Synthesis of 3-substituted (6-[(e)-2-(1-benzofuran-2-yl)ethenyl][1,2,4] triazolo[3,4-b][1,3,4]thiadiazoles

Article abstract of DOI:10.1080/10426500701557310

The reaction of (2E)-3-(1-benzofuran-2-yl)-2-propenoic acid with 4-amino-5-R-1,2,4-triazole-3-thioles has been investigated. It has been established, that 6-[(E)-2-(1-benzofuran-2-yl)ethenyl][1,2,4]triazolo[3,4-b][1, 3,4] thiadiazole were formed as the result of heterocyclization.

Full text of DOI:10.1080/10426500701557310

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Synthesis of 3-Substituted
(6-[(E)-2-(1-Benzofuran-2-
yl)ethenyl][1,2,4]triazolo[3,4-
b][1,3,4]thiadiazoles
Mykola D. Obushak a , Nazariy T. Pokhodylo a , Yuri V.
Ostapiuk a & Vasyl S. Matiychuk a
a Department of Organic Chemistry , Ivan Franko
National University of Lviv , Lviv, Ukraine
Published online: 20 Feb 2008.
To cite this article: Mykola D. Obushak , Nazariy T. Pokhodylo , Yuri V. Ostapiuk
& Vasyl S. Matiychuk (2007) Synthesis of 3-Substituted (6-[(E)-2-(1-Benzofuran-2-
yl)ethenyl][1,2,4]triazolo[3,4-b][1,3,4]thiadiazoles, Phosphorus, Sulfur, and Silicon
and the Related Elements, 183:1, 136-143, DOI: 10.1080/10426500701557310
To link to this article: http://dx.doi.org/10.1080/10426500701557310
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Phosphorus, Sulfur, and Silicon, 183:136–143, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701557310
Synthesis of 3-Substituted (6-[(E)-2-(1-Benzofuran-2-
yl)ethenyl][1,2,4]triazolo[3,4-b][1,3,4]thiadiazoles
Mykola D. Obushak, Nazariy T. Pokhodylo,
Yuri V. Ostapiuk, and Vasyl S. Matiychuk
Department of Organic Chemistry, Ivan Franko National University
of Lviv, Lviv, Ukraine
The reaction of (2E)-3-(1-benzofuran-2-yl)-2-propenoic acid with 4-amino-5-R-
1,2,4-triazole-3-thioles has been investigated. It has been established, that 6-[(E)-2-
(1-benzofuran-2-yl)ethenyl][1,2,4]triazolo[3,4-b][1,3,4] thiadiazole were formed as
the result of heterocyclization.
Keywords 1,3,4-thiadiazole; 1,2,4-triazole;; [1,2,4]triazolo[3,4-b] [1,3,4]thiadiazole; 4-
amino-4H-1,2,4-triazole-3-thiol; benzofuran derivates; heterocyclization
INTRODUCTION
A large number of [1,2,4]triazolo[3,4-b][1,3,4]thiadiazoles show a
broad spectrum of biological activity—ntibacterial,1–5 antimicrobial,6−8
antifungal,5,9–11 antihelmintic,12,13 anticancer,14−16 antiviral.17
The s-triazolo[3,4-b][1,3,4]thiadiazole ring system (I) can be success-
fully synthesized by the following routes. The first one is the cyclocon-
densation of 4-amino-5-aryl-1,2,4-triazole-3-thiones II (Scheme 1) with
acids in the presence of phosphorus oxychloride.8−10,14,15,17−20 The sec-
ond route involves a ring formation by the 5-substituted 2-hydrazino-
1,3,4-thiadiazole III reaction with acids by reflux in methanol or
xylene11 (Scheme 1).
As a rule, 4-amino-5-aryl-1,2,4-triazole-3-thiones are used. They eas-
ily can be prepared from respective acids using previously reported
methods.5,18
Received 31 March 2007; accepted 5 June 2007.
Address correspondence to Mykola D. Obushak, Department of Organic Chemistry,
Ivan Franko National University of Lviv, Kyryla & Mefodiya Str. 6, 79005 Lviv, Ukraine.
E-mail: obushak@in.lviv.ua
136

Substituted Thiadiazoles
137
SCHEME 1
On the other hand heterocyclic compounds of the benzofuran and
benzopyran series are important aromatic rings.21−25 They occur in na-
ture and exhibit wide range of valuable pharmacological properties. So,
we decided to combine benzofuran heterocycle with 1,3,4-triazolo[3,4-
b]-1,3,4-thiadiazole ring in order to obtain new heterocyclic compounds
exhibiting biological activity.
RESULTS AND DISCUSSION
We have synthesized (2E)-3-(1-benzofuran-2-yl)-2-propenoic acid 4
(Scheme 2) in general good yield. Formylation of the 1-benzofuran 2,
which was prepared according to literature procedure26 from salicy-
laldehyde by Vilsmeier–Haack reagent, afforded the 1-benzofuran-2-
carbaldehyde27 3 in 71% yield. By the reaction of aldehyde 3 with mal-
onic acid the corresponding (2E)-3-(1-benzofuran-2-yl)-2-propenoic acid
4 was obtained.
A new efficient synthesis of heterocycles that contain 1-benzofuran
and triazolo[3,4-b][1,3,4]thiadiazole cycle compounds 9a–h was devel-
oped by the cyclocondensation of the 5-substituted-4-amino-4H-1,2,4-
triazole-3-thiol 7a–h with acid 4a or acid chloride 4b. The first step of
synthesis involves the reaction acylation of aminotriazole, the second
one—intramolecular ring formation as shown in Scheme 2. Although
compounds 8a–h can be isolated and characterized, it is more conve-
nient to treat them without isolating.
The structures of compounds 9a–i were substantiated from micro-
analytical and spectral data. Analytical, 1H-NMR, 13C-NMR and mass
1
spectral data are in agreement with the proposed structures. In H-
NMR spectrum compounds 9a–i benzofuranethenyle fragments are
showed as two triplets, two doublets and singlet (3-H) of benzofurane
and two doublets (J ≈ 16 Hz) of protons of the external double bond.
As a result we have developed a new method for the synthesis of
6-[(E)-2-(1-benzofuran-2-yl)ethenyl][1,2,4]triazolo[3,4-b][1,3,4] thiadi-
azoles by the cycloaddition of (1-benzofuran-2-yl)-2-propenoic acid and
4-amino-5-aryl-1,2,4-triazole-3-thiones derivatives. The convenient

138
M. D. Obushak et al.
SCHEME 2
synthesis provides access to polynuclear heterocycles which contain the
basic ring structure found in molecules that exhibit biological activity.
EXPERIMENTAL
All melting points are uncorrected. The 1H and 13C NMR spectra were
run on a Mercury 400 MHz and Bruker 200 (200 MHZ) instruments
respectively using TMS as an internal reference and DMSO-d6 sas

Substituted Thiadiazoles
139
solvent. The mass spectra were performed on a Agilent 1100 chro-
matomass spectrometer at 70 eV. The starting materials were commer-
cially available and/or prepared in accordance to literature procedures
7a–h.5,18 Yields of products were not optimized.
Synthesis of 4-Amino-5-R-4H-1,2,4-triazole-3-thioles 7a–h
Acid ester 5 (0.01 mol) was refluxed with 20 ml of 85% NH2NH2·H2O
for 2 h; it was then cooled and diluted with water. Solid collected by fil-
tration and dried. To 250 ml KOH (0.015 mol) absolute ethanol solution
acid hydrazide (0.01 mol) and CS2 (0.015 mol) were added. The solution
was stirred at room temperature for 24 h, and then absolute ether (50
mL) was added to it and left as such for 2 h. The residue was filtered,
washed (with ethanol and ether), and dried. This solid was dissolved in
excess hydrazine hydrate (85%). The mixture was heated and refluxed
for 6 h, cooled and poured into acidic ice water (pH = 2) to give solid
which was filtered, dried and recrystallized.
Compounds 7a–h were prepared as a white powdered solid in 56–
74% yields count on acid ester: 7a, m.p. 192–193◦C (ethanol–H2O), yield
56%; 7b, m.p. 112–113◦C (ethanol–H2O), yield 58%; 7c, m.p. 180◦C
(ethanol), yield 62%; 7d, m.p. 148–149◦C (ethanol) yield 67%; 7e, m.p.
188◦C (ethanol) yield 74%; 7f, m.p. 194◦C (ethanol) yield 69%; 7g, m.p.
197◦C (ethanol) yield 60%; 7h, m.p. 214◦C (ethanol) yield 71%; 7i, m.p.
127–128◦C (ethanol), yield 60%.
(2E)-3-(1-Benzofuran-2-yl)-2-propenoic Acid 4a
In a 100-ml round-bottomed flask, fitted with a reflux condenser, are
placed 10.4 g (0.1 mol) of malonic acid, 14.6 g (0.1 mol) of 1-benzofuran-
2-carbaldehyde 3, and 40 ml of pyridine. Piperidine (0.7 ml) is then
added and the mixture is heated to 80◦. An internal temperature of 80–
85◦ is maintained for 1 hour, and the material is finally heated under
reflux 109–115◦C for an additional 3 h. After being cooled the reaction
mixture is poured into a large beaker containing 200 ml of cold water.
The mixture is acidified by adding with stirring 50 ml of concentrated
hydrochloric acid; it should be strongly acidic at this point. The crystals
are separated by suction filtration and washed 4 times with 15 ml por-
tions of cold water. The product is dried at 60–70◦C. M.p. 221–222◦C;
yield 15,7 g. (84%). 1H NMR ppm: δ 6.44 (d, J = 15.8, 1H, CH), 7.29 (t,
J = 7.5, 1H, benzofuran), 7.36 (s, 1H, 3–H benzofuran), 7.42 (t, J = 7.1,
1H, benzofuran), 7.60 (d, J = 15.8, 1H, CH ), 7.62 (d, J =7.5, 1H, benzo-
furan), 7.71 (d, J = 7.5, 1H, benzofuran).13C NMR ppm: δ 111.3 (C-3),
111.6 (C-7), 119.5 (C-4), 122.1 (C-5), 123.5 (C-6), 126.6 (C-3a), 128.0

140
M. D. Obushak et al.
(C C), 130.9(C C), 152.0 (C-2), 154.5 (C-7a), 167.0 (C 0). MS m/z: 188
(M+). Anal. requires for C11H8O3 (188.18) calcd./found: C, 70.21/70.30;
H, 4.29/4.22.
(2E)-3-(1-Benzofuran-2-yl)-2-propenoyl Chloride 4b
A mixture of acid 6 (18.8 g, 0.1 mol) and thionyl chloride (17.8 g, 0.15
mol) in benzene was refluxed for 3 h; the product was distilled in vacuo.
Yield 56%.
General Procedure for the Synthesis of Compounds (9a–i)
A mixture of 4-amino-5-R-1,2,4-triazole-3-thione 8 (5.0 mmol) and acid
4a or acid chloride 4b (5.5 mmol) in POCl3 (20 ml) was refluxed for 1
h. The reaction mixture was gradually poured onto crushed ice with
stirring. Some solid K2CO3 was added to the mixture with stirring, and
then an appropriate amount of solid KOH was added to pH 8. The solid
which separated after standing overnight was filtered, washed with cold
water, dried, and recrystallized from DMF. The physical and spectral
properties the compounds (9 a–i) are given below.
6-[( E)-2-(1-benzofuran-2-yl)ethenyl]-3-methyl[1,2,4]triazolo-
[3,4- b] [1,3,4]thiadiazole (9a)
This compound was isolated as a reddish brown powdered solid, m.p.
1
257–258◦C (ethanol–DMF) in 76% yield. H NMR ppm: δ 2.66 (s, 3H,
CH3), 7.22–7.29 (m, 2H, benzofuran), 7.33 (d, J =16.6, 1H, CH ), 7.38
(pseudo t, 1H, benzofuran), 7.53 (d, J =7.8, 1H, benzofuran), 7.65 (d,
J =16.6, 1H, CH), 7.66 (d, J =7.8, 1H, benzofuran). MS m/z: 282 (M+).
Anal. requires for C14H10N4OS (282.32) calcd./found: C, 59.56/59.39; H,
3.57/3.46; N, 19.85/19.91; S, 11.36/11.31.
6-[( E)-2-(1-benzofuran-2-yl)ethenyl]-3-propyl[1,2,4]triazolo-
[3,4-b] [1,3,4]thiadiazole (9b)
This compound was isolated as brown needles, m.p. 199–199◦C
(ethanol–DMF) in 71% yield. 1H NMR ppm: δ 1.04 (t, J = 6.8, 3H,
CH3), 1.87 (6 lines, 2H, CH2), 2.99 (t, J = 7.8, 2H, CH2), 7.26 (t, J = 7.8,
1H, benzofuran), 7.27 (s, 1H, 3–H benzofuran), 7.34 (d, J = 16.1, 1H,
CH ), 7.38 (pseudo t, 1H, benzofuran), 7.52 (d, J = 8.8, 1H, benzofu-
ran), 7.64 (d, J = 16.1, 1H, CH ), 7.67 (d, J = 7.8, 1H, benzofuran). MS
m/z: 310 (M+). Anal. requires for C16H14N4OS (310.38) calcd./found: C,
61.92/61.82; H, 4.55/4.38; N, 4.55/4.76; S, 10.33/10.36.

Substituted Thiadiazoles
141
6-[( E)-2-(1-benzofuran-2-yl)ethenyl]-3-benzyl[1,2,4]triazolo
[3,4-b][1,3,4]thiadiazole (9c)
This compound was isolated as a light yellow solid, m.p. 221–
222◦C (ethanol–DMF) in 81% yield. 1H NMR ppm: δ 4.39 (s, 2H,
CH2), 7.21–7.41 (m, 9H, aromatic + CH ), 7.51 (d,J = 8.8, 1H,
benzofuran), 7.64 (d,J = 16.6, 1H, CH), 7.66 (d,J = 7.8, 1H,
benzofuran). MS m/z: 358 (M+). Anal. requires for C20H14N4OS
(358.42) calcd./found: C, 67.02/66.72; H, 3.94/3.89; N, 15.63/15.57; S,
8.95/8.81.
6-[( E)-2-(1-benzofuran-2-yl)ethenyl]-3-(3-methylphenyl)[1,2,4]
triazolo[3,4-b][1,3,4]thiadiazole (9d)
This compound was isolated as a light yellow powdered solid,
m.p. 215–216◦C (ethanol–DMF) in 84% yield. 1H NMR ppm: δ 2.54
(s, 3H, CH3), 7.21–7.54 (m, 7H, aromatic + CH ), 7.68 (d,J =
7.8, 1H, benzofuran), 7.75 (d,J = 16.6, 1H, CH), 8.08 (br. s,
2H, C6H4). MS m/z: 358 (M+). Anal. requires for C20H14N4OS
(358.42) calcd./found: C, 67.02/66.67; H, 3.94/3.91; N, 15.63/15.74; S,
8.95/8.85.
6-[( E)-2-(1-benzofuran-2-yl)ethenyl]-3-(4-methylphenyl)[1,2,4]
triazolo[3,4- b][1,3,4]thiadiazole (9e)
This compound was isolated as a white powdered solid, m.p. 191–
193◦C (ethanol-DMF) in 87% yield. 1H NMR ppm: δ 2.39 (s, 3H, CH3),
7.29 (pseudo t, 1H, benzofuran), 7.36–7.48 (m, 5H, aromatic), 7.62 (d,
J = 8.6, 1H, benzofuran), 7.73 (d, J = 8.6, 1H, benzofuran), 7.78 (d,
J = 15.6, 1H, CH ), 8.14 (d, J = 8.6, 2H, C6H4). MS m/z: 358 (M+).
Anal. requires for C20H14N4OS (358.42) calcd./found: C, 67.02/67.10; H,
3.94/3.88; N, 15.63/15.55; S, 8.95/9.04.
6-[( E)-2-(1-benzofuran-2-yl)ethenyl]-3-(4-bromophenyl)[1,2,4]-
triazolo[3,4-b][1,3,4]thiadiazole (9f)
This compound was isolated as a light yellow powdered solid, m.p.
224–226◦C (ethanol–DMF) in 81% yield. 1H NMR ppm: 7.27 (pseudo t,
1H, benzofuran), 7.31 (s, 1H, 3–H benzofuran), 7.40 (pseudo t, 1H, ben-
zofuran), 7.47 (d, J = 15.6, 1H, CH ), 7.53 (d,J = 7.8, 1H, benzofuran),
7.68 (d, J = 7.8, 1H, benzofuran), 7.74 (d, J = 8.8, 2H, C6H4) 7.79 (d,
J = 15.6, 1H, CH), 8.22 (d, J = 8.8, 2H, C6H4). MS m/z: 423 (M+).
Anal. requires for C19H11BrN4OS (423.29) calcd./found: C, 53.91/54.02;
H, 2.62/2.69; N, 13.24/13.30; S, 7.58/7.47.

142
M. D. Obushak et al.
6-[( E)-2-(1-benzofuran-2-yl)ethenyl]-3-(2-methyl-3-
furyl)[1,2,4]-triazolo-[3,4-b][1,3,4]thiadiazole (9g)
This compound was isolated as brown needles, m.p. 204–205◦C
1
(ethanol–DMF) in 64% yield. H NMR ppm: δ 2.70 (s, 3H, CH3), 7.08
(br. s, 1H, 4–H furan), 7.26 (pseudo t, 1H, benzofuran), 7.28 (s, 1H,
3–H benzofuran), 7.38 (pseudo t, 1H, benzofuran), 7.40 (d, J = 15.6,
1H, CH ), 7.52 (d,J = 7.8, 1H, benzofuran), 7.61 (br. s, 1H, 5–H fu-
ran), 7.67 (d,J =7.8, 1H, benzofuran), 7.70 (d, J = 15.6, 1H, CH). MS
m/z: 348 (M+). Anal. requires for C18H12N4O2S (348.38) calcd./found:
C, 62.06/61.97; H, 3.47/3.36; N, 16.08/15.95; S, 9.20/9.29.
6-[(E)-2-(1-benzofuran-2-yl)ethenyl]-3-(2-furyl)[1,2,4]-triazolo-
[3,4-b][1,3,4]thiadiazole (9h)
This compound was isolated as a yellow-powdered solid, m.p. 256–
257◦C (ethanol–DMF) in 57% yield. 1H NMR ppm: 6.73 (br. s, 1H, 4-H
furan), 7.23–7.32 (m, 3H, aromatic), 7.38 (pseudo t, 1H, benzofuran),
7.43 (d, J = 16.1, 1H, CH ), 7.53 (d, J = 7.8, 1H, benzofuran), 7.68 (d,
J = 7.8, 1H, benzofuran), 7.76 (d, J = 16.1, 1H, CH), 7.90 (br. s, 1H,
5–H furan). MS m/z: 334 (M+). Anal. requires for C17H10N4O2S (334.35)
calcd./found: C, 61.07/60.90; H, 3.01/3.11; N, 16.76/16.95; S, 9.59/9.63.
6-[(E)-2-(1-benzofuran-2-yl)ethenyl]-3-(3-methyl-2-furyl)[1,2,4]-
triazolo[3,4-b][1,3,4]thiadiazole (9i)
This compound was isolated as a reddish brown powdered solid, m.p.
222–223◦C (ethanol–DMF). 1H NMR ppm: δ 2.40 (s, 3H, CH3), 6.57 (br. s,
1H, 4–H furan), 7.26 (t, J = 7.8, 1H, benzofuran), 7.29 (s, 1H, 3–H ben-
zofuran), 7.39 (t, J = 7.8, 1H, benzofuran), 7.42 (d, J = 16.1, 1H, CH ),
7.52 (d, J = 7.8, 1H, benzofuran), 7.66 (d, J = 7.8, 1H, benzofuran), 7.70
(d, J = 16.1, 1H, CH), 7.76 (br. s, 1H, 5–H furan). MS m/z: 348 (M+).
Anal. requires for C18H12N4O2S (348.38) calcd./found: C, 62.06/61.83;
H, 3.47/3.44; N, 16.08/15.96; S, 9.20/9.16.
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