One-flask synthesis of 1,3,5-trisubstituted 1,2,4-triazoles from nitriles and hydrazonoyl chlorides via 1,3-dipolar cycloaddition

Article abstract of DOI:10.1039/c4ra00113c

A one-flask strategy for the synthesis of 1,3,5-trisubstituted 1,2,4-triazoles 4a-s and 8a and b from nitriles 5a-i with N-arylhydrazonoyl hydrochlorides 3a-h and 7a and b under basic conditions was developed. The reaction provided the desired 1,2,4-triazoles in moderate to excellent yields (56-98%), and was applicable to aliphatic and aromatic nitriles as well as N-phenylhydrazonoyl hydrochlorides bearing ester and acetyl functionalities. A 1,3-dipolar cycloaddition between imidate and nitrilimine generated from the respective nitrile and N-arylhydrazonoyl chloride in one flask was proposed for the new transformation.

Full text of DOI:10.1039/c4ra00113c

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One-flask synthesis of 1,3,5-trisubstituted 1,2,4-
triazoles from nitriles and hydrazonoyl chlorides via
1,3-dipolar cycloaddition†
Cite this: RSC Adv., 2014, 4, 14215
Li-Ya Wang,ab Henry J. Tsai,c Hui-Yi Lin,d Kimiyoshi Kaneko,e Fen-Ying Cheng,f
Received 6th January 2014
Accepted 6th March 2014
f
a
f
*
*
Hsin-Siao Shih, Fung Fuh Wong and Jiann-Jyh Huang
DOI: 10.1039/c4ra00113c
www.rsc.org/advances
A one-flask strategy for the synthesis of 1,3,5-trisubstituted 1,2,4-tri-
In our previous study,7 we report the synthesis of 1,3,5-
azoles 4a–s and 8a and b from nitriles 5a–i with N-arylhydrazonoyl trisubstituted 1,2,4-triazole 4 by a new 1,3-dipolar cycloaddition
hydrochlorides 3a–h and 7a and b under basic conditions was between oxime 2 and hydrazonoyl chloride8 3 under basic
developed. The reaction provided the desired 1,2,4-triazoles in conditions (eqn (1) in Fig. 1). In this reaction, compound 2
moderate to excellent yields (56–98%), and was applicable to aliphatic serves as the dipolarophile and 3 serves as the precursor to 1,3-
and aromatic nitriles as well as N-phenylhydrazonoyl hydrochlorides dipole nitrilimine.9 In a further study we present that 1,2,4-tri-
bearing ester and acetyl functionalities. A 1,3-dipolar cycloaddition azole 4 can be obtained in “one ask” by reacting aldehyde 1,
between imidate and nitrilimine generated from the respective nitrile the precursor to 2, with hydroxylamine hydrochloride following
and N-arylhydrazonoyl chloride in one flask was proposed for the new with 3.7c The two reactions proceed under mild conditions and
transformation.
produce 4 regiospecically in good to excellent yields. We also
demonstrate that triazoles 4 inhibit the proliferation of NCI-
H226, NPC-TW01, and Jurkat cancer cells at low micromolar
concentrations.7a
Introduction
As imidate 6 is electronically analogous to oxime 2, we
envisage that 6 could substitute oxime 2 as the dipolarophile for
the synthesis of 1,3,5-trisubstituted 1,2,4-triazole 4 (see eqn (2)
in Fig. 1). Moreover, due to the preparation of 6 can be achieved
by partial alcoholysis (Pinner reaction)10 of the precedent nitrile
5 under acidic conditions, an “one-ask” synthesis of triazole 4
can thus be achieved from nitrile 5 without the isolation of
imidate 6. The use of unstable aldehydes and toxic hydroxyl-
amine as the starting materials can be avoided. Despite direct
cycloaddition of nitriles with hydrazonoyl chlorides is also
reported to provide 1,2,4-triazoles, the substrates are limited to
nitriles such as ethyl carbonocyanidate (EtO2C–CN), benzyl
carbonocyanidate (BnO2C–CN), and 2,2,2-trichloroacetonitrile
(Cl3C–CN) that bear a strong electron-withdrawing group.11
Herein, we present our investigation on the one-ask synthesis
of 1,3,5-trisubstituted 1,2,4-triazoles 4 from nitriles 5 with
hydrazonoyl chlorides 3 thorough imidates 6. The reaction
provided the desired products in moderate to excellent yields
and was applicable to simple aliphatic and aromatic nitriles.
Substituted 1,2,4-triazoles are considered an important class
of heterocyclic compounds as they show various biological
activities.1 Among the traditional methods for their prepa-
ration,2 the dehydrative cyclization of acylamidrazone inter-
mediates,3 formed from the reaction of hydrazides and
the activated nitriles with amides including imidates or
thioamides at high temperature, are frequently used.4
However, the harsh reaction conditions including the use of
corrosive POCl3 or SOCl2 limit their application to sensitive
substrates.5,6
aGraduate Institute of Pharmaceutical Chemistry, China Medical University, No. 91,
Hsueh-Shih Rd., Taichung 40402, Taiwan
bThe Ph.D. Program for Cancer Biology and Drug Discovery, China Medical University,
No. 91, Hsueh-Shih Rd., Taichung 40402, Taiwan
cDepartment of Health and Nutrition Biotechnology, Asia University, Taichung 41354,
Taiwan
dSchool of Pharmacy, China Medical University, No. 91, Hsueh-Shih Rd., Taichung
40402, Taiwan
eDepartment of Medico Pharmaceutical Science, Nihon Pharmaceutical University,
10281, Komuro, Inamachi, Kita-Adachigun, Saitama, Japan
fDepartment of Applied Chemistry, National Chiayi University, No. 300, Syuefu Rd.,
Chiayi City 60004, Taiwan. E-mail: lukehuang@mail.ncyu.edu.tw; Fax: +886 5 271
7901; Tel: +886 5 271 7959
Results and discussion
For the nitrilimine as the 1,3-dipole is generated from hydra-
zonoyl chloride under basic conditions,9 we rst screened the
suitable base for the new 1,3-dipolar cycloaddition by use of
† Electronic supplementary information (ESI) available: General experimental
details, procedures, spectroscopic data for new compounds. See DOI:
10.1039/c4ra00113c
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Fig. 1 One-flask 1,3-dipolar cycloaddition strategies for the synthesis of 1,3,5-trisubstituted 1,2,4-triazole 4.
Table 1 Screening of a suitable base (3.0 equivalents) for the one-flask synthesis of 1,3,5-trisubstituted 1,2,4-triazole 4a via 1,3-dipolar
cycloaddition
Yield (%) of 4a
Entry
Base
From 6a
From 5a
1
2
3
4
5
6
—
N.D.a
N.D.
N.D.
N.D.
23
N.D.
N.D.
N.D.
N.D.
21
Diethylamine
N,N-Diisopropylethylamine
Pyridine
DBU
Triethylamine
77
76
a
N.D.: not detectable.
commercially available ethyl acetimidate hydrochloride (6a) materials. The result suggested the higher reactivity of imidate
with N-phenylhydrazonoyl chloride 3a as the model (Table 1). 6a than nitrile 5a as the dipolarophile to react with hydrazonoyl
Reaction of acetonitrile (5a) with HCl(g) and ethanol in CH2Cl2, chloride 3a.
then with 3a in one ask was also studied for comparison.12 For
We then focused on the one-ask 1,3-dipolar cycloaddition
the reaction of imidate 6a with hydrazonoyl chloride 3a, we strategy for the synthesis of 1,2,4-triazole 4 (eqn (2) in Fig. 1). We
observed that the 1,3-dipolar cycloaddition did not take place prepared various N-phenylhydrazonoyl chlorides 3a–h8 bearing
without a base (entry 1 in Table 1), neither did the use of different substituents on the phenyl ring and allowed them to
diethylamine or N,N-diisopropylethylamine or pyridine (entries react with alkyl nitriles 5a–e (see Table 2). For the reaction of
2–4). Use of DBU only gave 4a in low yield (23%, entry 5). Among acetonitrile (5a) with hydrazonoyl chlorides 3a–h bearing o-CF3,
the bases we have tried, only the use of triethylamine displayed m-Br, p-Me, p-CF3, p-OMe, p-F, and p-Cl on the phenyl group, the
a satisfactory result (77% yield, entry 6). For the one-ask reaction readily gave the corresponding 1,2,4-triazoles 4b–h in
transformation from acetonitrile (5a) with hydrazonoyl chloride 56–91% yields (entries 2–8 in Table 2). The use of compound 3h
3a to 1,2,4-triazole 4a, the reaction provided almost the iden- (X ¼ p-Cl) gave the best result (91% yield) nevertheless 3d (X ¼
tical results and the use of triethylamine also provided the best p-Me) gave a poor result (56% yield).
yield of 4a (76%, see entry 6). Triethylamine was thus used as
The one-ask 1,3-dipolar cycloaddition strategy was also
the base for all the following studies.
applicable to various aliphatic nitriles. Reaction of ethyl,
In control experiment, 5a was directly reacted with 3a in the i-propyl, n-butyl, and cyclopentyl nitriles 5b–e with p-chloro-N-
presence of Et3N without being reacted preferentially with HCl phenylhydrazonoyl chloride 3h in the presence of triethylamine
to form imidate 6a. The desired 1,3,5-trisubstituted 1,2,4-tri- provided the corresponding 1,3,5-trisubstituted 1,2,4-triazoles
azoles 4a was not obtained with the decomposition of starting 4i–l in 57–98% yields (see entries 9–12 in Table 2). The results in
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Table 2 One-flask synthesis of 1,2,4-triazoles 4a–l from alkyl nitriles 5a–e and N-phenylhydrazonoyl chlorides 3a–h via 1,3-dipolar
cycloaddition
Nitrile 5a–e
Hydrazone 3a–h
Entry
R
No.
X
No.
Triazole 4a–l
Reaction time (h)
Yield (%)
1
2
3
4
5
6
7
8
Me
Me
Me
Me
Me
Me
Me
Me
Et
i-Pr
n-Bu
Cyclopentyl
5a
5a
5a
5a
5a
5a
5a
5a
5b
5c
5d
5e
H
3a
3b
3c
3d
3e
3f
3g
3h
3h
3h
3h
3h
4a
4b
4c
4d
4e
4f
4g
4h
4i
3.0
3.0
4.0
5.0
3.0
2.5
3.5
2.0
3.5
2.5
4.5
5.0
76
71
66
56
81
90
77
91
93
98
76
57
o-CF3
m-Br
p-Me
p-CF3
p-OMe
p-F
p-Cl
p-Cl
p-Cl
p-Cl
p-Cl
9
10
11
12
4j
4k
4l
Table
2
indicated that aliphatic nitriles and N-phenyl-
We turned to study the reactivity of aromatic nitriles including
hydrazonoyl hydrochlorides bearing different substituents on benzonitrile (5f), 2-furonitrile (5g), thiophene-2-carbonitrile (5h),
the phenyl group were tolerable in the new 1,3-dipolar cyclo- and 1H-pyrrole-2-carbonitrile (5i) for the one-ask 1,3-dipolar
addition. The structures of 1,2,4-triazoles 4a–l were fully char- cycloaddition (Table 3). Reaction of compounds 5f–i with N-phe-
acterized by spectroscopic methods and consistent with the nylhydrazonoyl hydrochloride 3h bearing the p-Cl substituent
data in our previous studies.7
gave the corresponding 1,2,4-triazoles 4m–p in 56–86% yields
Table 3 Synthesis of 1,2,4-triazoles 4m–s from aromatic nitriles 5f–i with substituted N-phenylhydrazonoyl chlorides 3b, 3e, 3h, and 3i
Nitrile 5f–i
Hydrazone 3h–e
Entry
R
No.
X
No.
Triazole 4m–s
Reaction time (h)
Yield (%)
1
2
3
4
5
6
7
Phenyl
2-Furyl
5f
5g
5h
5i
5i
5i
5i
p-Cl
p-Cl
p-Cl
p-Cl
o-CF3
m-CF3
p-CF3
3h
3h
3h
3h
3b
3i
4m
4n
4o
4p
4q
4r
3.0
3.0
4.0
5.0
3.0
2.5
3.5
86
61
82
56
43
44
75
2-Thienyl
2-Pyrrolyl
2-Pyrrolyl
2-Pyrrolyl
2-Pyrrolyl
3e
4s
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Table 4 Synthesis of 1,2,4-triazoles 8a–e bearing various C-3 substituents
Hydrazone 7a–e
Entry
X
R
No.
Triazole 8a–e
Yield (%)
1
2
3
4
5
CF3
H
H
H
H
–CO2Et
–(C]O)Me
–CONMe2
–Et
7a
7b
7c
7d
7e
8a
8b
8c
8d
8e
81
79
N.D.a
N.D.
N.D.
–Ph
a
N.D.: not detectable.
(entries 1–4 in Table 3). In comparison with the results from cyclic aliphatic, and aromatic nitriles with various N-phenyl-
aliphatic nitriles 5a–e (entries 8–12 in Table 2), use of aromatic hydrazonoyl chlorides to synthesize 1,3,5-trisubstituted 1,2,4-tri-
nitriles 5f–i demonstrated slightly poorer results. The lower yields azoles in moderate to excellent yields.
of 4m–p might come from the conjugation of the double bond of
Table 4 presents the results of the one-ask 1,3-dipolar
imidate with the p-system in the aryl group to alter the HOMO– cycloaddition strategy for the synthesis of 1,2,4-triazoles
LUMO interactions of imidate with nitrilimine,13 or the instability bearing different C-3 substituents. The reaction of acetonitrile
of p-excessive furyl, thienyl, and pyrrolyl groups in acidic condi- (5a) with N-phenylhydrazonoyl hydrochloride 7a–e was studied
tions.14 Reaction of 1H-pyrrole-2-carbonitrile (5i) with N-phenyl- using the optimized reaction conditions from Table 1. Not
hydrazonoyl hydrochloride 3b, 3i, or 3e bearing ortho-, meta-, or surprisingly, reaction of 5a with 7a bearing ethoxycarbonyl
para-triuoromethyl group on the phenyl group also provided the group also provide the corresponding 1,2,4-triazole 8a in 81%
corresponding 1,2,4-triazoles 4q–s in 43–75% yields. The experi- yield (see entry 1 in Table 4). Reaction of N-phenylhydrazonoyl
mental results presented in Tables 2 and 3 demonstrated that the hydrochloride 7b bearing acetyl group also gave the corre-
one-ask 1,3-dipolar cycloaddition was applicable to aliphatic, sponding 8b in 79% yield. However, reaction of
Scheme 1 Proposed mechanism of the one-flask synthesis of 1,2,4-triazole 4 from nitrile 5 and hydrazonoyl chloride 3 via 1,3-dipolar
cycloaddition.
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N-phenylhydrazonoyl hydrochloride 7c with amido group, 7d
with ethyl group, and 7e with phenyl group did not give the
corresponding 1,2,4-triazole 8c–e (entries 3–5 in Table 4).
Decomposition of the starting materials was observed. The
results in Table 4 indicated the one-ask 1,3-dipolar cycload-
dition strategy was applicable to N-phenylhydrazonoyl hydro-
chlorides bearing ester and acetyl functionalities.
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We proposed a plausible mechanism for the one-ask
synthesis of 1,3,5-trisubstituted 1,2,4-triazoles from nitriles
and N-arylhydrazonoyl chlorides through a new 1,3-dipolar
cycloaddition in Scheme 1. Upon reaction with EtOH in the
presence of HCl(g), nitrile 5 was converted to its corre-
sponding imidate 6. N-Arylhydrazonoyl 3 was converted to
the corresponding nitrilimine 8 by Et3N in the same ask,
then the 1,3-dipolar cycloaddition between dipolarophile 6
and 1,3-dipole 9 took place to generate cyclic intermediate
10. Aromatization of 10 by releasing EtOH generated 1,2,4-
triazole 4 in one ask.
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Conclusions
We developed a one-ask methodology for the synthesis 1,3,5-
trisubstituted 1,2,4-triazoles using nitriles and N-arylhy-
drazonoyl chlorides as the starting materials. The reaction was
applicable to aliphatic and aromatic nitriles with N-arylhy-
drazonoyl chlorides bearing various substituents on the phenyl
group. A plausible 1,3-dipolar cycloaddition mechanism was
proposed for the reaction of imidate from nitrile with nitrili-
mine from hydrazonoyl chloride in one ask to generate the
desired 1,2,4-triazole.
Acknowledgements
We are grateful to the China Medical University (CMU100-ASIA-
17 and CMU101-S-23), Tsuzuki Institute for Traditional Medi-
cine, and the National Science Council of Republic of China for
nancial support (NSC-99-2320-B-039-014-MY3 and 102-2113-
M-415-005-MY2).
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