
68
S
S
2s
1s
a
Reaction conditions: pyrazoline (1.0 mmol), 3,4-dihydro-2H-pyran (1.5 mmol), acetonitrile (4 mL), 60 °C.
Yields of pure and isolated products.
b
Table 3
promising depressor effect and relatively good tolerability. These
compounds have been extensively studied due to their biological
significance in NADH redox process and also for therapeutic
functions, for example, calcium channel blockers for the treatment
of cardiovascular diseases,12 cancer, and AIDS.13 Aromatization of
1,4-DHPs has been thoroughly studied using various oxidants
because the oxidized products show antihypoxic and antiischemic
activities.14 This process has been conventionally done using an
excess of oxidizing reagents such as HNO3,15a DDQ,15b NaNO2,15c
Oxidation of dihydropyridines to pyridinesa
O
R
O
O
R
N
O
3,4-dihydro-2H-pyran (1.5 eqv.)
EtO
OEt
EtO
OEt
AcOH, 80 °C
N
H
3
4
Entry Substrates (1)
Products (2)
Time (h) Yieldb (%)
9i
(NH4)2Ce(NO3)6,15d Cu(NO3)2,15e Mn(OAc)3,15f and Zr(NO3)4.
Moreover, other environmentally benign processes, such as elec-
O
O
O
O
trochemical oxidation16a,b and catalytic aerobic oxidation using
1
2
8
95
92
activated carbon,16d Fe(ClO4)3,16e human hemoglobin–
EtO
Me
OEt EtO
OEt
16c
RuCl3
H2O2,16f or NHPI11e as the catalyst and HIO3 and I2O5 in water were
N
H
Me
Me
N
4a
Me
3a
also known in the literature for this type of reaction.10d
We extend the scope of this protocol further to oxidize the Han-
tzsch 1,4-dihydropyridines to pyridines using the present method.
In case of oxidation of 1,4-DHP it was found that acetic acid and
3,4-dihydro 2H-pyran combination gave better results compared
to acetonitrile and 3,4-dihydro 2H-pyran combination.
Me
Me
O
O
O
O
10
EtO
OEt
EtO
OEt
Me
N
Me
Me
N
Me
Thus, the treatment of diethyl-4-phenyl-2,6-dimethyl-1,4-dihy-
dropyridine-3,5-dicarboxylate (3a) with 1.5 equiv of 3,4-dihydro-
2H-pyran in acetic acid at 80 °C for 8 h afforded the corresponding
pyridine derivative (4a) in 95% yield. In this case acetonitrile and
ethanol solvents were almost ineffective (only 8% yield at 60 °C
after 12 h). The Hantzsch 1,4-dihydropyridines possessing variety
of substituents were oxidized to the corresponding pyridine deriv-
atives in excellent yields (92–96%) under this reaction condition
(Table 3).
It was realized from the study that 3,4-DHP and air/oxygen have
definitely a certain role in the conversion. But, their role could not
be understood in this stage and hence, the probable mechanism for
the conversion could not be suggested. It needs further extensive
studies which is beyond the scope in this stage.
In conclusion, we have demonstrated that 3,4-dihydro-2H-
pyran promoted aerial oxidation of pyrazolines and dihydropyri-
dines to the corresponding aromatized product pyrazoles and
pyridines respectively in good to excellent yield. The reaction is
simple, mild, highly efficient and products are easily separable
from the reaction mixture. Due to the numerous advantages the
present method might be complementary to the existing reported
methods and may find wide usage in generating diversity based
library of molecules related to pyrazoles and pyridines.
H
4b
Cl
3b
Cl
O
O
O
O
3
6
96
EtO
OEt EtO
OEt
Me
Me
N
H
3c
Me
Me
N
4c
OMe
OMe
O
O
O
O
4
7
94
EtO
Me
OEt EtO
Me
Me
OEt
Me
N
N
4d
H
3d
a
Reaction condition: 1,4-dihydropyridines (1.0 mmol), 3,4-dihydro-2H-pyran
(1.5 mmol), acetic acid (2 mL), 80 °C.
b
Yields of pure and isolated products.
to reaction time and yield of the product. Various functionalities
such as nitro, halogen, methoxy, and unsaturation group were
not affected in the reaction conditions. Only in a few cases yields
were moderate (entries 17, 18, and 19, Table 2) and required longer
reaction time.
Acknowledgments
Hantzsch 1,4-dihydropyridines (1,4-DHPs) are one of the first
line drugs for treatment of hypertension because of their
D.B., U.K. and R.K. thank CSIR, New Delhi for award of their
fellowships.

D. Banerjee et al. / Tetrahedron Letters 55 (2014) 5333–5337
5337
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17. Representative experimental procedure: To a magnetically stirred solution of
1,3,5-triphenyl-1H-pyrazoline 1a (300 mg, 1.01 mmol) in acetonitrile (4 mL),
3,4-dihydro-2H-pyran (127 mg, 1.51 mmol), was added. After stirring for
10 min at room temperature the reaction mixture was warmed to 60 °C and
stirred for 2 h at this temperature. After completion of the reaction (monitor by
TLC), acetonitrile was removed under reduced pressure. The crude residue was
purified by column chromatography over silica gel (100–200 mesh) eluting
with ethyl acetate–petroleum ether (1%) to afford 1,3,5-triphenyl-1H-pyrazole
2a (292 mg, 98 %) as a light yellow solid. Mp 136–138 °C, (lit. 139–140 °C), 1H
NMR (300 MHz, CDCl3) d ppm: 6.78 (s, 1H), 7.35–7.50 (m, 13H), 7.97–7.99 (m,
2H); 13C NMR (75 MHz, CDCl3) d ppm: 105.3, 125.4, 125.9, 127.5, 128.1, 128.4,
Supplementary data
Supplementary data (experimental procedures, compound
characterization data and spectra) associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2014.07.051.
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