Selenium Dioxide-Mediated Synthesis of Fused 1,2,4-Triazoles as Cytotoxic Agents

Article abstract of DOI:10.1080/00397911.2015.1111384

A series of fused 1,2,4-triazoles has been prepared by oxidative intramolecular cyclization of heterocyclic hydrazones with selenium dioxide. General applicability of this practical protocol was confirmed by the synthesis of moderate to good yields of 1,2

Full text of DOI:10.1080/00397911.2015.1111384

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: http://www.tandfonline.com/loi/lsyc20
Selenium Dioxide–Mediated Synthesis of Fused
1,2,4-Triazoles as Cytotoxic Agents
Heng Zheng, Kang Wang, Wei Zhang & Ruihuan Liu
To cite this article: Heng Zheng, Kang Wang, Wei Zhang & Ruihuan Liu (2015) Selenium
Dioxide–Mediated Synthesis of Fused 1,2,4-Triazoles as Cytotoxic Agents, Synthetic
Communications, 45:24, 2849-2856, DOI: 10.1080/00397911.2015.1111384
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Date: 15 December 2015, At: 19:14

Synthetic Communications1, 45: 2849–2856, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397911.2015.1111384
SELENIUM DIOXIDE–MEDIATED SYNTHESIS OF
FUSED 1,2,4-TRIAZOLES AS CYTOTOXIC AGENTS
Heng Zheng,1 Kang Wang,1 Wei Zhang,1 and Ruihuan Liu2
1Department of Gastrointestinal Surgery, Sichuan Academy of Medical
Sciences and Sichuan Provincial People's Hospital, Chengdu, China
2School of Pharmaceutical Sciences, Central South University, Changsha,
China
GRAPHICAL ABSTRACT
Abstract A series of fused 1,2,4-triazoles has been prepared by oxidative intramolecular
cyclization of heterocyclic hydrazones with selenium dioxide. General applicability of this
practical protocol was confirmed by the synthesis of moderate to good yields of 1,2,4-
triazolo[4,3-a]pyridines, 1,2,4-triazolo[4,3-a]pyrimidines, 1,2,4-triazolo[4,3-a]pyramidines,
and 1,2,4-triazolo-[4,3-a]quinoxa lines. All compounds were tested in vitro for their
cytotoxic activity against HCT-116, A549, and Colo-205 cell lines. Two compounds, 3-(4-
methoxyphenyl)-7-methyl-[1,2,4]triazolo[4,3-a]pyridine and 1-(4-methoxyphenyl)-[1,2,4]
triazolo[4,3-a]quinoxaline, showed potent antiproliferative activity against the three cell lines.
Keywords Antiproliferative activity; selenium dioxide; 1,2,4-triazole
INTRODUCTION
Triazoles are an important class of heterocyclic compounds. In particular,
fused 1,2,4-triazoles express antibactericidal,[1,2] antianxiety,[3] anticancer,[4,5]
and antimicrobial activities[6] and act as inhibitors of kinase[7] and fibrinogen
receptors.[8] In the past few decades, the chemistry of fused 1,2,4-triazoles has
received considerable attention because of their synthetic and effective biological
importance. Most methods for the preparation of fused 1,2,4-triazoles are based
on hydrazones or hydrazides as precursors.[2,9–15]
As part of our ongoing studies exploring novel antitumor drugs, a series of
fused 1,2,4-triazole derivatives was designed. Initially, we planned to synthesize
Received April 28, 2015.
Address correspondence to Heng Zheng, Department of Gastrointestinal Surgery, Sichuan
Academy of Medical Sciences and Sichuan Provincial People's Hospital, Chengdu 610072, P. R. China.
E-mail: zhengheng_1017@126.com
2849

2850
H. ZHENG ET AL.
Scheme 1. Synthesis of 1,2,4-triazoles 5–8.
1,2,4-triazoles via oxidative cyclization of hydrazones with oxidizing agents such as
copper(II) chloride,[9] [bis(trifluroacetoxy)iodo]benzene (PIFA),[12] iodobenzene dia-
cetate (IBD),[13] and lead(IV) acetate.[15] Unfortunately, the results obtained
with these oxidative agents were not satisfactory. Therefore, we developed a simple
and practical protocol for preparing 1,2,4-triazoles by oxidative cyclization of
hydrazones using SeO2 (Scheme 1).
RESULTS AND DISCUSSION
The precursors, hydrazones, were obtained by treating the corresponding
hydrazino heterocycles with aldehydes.
To achieve a generally accepted oxidizing agent for the cyclization of hydra-
zine, a series of additives was examined for efficiency in the heterocyclization of
hydrazone (1f). The results listed in Table 1 indicate that the SeO2 is more effective
than others, although greater reaction temperature was required. Compared with
Table 1. Catalyst exploring for the synthesis of triazolopyridine 5f
Entry
Catalyst
Temp. (°C)
Yield (%)a
b [9]
1
2
3
4
5
6
7
CuCl2
100
25
25
64
57
78
45
8
PIFAb [12]
IBDb [13]
b [15]
Pb(OAc)4
Ce(SO4)2
25
c
110
110
110
c
Fe2O3
—
c
SeO2
98
aIsolated yields.
bThe reaction was performed according to the literature.
cReaction conditions: 1f (10 mmol) and catalyst (20 mmol) in DMF (10 mL) was stirred at 110 °C for 2 h.

SYNTHESIS OF FUSED 1,2,4-TRIAZOLES
2851
Table 2. Optimization of reaction conditions for the synthesis of triazolopyridine 5f
Entry
Cat. (equiv)
Temp. (°C)
Solvent
Yield (%)a
1
2
3
4
5
6
7
8
1
2
3
2
2
2
2
2
2
2
2
90
90
90
100
110
120
140
Reflux
Reflux
Reflux
110
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DCM
38
79
80
86
98
96
82
—
—
—
90
9
10
11
Ethanol
Acetonitrile
DMSO
aIsolated yields. Conditions: 1f (10 mmol) in DMF (10 mL) was heated for 2 h.
other additives, the use of SeO2 as a heterogeneous oxidant in this reaction has
advantage of good yield, simple workup, and low cost.
In the primary experiment, the heterocyclization was performed in dimethylfor-
mamide (DMF) under air. To identify an optimal reaction condition for oxidative
heterocyclizaiton, the influence of oxidant amount was investigated (Table 2) at dif-
ferent temperatures with heterocyclization of hydrazine 1f as the model reaction.
Encouragingly, nearly quantitative conversion of 1f to 5f was achieved when 1f
was treated with 2 equiv SeO2 at 110 °C for 2 h (Table 2, entry 5). No further
improvement was obtained when the reaction was performed in different solvents
(Table 2, entries 8–11).
To further probe the synthetic scope of this reagent, we investigated the oxidat-
ive cyclization of other heterocyclic hydrazones to afford the corresponding 1,2,4-
triazolo derivatives. Hydrazones 1–4 of aromatic and aliphatic aldehydes with both
electron-withdrawing and electron-donating substituents were oxidized to give the
corresponding 1,2,4-triazolo[4,3-a]pyridines (5), 1,2,4-triazolo[4,3-a]pyrimidines (6),
1,2,4-triazolo[4,3-a]quinolone (7), and 1,2,4-triazolo[4,3-a]quinoxalines (8) in good
yields (Table 3). The plausible mechanism for the oxidation of hydrazones is depicted
in Scheme 2 with the oxidation of 1 to 5 as an example.
The target compounds were evaluated for cytotoxic activity against three
human cancer cell lines using the sulforhodamine B (SRB) assay.[16] 5-Fluorouracil
(5-FU), which has a broad spectrum of anticancer activity, was used as reference
drug. The three cell lines are human colon cancer HCT-116 and Colo-205 cell lines
and non-small-cell lung carcinoma A549 cell line. The IC50 values of all targeted
compounds are assessed and summarized in Table 3.
As shown in Table 3, most compounds were able to inhibit the growth of the
three cancer cells with IC50 values between 2 and 70 µM. Among them, compounds
5a, 5b, and 8a (R ¼ alkyl groups) were found to be inactive against all tumor cell lines
(IC50 > 100 lM). The cytotoxic activities of 4-methoxylphenyl analogs (5f, 6a and
8b) was superior to those of other substituted-phenyl analogs. The antiproliferative
activities of 1,2,4-triazolo[4,3-a]quinoxalines seemed to be stronger than those of
other triazoles. In the whole series, compound 8b showed significant growth

2852

SYNTHESIS OF FUSED 1,2,4-TRIAZOLES
2853
Scheme 2. Mechanism for the oxidation of 1 to 5.
inhibition with IC50 values of 2.36 and 5.76 mM against HCT-116 and A549 cell lines
respectively, which is almost equipotent to 5-fluorouracil.
CONCLUSIONS
In conclusion, we have developed a practical and efficient protocol for the syn-
thesis of fused 1,2,4-triazoles by oxidation of heterocyclic substituted hydrazones
using SeO2 as oxidizing agent. The presence of several functionalities in the substrate
is tolerated and almost does not affect the yield of the desired product. Most of the
target compounds displayed potent antitumor activity. Among them, compound 8b
exhibited significant growth inhibition with IC50 values of 2.36 and 5.76 mM against
HCT-116 and A549 cells respectively, which is almost equipotent to 5-fluorouracil.
EXPERIMENTAL
All the reagents and solvents for the synthesis and analysis were commercially
available and used directly. Melting points were recorded on a YRT-3 melting-point
1
apparatus and are uncorrected. H NMR and 13C NMR spectra were measured
1
using a Bruker DRX 400 instrument [400 MHz for H NMR and 100 MHz for the
13C NMR (proton decoupled)] using dimethylsulfoxide (DMSO-d6) as solvent and
tetramethylsilane (TMS) as internal standard, chemical shifts (d scale) are reported
in parts per million (ppm). Data are reported as follows: chemical shift in ppm
(d), multiplicity (s ¼ singlet, d ¼ doublet, t ¼ triplet, brs ¼ broad singlet, m ¼ multi-
plet), coupling constant (Hz), integration. Mass spectra analyses of all the
compounds were performed on a high-resolution q-TOF.
Typical Procedure for the Synthesis of Hydrazones (1–4)
Hydrazones 4b and 4c were prepared according to the literature procedure.[17]
The new hydrazones are prepared as follows: The hydrazine (15 mmol) was dissolved
in boiling ethanol (20 ml) and the aldehyde (15 mmol) dissolved in ethanol (20 ml)
was added dropwise. After that, the solution was stirred and heated under reflux
for 1 h. The formed hydrazone was filtered from the cooled solution and used in
the next reaction without any purification.
Typical Procedure for the Synthesis of Fused 1,2,4-Triazoles (5–8)
To a solution of hydrazone (10.0 mmol) in DMF (10 ml), selenium dioxide
(2.20 g, 20.0 mmol) was added while stirring. The mixture was stirred at 110 °C

2854
H. ZHENG ET AL.
and monitored by thin-layer chromatography (TLC). After the completion of
the reaction, the reaction mixture was filtered, and the filtrate was evaporated in
vacuum. The crude product was purified by recrystallization or by flash column
chromatography to give the desired product.
Spectral Data for Selected Compounds
(E)-2-(2-(4-Methoxybenzylidene)hydrazinyl)-4-methylpyridine (1f). Yellow
solid, mp 234–236 °C; 1H NMR (400 MHz, DMSO-d6): d ¼ 2.31 (s, 3 H), 3.95 (s, 3 H),
6.73 (dd, J ¼ 1.3Hz, 7.3Hz, 1 H), 7.10 (d, J ¼ 8.5 Hz, 2 H), 7.56 (d, J ¼ 1.3Hz, 1 H), 7.69
(d, J ¼ 8.3 Hz, 2 H), 7.90 (s, 1 H), 8.38 (d, J ¼ 7.1 Hz, 1 H), 10.90 (s, 1 H) ppm. HRMS-
ESI (m=z): [MþH]þ calcd. for C14H16N3O, 242.1288; found, 242.1267.
(E)-2-(2-(4-Methoxybenzylidene)hydrazinyl)pyrimidine (2a)
Yellow solid, mp 192–193 °C; 1H NMR (400 MHz, DMSO-d6): d ¼ 3.83 (s, 3 H),
7.20 (d, J ¼ 8.5 Hz, 2 H), 7.45 (t, J ¼ 8.2 Hz, 1 H), 7.61 (d, J ¼ 8.5 Hz, 2 H), 8.12 (d,
J ¼ 8.2 Hz, 2 H), 7.97 (s, 1H), 11.07 (s, 1 H) ppm. HRMS-ESI (m=z): [MþH]þ calcd
for C12H13N4O, 229.1084; found, 229.1059.
(E)-2-(2-(2-Nitrobenzylidene)hydrazinyl)quinoline (3)
1
Yellow solid, mp 227–229 °C; H NMR (400 MHz, DMSO-d6): d ¼ 7.13 (d, J
¼ 8.0 Hz, 1H), 7.40 (t, J ¼ 8.1 Hz, 1 H), 7.57 (t, J ¼ 7.3 Hz, 1 H), 7.73 (d, J ¼ 8.6 Hz,
1 H), 7.87 (d, J ¼ 8.6 Hz, 1 H), 8.01 (d, J ¼ 8.0 Hz, 1 H), 8.07–8.11 (m, 3 H), 8.23 (s, 1
H), 8.36 (d, J ¼ 7.3 Hz, 1 H), 10.56 (s, 1 H) ppm. HRMS-ESI (m=z): [M þH]þ calcd.
for C16H13N4O2, 293.1033; found, 293.1028.
(E)-2-(2-Propylidenehydrazinyl)quinoxaline (4a)
1
Yellow solid, mp 212–213 °C; H NMR (400 MHz, DMSO-d6): d ¼ 1.45 (t, J
¼ 7.2 Hz, 3 H), 3.30 (m, 2 H),7.68 (td, J ¼ 1.0 Hz, 8.5 Hz, 1 H), 7.77 (td, J ¼ 1.2 Hz,
8.5 Hz, 1 H), 7.95 (t, J ¼ 7.2 Hz, 1 H), 8.06 (dd, J ¼ 1.2 Hz, 8.0 Hz, 1 H), 8.06 (dd,
J ¼ 1.0 Hz, 8.0 Hz, 1 H), 8.46 (s, 1 H), 11.97 (s, 1H) ppm. HRMS-ESI (m=z): [M
þH]þ calcd. for C11H13N4, 201.1135; found, 201.1122.
3-(4-Methoxyphenyl)-7-methyl-[1,2,4]triazolo[4,3-a]pyridine (5f)
White solid, mp 214–216 °C; 1H NMR (400 MHz, DMSO-d6): d ¼ 2.38 (s, 3 H),
3.85 (s, 3 H), 6.83 (dd, J ¼ 1.4 Hz, 7.2 Hz, 1 H), 7.15–7.17 (m, 2 H), 7.56 (d, J ¼ 0.8
Hz, 1 H), 7.79–7.81 (m, 2 H), 8.38 (d, J ¼ 7.2 Hz, 1 H) ppm. 13C NMR (100 MHz,
DMSO-d6): d ¼ 21.2, 55.8, 113.6, 115.1, 117.4, 119.4, 123.4, 129.9, 138.7, 145.9,
150.7, 160.8 ppm. HRMS-ESI (m=z): [M þH]þ calcd. for C14H14N3O, 240.1131;
found, 240.1160.

SYNTHESIS OF FUSED 1,2,4-TRIAZOLES
2855
3-(4-Methoxyphenyl)-[1,2,4]triazolo[4,3-a]pyrimidine (6a)
1
Yellow solid, mp 152–153 °C; H NMR (400 MHz, DMSO-d6): d ¼ 3.91 (s, 3
H), 7.23 (d, J ¼ 8.7 Hz, 2 H), 7.50 (dd, J ¼ 1.0 Hz, 8.3 Hz, 1 H), 7.55 (td, J ¼ 1.3 Hz,
8.4 Hz, 1 H), 7.72 (d, J ¼ 10.0 Hz, 2 H), 8.11 (dd, J ¼ 1.0 Hz, 8.0 Hz, 1 H) ppm. 13C
NMR (100 MHz, DMSO-d6): d ¼ 55.9, 115.1, 116.2, 120.6, 126.6, 127.9, 129.8, 130.8,
132.0, 136.6, 144.5, 144.8, 149.4, 161.6 ppm. HRMS-ESI (m=z): [M þH]þ calcd. for
C12H11N4O, 227.0927; found, 227.0939.
1-(2-Nitrophenyl)-[1,2,4]triazolo[4,3-a]quinoline (7)
1
Yellow solid, mp 210–212 °C; H NMR (400 MHz, DMSO-d6): d ¼ 7.18 (d, J
¼ 8.4 Hz, 1H), 7.48 (t, J ¼ 8.1 Hz, 1 H), 7.56 (t, J ¼ 7.2 Hz, 1 H), 7.83 (d, J ¼ 9.6 Hz,
1 H), 7.91 (d, J ¼ 9.6 Hz, 1 H), 8.00 (d, J ¼ 8.4 Hz, 1 H), 8.03–8.10 (m, 3 H), 8.46 (d,
J ¼ 7.5 Hz, 1 H) ppm. 13C NMR (100 MHz, DMSO-d6): d ¼ 114.9, 115.7, 124.5,
124.8, 126.1, 126.8,130.2, 130.3, 130.8, 131.6, 133.2, 133.7, 135.5, 144.8, 148.6,
149.6 ppm. HRMS-ESI (m=z): [MþH]þ calcd. for C16H11N4O2, 291.0877; found,
291.0832.
1-(4-Methoxyphenyl)-[1,2,4]triazolo[4,3-a]quinoxaline (8b)
Yellow solid, mp 182.3–182.7 °C; 1H NMR (400 MHz, DMSO-d6): d ¼ 3.91 (s,
3 H), 7.24 (d, J ¼ 8.5 Hz, 2 H), 7.49–7.51 (m, 1 H), 7.55 (td, J ¼ 1.4 Hz, 8.5 Hz, 1 H),
7.65 (td, J ¼ 1.4 Hz, 8.0 Hz, 1 H), 7.71–7.73 (m, 2 H), 8.10 (dd, J ¼ 1.2 Hz, 8.0 Hz, 1
H), 8.71 (s, 1 H) ppm. 13C NMR (100 MHz, DMSO-d6): d ¼ 55.9, 115.1, 116.2, 120.6,
126.6, 127.9, 129.8, 130.8, 132.0, 144.5, 144.8, 149.4, 152.5, 161.6 ppm. HRMS-ESI
(m=z): [MþH]þ calcd. for C16H13N4O, 277.1084; found, 277.1108.
In Vitro Antiproliferative Activity Assays
The antiproliferative activity of the compounds was tested on HCT-116, A549,
and Colo-205 cells. Cells were seeded in 96-well plates at a density of 5000 cells=well
and incubated for 24 h. The cells were then incubated with serially diluted com-
pounds for another 48 h. The final number of cells per well was assessed using the
sulforhodamine B dye (SRB, Sigma–Aldrich) following the standard procedure,[16]
and absorbance was measured using a microplate reader (SpectraMax M5) at a
wavelength of 515 nm. The results were expressed as IC50 calculated using the
Easy-fit software.
SUPPLEMENTAL MATERIAL
Experimental procedures, 1H NMR and 13C NMR spectra data, and references
for all the products for this article can be accessed on the publisher’s website.
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