Synthesis of 3,5-Disubstituted 1,2,4-Oxadiazoles from Amidoximes and Aldehydes in the Superbasic System NaOH/DMSO

Article abstract of DOI:10.1134/S107042802007009X

Abstract: A new procedure has been proposed for the synthesis of 3,5-disubstituted 1,2,4-oxadiazoles by reaction of amidoximes with aldehydes in the superbasic system NaOH/DMSO at room temperature. The scope of the proposed procedure has been demonstrated by 15 syntheses from various amidoximes and aromatic aldehydes with 27–76% yields. The procedure is inapplicable to aliphatic aldehydes.

Full text of DOI:10.1134/S107042802007009X

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2020, Vol. 56, No. 7, pp. 1181–1186. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Zhurnal Organicheskoi Khimii, 2020, Vol. 56, No. 7, pp. 1064–1070.
Synthesis of 3,5-Disubstituted 1,2,4-Oxadiazoles
from Amidoximes and Aldehydes in the Superbasic
System NaOH/DMSO
A. A. Shetneva,*, V. E. Pankratievab, A. S. Kunichkinab, A. S. Vlasova, I. K. Proskurinab,
A. D. Kotovb, and M. K. Korsakova
a Dorogov Pharmaceutical Technology Transfer Center, Ushinsky Yaroslavl State Pedagogical University,
Yaroslavl, 150000 Russia
b Ushinsky Yaroslavl State Pedagogical University, Yaroslavl, 150000 Russia
*e-mail: zlodeus@gmail.com
Received March 10, 2020; revised April 22, 2020; accepted April 22, 2020
Abstract—A new procedure has been proposed for the synthesis of 3,5-disubstituted 1,2,4-oxadiazoles by
reaction of amidoximes with aldehydes in the superbasic system NaOH/DMSO at room temperature. The scope
of the proposed procedure has been demonstrated by 15 syntheses from various amidoximes and aromatic
aldehydes with 27–76% yields. The procedure is inapplicable to aliphatic aldehydes.
Keywords: aldehyde, amidoxime, oxadiazole, superbasic medium, condensation, dimethyl sulfoxide, sodium
hydroxide
DOI: 10.1134/S107042802007009X
1,2,4-Oxadiazoles possess various useful properties,
and they occupy an important place in applied chem-
istry. Derivatives of 1,2,4-oxadiazoles are components
of many drugs [1–4] and are used as base structures of
liquid crystals, energetic compounds [5], and lumino-
phores [6]. Several methods of synthesis of 1,2,4-oxa-
diazoles have been reported [7–12], the most practical
of which are those based on the condensation of
amidoximes with carboxylic acids and their derivatives.
Superbase-catalyzed reactions of amidoximes with
electrophiles, in particular with acetylene [13, 14],
were studied in [15–20], where 1,2,4-oxadiazoles were
synthesized from esters, imidazolides, carboxylic acid
anhydrides, and other carbonyl compounds in the
system NaOH/DMSO. The products of these reactions
were often found to possess valuable biological
properties [21–25]. However, until recently, reactions
of amidoximes with aldehydes have been reported in
a few publications; these reactions were catalyzed by
4-toluenesulfonic [26] or acetic acid or carried out
under microwave irradiation [27], and the yields were
generally moderate to low. In our opinion, the lack of
reliable methods of synthesis of needed heterocyclic
systems of the 1,2,4-oxadiazole series using aldehydes
as initial compounds is a serious gap in the set of tools
of modern organic chemistry since aromatic and ali-
phatic aldehydes are synthetic precursors of carboxylic
acids and their derivatives. In some cases, the use of
aldehydes for the synthesis of practically important
1,2,4-oxadiazoles could be preferable from the eco-
nomic viewpoint. An example is the large-scale pro-
duction of such aldehydes as 4-hydroxymethylfurfural,
furfural, fructose, formaldehyde, propanal, acetalde-
hyde, vanillin, and many others from natural raw
materials.
Cesium carbonate-catalyzed thermal oxidative con-
densation of aldehydes and amidoximes [28] has
become an important step in filling the above gap in
the synthesis of 1,2,4-oxadiazoles. In this reaction, the
aldehyde molecule acts as not only electrophile but
also oxidant for initially formed dihydrooxadiazole
ring. Further to this study, herein we report the reaction
of amidoximes with aromatic and aliphatic aldehydes
in the system NaOH/DMSO as a part of our research
on superbase-catalyzed reactions of amidoxime. We
found that amidoximes reacted with aromatic alde-
hydes in DMSO to give 1,2,4-oxadiazoles even at room
temperature and that the target products can be isolated
1181

SHETNEV et al.
1182
Scheme 1.
O
N
O
NOH
NH2
N
PhCHO (2a)
NaOH, DMSO, r.t.
[O], NaOH
DMSO, r.t.
Ph
Ph
N
H
N
F3C
F3C
F3C
1a
4a
3a
in acceptable yields despite the possibility of concur-
rent base-catalyzed Cannizzaro disproportionation of
the aldehyde.
chromatographic purification from products of alde-
hyde disproportionation in alkaline medium. Apart
from excess aldehyde, Oxone (entry no. 10), N-chloro-
succinimide (NCS, entry no. 11), atmospheric oxygen
(entry no. 12), and dichlorodicyanoquinone (DCQ,
entry no. 13) were tried as oxidants. Replacement of
the solvent (DMSO) by N,N-dimethylformamide (entry
no. 8) or N-methylpyrollidin-2-one (entry no. 9) did
not affect the product yield to an appreciable extent. In
all cases, the yields of 3a were lower than in the model
reaction with 2.2 equiv of benzaldehyde (entry no. 7),
and we failed to reduce the consumption of aldehyde in
the system NaOH/DMSO.
The reaction of N′-hydroxy-4-(trifluoromethyl)-
benzimidamide (1a) with benzaldehyde (2a) was used
as a model to optimize the conditions (Scheme 1); the
results are summarized in Table 1.
We believe that the reaction proceeds in two steps.
In the first step, amidoxime 1 reacts with aldehyde 2 in
DMSO to give intermediate 4. Under superbasic condi-
tions, this reaction is complete in 1 h. The second step
is oxidation of dihydrooxadiazole 4 to oxadiazole 3,
which takes 18–24 h. In order to improve the yield of
target 1,2,4-oxadiazoles 3, such conditions as reaction
time, amount of the base, and reactant ratio were
varied. In most experiments (Table 1, entry nos. 1–9),
the dihydrooxadiazole ring in 4 was oxidized with
excess aldehyde. In order to obtain satisfactory to good
yield, more than 2 equiv of aldehyde should be used.
However, in this case, the product isolation procedure
was complicated due to the necessity of resorting to
Thus, the optimal conditions included DMSO as
solvent, temperature 20–25°C, 1.8 equiv of NaOH,
amidoxime-to-aldehyde molar ratio 1:2.2, and reaction
time no less than 22 h. These conditions were applied
to reactions with other amidoximes 1b–1l and alde-
hydes 2a–2e to synthesize the corresponding 3,5-disub-
stituted 1,2,4-oxadiazoles 3b–3n in up to 69% yield
(Scheme 2). The substituent in the amidoxime fragment
Table 1. Condensation of N′-hydroxy-4-(trifluoromethyl)benzimidamide (1a) with benzaldehyde (2a) in superbasic systems at
room temperature
Entry no.
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSOa
DMSO
DMF
Base (equiv)
1.5
Aldehyde (equiv)
Oxidant
Time, h
2–48
22
Yield of 3a, %
1
1.1
1.2
1.5
1.2
1.5
1.5
2.2
1.5
1.5
1.1
1.1
1.1
1.1
–
21–24
23
28
45
52
50
76
6
2
1.5
–
3
1.5
–
22
4
1.8
–
22
5
1.8
–
2–22
2–22
22
6
1.8
–
7
1.8
–
8
1.8
–
22
9
10
DMA
1.8
–
22
16
49
31
52
61
DMSO
DMSO
DMSO
DMSO
1.8
KHSO5 (1 equiv)
NCSb (1 equiv)
O2
22
11
1.8
22
12
1.8
22
13
1.8
DCQc (1 equiv)
22
a
b
c
At 100°C.
N-Chlorosuccinimide.
Dichlorodicyanoquinone.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 7 2020

SYNTHESIS OF 3,5-DISUBSTITUTED 1,2,4-OXADIAZOLES
1183
Scheme 2.
N
O
NOH
NaOH, DMSO, r.t.
+
R2CHO
R1
R2
R1
NH2
1b–1l
N
2a–2e
3b–3n
1, R1 = 4-MeOC6H4 (b), PhCH2 (c), pyridin-2-yl (d), 4-MeC6H4 (e), 2-ClC6H4 (f), 4-FC6H4 (g), Ph (h), 4-O2NC6H4 (i), cyclo-
propyl (j), thophen-3-yl (k), 5-methylthiophen-2-yl (l); 2, R2 = Ph (a), thiophen-3-yl (b), furan-2-yl (c), 1H-pyrrol-2-yl (d),
5-bromofuran-2-yl (e).
O
O
O
N
N
N
N
O
Ph
Ph
Ph
N
Ph
Cl
N
N
N
Ph
Ph
N
MeO
Me
3b, 33%
3c, 35%
3d, 42%
3e, 56%
O
O
O
N
N
N
N
O
Ph
Ph
N
N
N
Ph
Ph
N
O2N
F
3f, 36%
3g, 36%
3h, 64%
3i, 41%
N
O
N
O
N
O
N
O
H
N
O
Ph
Ph
Ph
N
N
N
N
S
S
3j, 39%
3k, 69%
3l, 59%
3m, 27%
N
O
N
S
O
Me
Br
3n, 69%
weakly affected the yield of oxadiazoles 3. Neverthe-
less, somewhat lower yields were obtained in reactions
with amidoximes having an electron-withdrawing sub-
stituent in the aromatic ring. Despite generally modest
yields, the yields of compounds 3l and 3m in the reac-
tions of N′-hydroxybenzimidamide (1h) with furfural
(2c) and 1H-pyrrole-2-carbaldehyde (2d) were higher
than those reported in [29, 18].
EXPERIMENTAL
Unless otherwise stated, organic and inorganic
reagents and solvents were commercial products
(Aldrich, Vekton, Ekros) and were used without further
purification. The progress of reactions was monitored
by TLC on Silufol UV 254 plates using acetone–
toluene–n-hexane (5:3:5) as eluent.
The NMR spectra were recorded on a Varian XL-
400 spectrometer at 25°C from solutions in DMSO-d6
or CDCl3 using the residual proton and carbon signals
of the solvent as reference (DMSO-d5, δ 2.50 ppm;
DMSO-d6, δC 39.5 ppm). The high-resolution mass
spectra (electron impact, 70 eV, ion source tempera-
ture 100–220°C) were obtained with a Kratos MS-30
instrument (UK) at the Zelinsky Institute of Organic
Chemistry, Russian Academy of Sciences (Moscow).
Elemental analyses were carried out with a Perkin
Elmer 2400 analyzer. The melting points were meas-
ured with a Büchi M-560 apparatus.
Attempts to obtain 3,5-disubstituted 1,2,4-oxadi-
azoles from amidoximes and aliphatic aldehydes
(formaldehyde, acetaldehyde, propanal) were unsuc-
cessful, and no intermediate dihydrooxazole was
detected in the reaction mixtures by chromatography.
Neither the use of dehydrating agents (Na2SO4,
4-Å molecular sieves) nor elevated temperature gave
positive result. It should be noted than in the reaction
of N′-hydroxy-4-methylbenzimidamide with formal-
dehyde under the proposed conditions, we isolated
54% of 4-methylbenzonitrile instead of desired
oxadiazole 3.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 7 2020

SHETNEV et al.
1184
Initial amidoximes were synthesized according to
ppm: 7.59–7.62 m (4H), 7.67–7.74 m (2H), 8.08 d (1H,
the procedure described in [30].
J = 7.6 Hz), 8.08 d (1H, J = 7.6 Hz).
3-(4-Fluorophenyl)-5-phenyl-1,2,4-oxadiazole
General procedure for the synthesis of 3,5-disub-
stituted 1,2,4-oxadiazoles from amidoximes and
aldehydes in the superbasic system NaOH/DMSO.
A 10-mL flask was charged with 1 mmol of amidoxime
which was dissolved in 1.5 mL of DMSO, and
2.2 mmol of aldehyde 2 and 0.05 mmol (2 mg) of
powdered sodium hydroxide were added each in one
portion. The mixture was stirred for 1 h, 1.75 mol
(70 mg) of powdered sodium hydroxide was added,
and the mixture was stirred for 21 h more. After com-
pletion of the reaction (TLC), the resulting suspension
was diluted with 7 mL of distilled water, and the
precipitate was filtered off and washed with 2 mL of
water. If a tarry material was formed, the supernatant
was separated by decanting, and the residue was
washed with three 2–4-mL portions of water. The
products were purified by chromatography using
acetone–toluene–petroleum ether (5:3:5) as eluent.
(3g) [33]. Yield 86 mg (36%), pinkish white powder,
1
mp 132–134°C. H NMR spectrum (400 MHz,
DMSO-d6), δ, ppm: 7.46 t (2H, J = 11.4 Hz), 7.63 s
(2H), 8.09 d (2H, J = 8.6 Hz), 8.14–8.19 m (2H).
3,5-Diphenyl-1,2,4-oxadiazole (3h) [34]. Yield
1
142 mg (64%), white powder, mp 91–93°C. H NMR
spectrum (400 MHz, DMSO-d6), δ, ppm: 7.64 s (5H),
8.08–8.13 m (4H). 13C NMR spectrum (101 MHz,
DMSO-d6), δC, ppm: 126.0, 126.1, 126.8, 127.2, 128.8,
129.4, 131.9, 128.0, 168.5, 174.5.
3-(4-Nitrophenyl)-5-phenyl-1,2,4-oxadiazole (3i)
[35]. Yield 109 mg (41%), yellow crystalline powder,
1
mp 131–133°C. H NMR spectrum (400 MHz,
DMSO-d6), δ, ppm: 7.83 d (3H, J = 8.0 Hz), 8.12 t (2H,
J = 8.4 Hz), 8.43–8.49 m (4H).
5-Cyclopropyl-3-(thiophen-3-yl)-1,2,4-oxadiazole
(3j) [23]. Yield 75 mg (39%), off-white powder,
mp 35–37°C. 1H NMR spectrum (400 MHz,
DMSO-d6), δ, ppm: 1.20–1.14 m (2H), 1.30–1.23 m
(2H), 2.33–2.40 m (1H), 7.54 d.d (1H, J = 5.1, 1.2 Hz),
7.77–7.71 m (1H), 8.24–8.18 m (1H). 13C NMR spec-
trum (101 MHz, DMSO-d6), δC, ppm: 7.6, 10.4, 126.1,
128.1, 128.8, 128.9, 164.6, 181.9.
5-Phenyl-3-[4-(trifluoromethyl)phenyl]-1,2,4-
oxadiazole (3a) [31]. Yield 221 mg (76%), white
1
powder, mp 93–95°C. H NMR spectrum (400 MHz,
DMSO-d6), δ, ppm: 7.53–7.77 m (3H), 7.91 d (2H, J =
7.7 Hz), 8.19 m (4H).
3-(4-Methoxyphenyl)-5-phenyl-1,2,4-oxadiazole
(3b) [32]. Yield 83 mg (33%), white powder, mp 92–
5-Phenyl-3-(thiophen-3-yl)-1,2,4-oxadiazole (3k)
[23]. Yield 75 mg (69%), white solid, mp 125–127°C.
1H NMR spectrum (400 MHz, DMSO-d6), δ, ppm:
7.70–7.61 m (3H), 7.73 t (1H, J = 7.4 Hz), 7.81 d.d
(1H, J = 5.0, 3.0 Hz), 8.22–8.12 m (2H), 8.37 d.d (1H,
J = 2.7, 0.9 Hz).
1
94°C. H NMR spectrum (400 MHz, DMSO-d6), δ,
ppm: 3.85 s (3H), 7.14 d (2H, J = 8.4 Hz), 7.60–7.78 m
(3H), 8.04–8.06 m (2H), 8.22–8.17 m (2H).
3-Benzyl-5-phenyl-1,2,4-oxadiazole (3c) [33].
Yield 83 mg (35%), off-white powder, mp 89–91°C.
1H NMR spectrum (400 MHz, DMSO-d6), δ, ppm:
4.11 s (2H), 7.13 d (2H, J = 7.8 Hz), 7.23 d (2H, J =
7.8 Hz), 7.64–7.91 m (5H), 8.06 d (2H, J = 7.4 Hz).
5-(Furan-2-yl)-3-phenyl-1,2,4-oxadiazole (3l)
[29]. Yield 125 mg (59%), off-white solid, mp 101–
1
102°C. H NMR spectrum (400 MHz, DMSO-d6), δ,
5-Phenyl-3-(pyridin-2-yl)-1,2,4-oxadiazol (3d)
ppm: 6.88–6.90 m (1H), 7.57–7.65 m (4H), 8.07–
8.09 m (2H), 8.18 s (1H).
[16]. Yield 94 mg (42%), white powder, mp 129–
1
131°C. H NMR spectrum (400 MHz, DMSO-d6), δ,
3-Phenyl-5-(1H-pyrrol-2-yl)-1,2,4-oxadiazole
(3m) [18]. Yield 214 mg (27%), off-white powder,
ppm: 7.41–7.73 m (5H), 8.04 s (1H), 8.17 t (3H, J =
8.2 Hz), 8.79 s (1H).
1
mp 131–132°C. H NMR spectrum (400 MHz,
3-(4-Methylphenyl)-5-phenyl-1,2,4-oxadiazole
DMSO-d6), δ, ppm: 6.34 t (1H, J = 4.4 Hz), 7.08 d (1H,
J = 3.6 Hz), 7.22 s (1H), 7.56–7.59 m (3H), 8.06 d.d
(2H, J = 8.0, 4.4 Hz), 12.44 s (1H).
(3e) [18]. Yield 116 mg (52%), white powder, mp 102–
1
104°C. H NMR spectrum (400 MHz, DMSO-d6), δ,
ppm: 2.38 s (3H), 7.38 d (2H, J = 8.1 Hz), 7.64 t (2H,
J = 7.4 Hz), 7.72 t (1H, J = 7.4 Hz), 7.96 d (2H, J =
8.1 Hz), 8.15 d (2H, J = 7.1 Hz).
5-(5-Bromofuran-2-yl)-3-(5-methylthiophen-2-
yl)-1,2,4-oxadiazole (3n). Yield 214 mg (69%), off-
1
white powder, mp 89–90°C. H NMR spectrum
3-(2-Chlorophenyl)-5-phenyl-1,2,4-oxadiazole
(400 MHz, CDCl3), δ, ppm: 7.65 d (1H, J = 3.5 Hz),
7.28 d.d (1H, J = 3.8, 1.6 Hz), 6.86–6.75 m (1H),
6.57 d.d (1H, J = 3.6, 1.6 Hz). 13C NMR spectrum
(3f) [18]. Yield 93 mg (36%), white powder, mp 87–
1
88°C. H NMR spectrum (400 MHz, DMSO-d6), δ,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 56 No. 7 2020

SYNTHESIS OF 3,5-DISUBSTITUTED 1,2,4-OXADIAZOLES
1185
Gray, J.L., Tetrahedron Lett., 2006, vol. 47, p. 3629.
https://doi.org/10.1016/j.tetlet.2006.03.155
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https://doi.org/10.1016/s0960-894x(98)00712-4
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(101 MHz, CDCl3), δC, ppm: 15.74, 76.91, 77.23,
77.54, 114.72, 118.96, 125.38, 126.60, 128.48, 130.61,
141.91, 145.13, 165.01, 166.39. Mass spectrum:
m/z 310.9479 [C11H7BrN2O2S + H]+. Calculated:
M + H 310.9485.
4-Methylbenzonitrile [36]. Yield 63 mg (54%),
1
light yellow crystals, mp 27–29°C. H NMR spectrum
https://doi.org/10.1070/rc2014v083n07abeh004425
(400 MHz, CDCl3), δ, ppm: 2.42 s (3H), 7.27 d (2H,
J = 1.0 Hz), 7.53 d (2H, J = 1.0 Hz).
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FUNDING
https://doi.org/10.1134/s1070428018080213
This study was performed under financial support by the
Russian Foundation for Basic Research (project no. 18-33-
01108).
16. Sharonova, T., Pankrat’eva, V., Savko, P., Baykov, S.,
and Shetnev, A., Tetrahedron Lett., 2018, vol. 59,
p. 2824.
https://doi.org/10.1016/j.tetlet.2018.06.019
CONFLICT OF INTEREST
The authors declare no conflict of interest.
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