Oxidative cyclization of N-aroylhydrazones to 2-(2-arylethenyl)-1,3,4-oxadiazoles using DDQ as an efficient oxidant

Article abstract of DOI:10.1016/j.tetlet.2015.09.018

A series of novel 5-aryl-2-(2-arylethenyl)-1,3,4-oxadiazoles were synthesized by the 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) promoted oxidative cyclization of N′-(arylmethylidene)-3-arylacrylohydrazides. A facile and efficient one-pot protocol using th

Full text of DOI:10.1016/j.tetlet.2015.09.018

Accepted Manuscript
Oxidative cyclization of N-aroylhydrazones to 2-(2-arylethenyl)-1,3,4-oxadia-
zoles using DDQ as an efficient oxidant
Karolina Jasiak, Agnieszka Kudelko
PII:
S0040-4039(15)30060-5
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.09.018
TETL 46690
Reference:
Tetrahedron Letters
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4 September 2015
Please cite this article as: Jasiak, K., Kudelko, A., Oxidative cyclization of N-aroylhydrazones to 2-(2-
arylethenyl)-1,3,4-oxadiazoles using DDQ as an efficient oxidant, TetrahedronLetters (2015), doi: http://dx.doi.org/
10.1016/j.tetlet.2015.09.018
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Oxidative cyclization of N-aroylhydrazones
to 2-(2-arylethenyl)-1,3,4-oxadiazoles using
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Karolina Jasiak, Agnieszka Kudelko

1
Tetrahedron Letters
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Oxidative cyclization of N-aroylhydrazones to 2-(2-arylethenyl)-1,3,4-oxadiazoles
using DDQ as an efficient oxidant
Karolina Jasiak, Agnieszka Kudelko∗
Department of Chemical Organic Technology and Petrochemistry, The Silesian University of Technology, Krzywoustego 4, PL-44100 Gliwice, Poland
ARTICLE INFO
ABSTRACT
Article history:
Received
A series of novel 5-aryl-2-(2-arylethenyl)-1,3,4-oxadiazoles were synthesized by the 2,3-
dichloro-5,6-dicyanobenzoquinone (DDQ) promoted oxidative cyclization of N'-
(arylmethylidene)-3-arylacrylohydrazides. A facile and efficient one-pot protocol using the
starting 3-arylacrylohydrazides and aromatic aldehydes is also reported. Short reaction times,
high yields, a simple work-up procedure and the possibility of regeneration of the oxidizing
agent make the reported method a promising alternative for the synthesis of 2,5-diaryl-1,3,4-
oxadiazoles.
Received in revised form
Accepted
Available online
Keywords:
1,3,4-Oxadiazoles
2009 Elsevier Ltd. All rights reserved.
N-Aroylhydrazones
2,3-Dichloro-5,6-dicyanobenzoquinone (DDQ)
Heterocycles
Oxidative cyclization
1. Introduction
The myriad of applications of 1,3,4-oxadiazoles has resulted
in the development of a number of synthetic methods for their
preparation. The most common methods can be divided into two
Among
five-membered
aromatic
heterocycles,
1,3,4-oxadiazoles have attracted considerable interest in recent
decades due to their broad spectrum of pharmaceutical and
biological activities.1 Many compounds containing this scaffold
groups: (a) oxidative cyclization of N-acylhydrazones using
reagents such as CAN,19 KMnO4,20 FeCl3,21 tetravalent lead
reagents,22 HgO/I2,23 Br2/NaOAc,24 chloramine T,25 and
hypervalent iodine reagents;26 (b) dehydrative cyclization of
N,N'-diacylhydrazines using PPA,27 H2SO4,28 SOCl2,29 POCl3,30
display
antimicrobial,2
antiviral,3
anti-inflammatory,4
antihypertensive,5 analgesic,6 anticonvulsant,7 antidiabetic8 and
antitubercular activities.9 They have also attracted attention in
medicinal chemistry as potential therapeutic agents for the
treatment of cancer10 and HIV infections.11 In addition, 1,3,4-
oxadiazole derivatives have been utilized as bioisosteric
replacements for carboxylic acid,12 ester13 and amide14 functional
groups in biologically active compounds. 2,5-Disubstituted 1,3,4-
oxadiazoles have found further application in agriculture as plant
growth regulators15 and crop protection agents.16 These
heterocyclic molecules have also been used to produce heat-
resistant polymers, optical brighteners, blowing agents and
corrosion inhibitors for mild steel.17 π-Conjugated arrangements
possessing the 1,3,4-oxadiazole moiety, such as 2-(4-biphenylyl)-
5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), are well known
as electron conducting and hole blocking (ECHB) materials in
polymer and material science due to their electron deficient
nature, high photoluminescence quantum yield as well as both
thermal and chemical stability.18 Therefore, they have been
applied as monomers in the production of fluorescent emitting
layers for organic electronics, particularly in photovoltaic cells,
organic light-emitting diodes (OLEDs) and photosensitive
materials.18
33
P2O5,31 (CF3SO2)2O,32 BF3·OEt2 and the Burgess reagent.34
Besides these reactions, 1,3,4-oxadiazole derivatives have also
been prepared by the one-pot reaction of acid hydrazides with
carboxylic acids,35 aromatic aldehydes19 or orthoesters,36 the
heterocyclization of semicarbazide, thiosemicarbazide and
selenosemicarbazide derivatives,37 photoisomerization of 1,2,4-
oxadiazoles,38 as well as N-acylation and subsequent ring
opening/closing of tetrazoles.39 In recent years, the solid phase
syntheses of 2,5-disubstituted 1,3,4-oxadiazoles has also been
reported.40 However, many of the above-mentioned methods
suffer from drawbacks, such as the use of expensive, hazardous
reagents, long reaction times, harsh reaction conditions, limited
substrate scope and difficult work-up procedures. Hence, the
development of a facile and efficient synthetic protocol to
overcome these difficulties is still highly desirable. In
continuation of our studies on the synthesis of 1,3,4-oxadiazoles
that are conjugated via an ethenyl linker to benzene, thiophene,
furan and pyridine rings,36c,41 we describe the efficient oxidative
cyclization of N-acylhydrazones using DDQ. To the best of our
knowledge, the application of DDQ to the preparation of 2,5-
disubstituted 1,3,4-oxadiazoles has not been reported.
———
∗ Corresponding author. Tel.: +48 32 2371729 ; fax: +48 32 2371021; e-mail: agnieszka.kudelko@polsl.pl (A. Kudelko)

2
Tetrahedron
2. Results and discussion
co-workers for N-acylhydrazones and by Kudelko and Zieliński
for N'-ethoxymethylene-2-(N-Boc-amino)propionohydrazides or
N-protected N'-ethoxymethylene phenylglycine hydrazides.42
The hydrazides of 3-arylacrylic acids 2a-c utilized as starting
materials were prepared from their respective aldehydes, using a
previously reported four-step synthetic protocol. Formation of
α,β-unsaturated carboxylic acids via the piperidine-catalyzed
Knoevenagel-Doebner reaction was followed by neutralization
with KOH. The resulting potassium salts were transformed into
the corresponding 3-arylacrylic acid hydrazides 2a-c in a one-pot,
two-step reaction sequence by treatment with ethyl chloroformate
in acetonitrile, followed by the addition of excess hydrazine
hydrate (Scheme 1).
N'-(4-Methoxybenzylidene)-3-phenylacrylohydrazide
(3e)
was selected as the model substrate in order to optimize the
reaction conditions. Initially, various oxidants, including KMnO4,
CAN, Oxone and DDQ, were screened for the reaction. Among
the oxidants tested, DDQ was found to give the highest yield of
the product, 5-(4-methoxyphenyl)-2-(2-phenylethenyl)-1,3,4-
oxadiazole 4e (67%, Table 1, entry 1-4). In addition, the reduced
by-product (DDQ-H2) could be easily removed from the reaction
mixture by filtration and recycled by oxidation with MnO2. It
should be noted that the starting N-aroylhydrazones contain an
unsaturated C=C double bond, that is susceptible to oxidation,
which is why effective and selective oxidation in this reaction
remains a challenge. The reactions of other oxidizing agents
resulted in the formation of side-products, where the desired
conjugated arrangement was destroyed. Hence, DDQ was
selected as the most efficient and selective oxidizing agent and
used throughout this work. Further optimization with respect to
the solvent showed that toluene gave the best result in terms of
both yield and time (95%, Table 1, entry 4-6). Finally, the
optimal procedure was established as follows: reflux in toluene
for 5 h in the presence of an equimolar amounts of DDQ.
The resulting hydrazides 2a-c were heated with the
appropriate aromatic aldehydes 1a-h in ethanol, in the presence
of catalytic HCl to yield N-aroylhydrazones 3a-p in high yields
(77-95%, Table 2). These acyclic products precipitated
immediately after mixing the reagents and were purified by
recrystallization from ethanol. The structures of all novel
compounds 3a-p were confirmed by elemental analysis and
spectroscopic methods (1H and 13C NMR, MS, UV, IR). Analysis
of the spectra of N-aroylhydrazones 3a-p in DMSO, showed
double the number of signals which was ascribed to the presence
of C=C and C=N double bonds, which are a source of geometric
isomerism. Similar observations were reported by Palla and
Scheme 1. Synthesis of the key N-aroylhydrazones 3a-p. Reagents and conditions: (i) CH2(COOH)2, pyridine, piperidine, reflux,
2 h; (ii) KOH, H2O; (iii) ClCOOEt, MeCN, pyridine, reflux, 2 h; (iv) N2H4⋅H2O, MeCN, 0 °C, 24 h; (v) aromatic aldehyde, cat.
HCl, EtOH, reflux, 2 h.
Table 1. Optimization of the reaction conditions for the synthesis of 5-(4-methoxyphenyl)-2-(2-phenylethenyl)-1,3,4-oxadiazole
(4e)a
Entry
Oxidant
Solvent
Time (h)
Yieldb (%)
1
2
KMnO4
CAN
CHCl3-H2Oc
CHCl3
20
15
40
22
3
Oxone
CHCl3
25
0
4
5
6
DDQ
DDQ
DDQ
CHCl3
MeCN
Toluene
8
8
5
67
71
95
aReaction conditions: 3e (5.0 mmol), oxidant (5.0 mmol, for KMnO4 15.0 mmol), solvent (30 mL).
bIsolated yield.
cSolvents used in 4:1 (v/v) ratio.

3
Table 2. Formation of 5-aryl-2-(2-arylethenyl)-1,3,4-oxadiazoles 4a-p using a stepwise synthesis and proceeding via
intermediate N-aroylhydrazones 3a-p and using a one-pot synthesis starting from 3-arylacrylohydrazides 2a-c
O
O
O
O
DDQ, toluene,
Ar'
HCl, EtOH,
N
Ar'
Ar
+
Ar
N
H
stepwise
Ar
NHNH2
Ar'
H
N
N
H
2a-c
1a-h
3a-p
4a-p
DDQ, p-TsOH, toluene,
one-pot
Stepwise synthesisa
1,3,4-Oxadiazole 4
Time (h)
One-pot synthesisb
Product
Ar
Ar'
Hydrazone 3
Yieldc (%)
1,3,4-Oxadiazole 4
Yieldc (%)
Yieldc (%)
95
92
96
84
89
86
77
81
86
93
95
83
90
95
98
86
a
b
c
d
e
f
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
73
6
75
86
90
49
91
75
40
75
88
90
94
79
96
92
97
84
2-furyl
82
87
43
95
69
26
75
91
86
89
78
94
89
96
80
5
5
2-thienyl
2-pyrrolyl
4-MeO-C6H4
4-O2N-C6H4
4-pyridyl
9-anthryl
4-MeO-C6H4
2-furyl
10
5
6
10
8
g
h
i
2-furyl
3
2-furyl
4
j
2-furyl
2-thienyl
9-anthryl
4-MeO-C6H4
2-furyl
3
k
l
2-furyl
6
m
n
o
p
2-thienyl
2-thienyl
2-thienyl
2-thienyl
3
3
2-thienyl
9-anthryl
2
5
aReaction conditions: 3 (5.0 mmol), DDQ (5.0 mmol), toluene (30 mL), reflux, 2-10 h.
bReaction conditions: 1 (5.0 mmol), 2 (5.0 mmol), p-TsOH (0.1 mmol), DDQ (5.0 mmol), toluene (30 mL), reflux, 3 h.
cIsolated yield.
With the optimized reaction conditions in hand, N'-
(arylmethylidene)-3-arylacrylohydrazides 3a-p were examined to
determine the scope and limitations of the reaction (Table 2). To
our delight, the reaction displayed a wide substrate scope and
proceeded smoothly to afford the 5-aryl-2-(2-arylethenyl)-1,3,4-
oxadiazoles 4a-p, largely, in good yields (Table 2). Initially, N-
aroylhydrazones 3a,e,f were tested, which indicated that this
3-(2-furyl)acrylic acid hydrazide (2b) or 3-(2-thienyl)acrylic acid
hydrazide (2c) were smoothly converted to the corresponding
2,5-disubstituted 1,3,4-oxadiazoles 4i-p in good to excellent
yields (78-96%, Table 2). Comparing the three series of
heterocyclic products (Ar = Ph, 4a-h; Ar = 2-furyl, 4i-l and Ar =
2-thienyl, 4m-p), the best results were observed in the derivatives
containing the thiophene ring. This trend was also observed for
the synthesis of the acyclic N-aroylhydrazones 3a-p (77-98%;
Table 2).
synthetic methodology was compatible with
a variety of
substituents on the benzene ring of the precursor aldehyde. The
2-styryl-1,3,4-oxadiazoles substituted at the 5-position with
heteroaryl groups 4b-d,g were synthesized with yields ranging
from 26% to 87%. It should be noted that although heterocycles
such as furan and thiophene were found to be stable under the
optimized reaction conditions, those possessing a pyrrole 3d or
pyridine ring 3g gave lower yields due to the formation of by-
products as indicated by TLC. N'-(9-Anthrylmethylidene)-3-
phenylacrylohydrazide (3h) was also converted to the
corresponding 1,3,4-oxadiazole 4h in 75% yield.
In addition, preliminary studies were conducted with aliphatic
hydrazides and aliphatic aldehydes to further explore the scope
and generality of this reaction. Two N-acylohydrazones derived
from crotonohydrazide and 4-methoxybenzaldehyde or 3-
phenylacrylohydrazide and propionaldehyde were selected. The
synthesis of these acyclic intermediates required longer reaction
times, giving lower yields with numerous by-products. Similar
trends were also observed for the synthesis of the corresponding
1,3,4-oxadiazoles, demonstrating that this methodology does not
work as well when either of the counterparts are aliphatic.
Further in-depth studies are currently in progress.
To further investigate the general utility of this
transformation, 3-phenylacrylohydrazide (2a) was replaced with
other α,β-unsaturated acid hydrazides. Substrates derived from

4
Tetrahedron
Scheme 2. Proposed mechanism for the DDQ-mediated cyclization of N-aroylhydrazones.
Additionally, we decided to explore the possibility of
conducting the reaction as a one-pot procedure, thus avoiding the
purification of the intermediate N-aroylhydrazones 3a-p. A
mixture of the appropriate 3-arylacrylic acid hydrazide 2a-c and
aromatic aldehyde 1a-h in dry toluene was heated at reflux with
DDQ in the presence of catalytic p-toluenesulfonic acid for 3 h.
To our satisfaction, we observed that all tested substrates reacted
smoothly to afford the 5-aryl-2-(2-arylethenyl)-1,3,4-oxadiazoles
4a-p in satisfactory yields (40-97%, Table 2). The developed
one-pot procedure gave results that were comparable to those
obtained using the stepwise protocol.
3. Conclusions
In conclusion, we have demonstrated the effectiveness and
selectivity of DDQ for the synthesis of a series of novel,
conjugated 5-aryl-2-(2-arylethenyl)-1,3,4-oxadiazoles proceeding
via the oxidative cyclization of N-aroylhydrazones. Commercial
availability, high reactivity, recyclability, low cost and low
toxicity make DDQ a versatile and useful oxidizing agent for this
reaction. In addition, we have developed a facile and efficient
method to synthesize 1,3,4-oxadiazoles directly from
α,β-unsaturated acid hydrazides and aromatic aldehydes in a one-
pot procedure. This methodology offers the advantage of short
reaction times, high yields and a wide substrate scope, which
makes it a promising alternative to the existing synthetic
protocols.
Based on literature reports,43 a plausible mechanism was
proposed (Scheme 2). We presume that N-aroylhydrazone 3
tautomerizes to hydroxyhydrazone A, containing a stable and
highly extended π-conjugated system. The reaction of A with
DDQ generates the resonance stabilized oxygen-centred radical
B. Subsequent 1,5-homolytic radical cyclization of B leads to the
formation of the final 1,3,4-oxadiazole ring 4 and reduced DDQ-
H2 which precipitated as a yellow solid. The presence of an
electron-donating substituent on the Ar′ ring stabilizes radical B,
resulting in a higher yield of the desired product (4e, 91%, Table
2). Conversely an electron-withdrawing substituent resulted in a
decrease in yield (4f, 75%, Table 2).
Acknowledgments
The authors thank Professor Wojciech Zieliński for
stimulating discussions and Roman Komor, Ph.D. student, from
The Silesian University of Technology for 2D NMR analysis.
References and notes
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The structures of the 5-aryl-2-(2-arylethenyl)-1,3,4-
oxadiazoles 4a-p were established by elemental analysis and
spectroscopic methods. The proton NMR spectrum confirmed the
successful synthesis of the desired products by the disappearance
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aryl substituent and appeared in the range of 163 to 165 ppm.
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