TiCl4 mediated facile synthesis of 1,3,4-oxadiazoles and 1,3,4-thiadiazoles

Article abstract of DOI:10.1080/00397911.2019.1700521

An efficient method for the synthesis of 2,5-disubstituted 1,3,4-oxadiazoles and 1,3,4-thiadiazoles has been developed. Various hydrazides or thionyl hydrazides readily react with DMA derivatives in the presence of TiCl₄ as a catalyst to afford the desired products. This protocol provides a simple and economical procedure that affords the target products with good yields and wide substrate scope.

Full text of DOI:10.1080/00397911.2019.1700521

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
TiCl mediated facile synthesis of 1,3,4-oxadiazoles
4
and 1,3,4-thiadiazoles
Lin Zhang, Yu Yu, Qiang Tang, Jianyong Yuan, Dongzhi Ran, Binghua Tian, Tao
Pan & Zongjie Gan
To cite this article: Lin Zhang, Yu Yu, Qiang Tang, Jianyong Yuan, Dongzhi Ran, Binghua Tian,
Tao Pan & Zongjie Gan (2019): TiCl mediated facile synthesis of 1,3,4-oxadiazoles and 1,3,4-
4
thiadiazoles, Synthetic Communications
To link to this article: https://doi.org/10.1080/00397911.2019.1700521
Published online: 17 Dec 2019.
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https://doi.org/10.1080/00397911.2019.1700521
TiCl mediated facile synthesis of 1,3,4-oxadiazoles and
4
1,3,4-thiadiazoles
a
a
a,b
a,b
a
ꢀ
ꢀ
Lin Zhang , Yu Yu , Qiang Tang , Jianyong Yuan , Dongzhi Ran ,
a
a
a,b
Binghua Tian , Tao Pan , and Zongjie Gan
a
Department of Medicinal Chemistry, College of Pharmacy, Chongqing Medical University, Chongqing,
b
People’s Republic of China; Chongqing Research Center for Pharmaceutical Engineering, Chongqing
Medical University, Chongqing, People’s Republic of China
ABSTRACT
ARTICLE HISTORY
An efficient method for the synthesis of 2,5-disubstituted 1,3,4-oxa-
diazoles and 1,3,4-thiadiazoles has been developed. Various hydra-
zides or thionyl hydrazides readily react with DMA derivatives in the
presence of TiCl4 as a catalyst to afford the desired products. This
protocol provides a simple and economical procedure that affords
the target products with good yields and wide substrate scope.
Received 21 October 2019
KEYWORDS
1
,3,4-oxadiazoles and 1,3,4-
thiadiazoles; synthesis; TiCl
4
GRAPHICAL ABSTRACT
Introduction
The 1,3,4-oxadiazole and 1,3,4-thiadiazole structural motifs are important heterocycles
found in a variety of molecules of critical use. Bioactive compounds featuring these scaf-
[
1]
folds, usually exhibit a wide range of pharmacological activities such as antitumor,
[
2]
[3]
[4]
anti-inflammatory, antimicrobial and anti-HIV in the pharmaceutical sciences. In
addition, significant molecules possessing 1,3,4-oxadiazole and 1,3,4-thiadiazole as the
[
5]
core unit are widely applied in organic synthesis as important building blocks.
Inspired by the versatile applications of 1,3,4-oxadiazoles and 1,3,4-thiadiazoles, the
development of simple and facile synthetic methods for these structures is of
great importance.
Currently, the strategies for the synthesis of 2,5-disubstituted 1,3,4-oxadiazoles and
1,3,4-thiadiazoles generally involve the dehydration of diacylhydrazines or oxidative
CONTACT Zongjie Gan
gzj@cqmu.edu.cn
Department of Medicinal Chemistry, College of Pharmacy, Chongqing
Medical University, Chongqing, 400016, People’s Republic of China.
ꢀ
These authors contributed equally to this work.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website
ß 2019 Taylor & Francis Group, LLC

2
L. ZHANG ET AL.
Scheme 1. Synthesis methods for 1,3,4-oxadiazoles and 1,3,4-thiadiazoles.
cyclization of hydrazones. Taking 1,3,4-oxadiazoles for example, coupling of hydrazines
with carboxylic acids or aryl chlorides has been reported to generate diacylhydrazines,
which are converted to the desired 1,3,4-oxadiazoles by cyclization in the presence of
[
6]
[7]
various dehydrated reagents including Brønsted acids,
phosphorus oxychloride,
[
8]
[9]
[10]
polyphosphate acid (PPA), Burgess’s reagent, H SO /SiO under MW condition
2
4
2
and so on. However, there are one or more limitations to the cyclization strategies pre-
sented above, like harsh reaction conditions, extended reaction times and low yields in
[
11]
the most of the cases.
Moreover, the acidic cyclization conditions caused by the large
amount of strong acids or POCl are generally not compatible with diverse functional-
3
ity. Alternatively, many improvements to the reaction have been made in recent years.
[
12]
Gong
and coworkers presented a flexible and elegant protocol for the regioselective
synthesis of 1,3,4-oxadiazole and 1,3,4-thiadiazole via reagent-based cyclization of thio-
semicarbazide intermediate. On the other hand, hydrazones generated from condensa-
tion of an aldehyde and hydrazines have also been proposed as a superior method to
afford 1,3,4-oxadiazoles after oxidative cyclization. Typically, the oxidation is carried out
[
13]
using stoichiometric amounts of oxidizing agents such as KMnO ,
Fe(NO ) /
3 3
4
[
14]
[15]
[16]
[17]
TEMPO,
ceric ammonium nitrate
or TBHP/I2.
In addition, Dabiri
reported an improved method to form 1,3,4-oxadiazoles from acyl hydrazides
and
[
18]
Bunce
and orthoesters in the presence of KAl(SO ) ꢁ 12 H O or NH Cl, respectively. Similarly,
4
2
2
4
the requirement of an expensive catalyst or commercially unavailable starting material
and the addition of hazardous or explosive solvents/regents also make these methods
unattractive for large scale application. Thus, we became interested in the development
of efficient synthesis of 1,3,4-oxadiazoles and 1,3,4-thiadiazoles utilizing a single dehy-
dration reagent for both coupling and cyclodehydration events.
Titanium tetrachloride (TiCl ) is a typical Lewis acid, and its strong affinity toward
4
oxygenated organic compounds has been widely used in various functional group trans-
formations. Besides, titanium tetrachloride is also a mild and powerful dehydrating
[
19]
agent which is usually used in the synthesis of numerous organic molecules.
Recently, our group successfully applied TiCl in the convenient synthesis of furan and
4
[
20]
benzofuran derivatives.
Herein in this paper, we would like to describe a facile and
general method for the preparation of 1,3,4-oxadiazoles and 1,3,4-thiadiazoles employ-
ing TiCl as the catalyst, which has not been disclosed in the literature (Scheme 1).
4
Results and discussion
In our preliminary study, the cyclization of benzohydrazide with N,N-dimethylaceta-
mide (DMA) was carried out as a model reaction. To our delight, when TiCl (2 equiv.)
4
was employed, the reaction afforded the desired product (2a) in good isolated yield
without any other additives or solvents (Table 1, entry 2). However, replacement of

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a
Table 1. Optimization of the reaction conditions.
ꢂ
b
Entry
Catalyst (equiv.)
Temp ( C)
Time (h)
Yield (%)
c
1
2
3
4
5
6
7
8
9
–
110
110
110
110
110
110
110
110
110
110
110
110
110
110
100
90
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
NR
85
24
Trace
Trace
Trace
NR
Trace
NR
NR
Trace
34
68
TiCl
SO
4
(2.0)
(2.0)
H
2
4
Conc. HCl (2.0)
HAc (2.0)
TsOH (2.0)
ZnCl
FeCl
Cu(OAc)
CaCl
2
(2.0)
(2.0)
3
2
(2.0)
10
11
12
13
14
15
16
a
2
(2.0)
CuBr (2.0)
TiCl
TiCl
TiCl
TiCl
TiCl
4
4
4
4
4
(0.5)
(1.0)
(3.0)
(2.0)
(2.0)
86
63
40
Reactions were carried out with benzohydrazide (0.3 g, 2.2 mmol, 1 equiv.) and DMA (3 mL).
b
Isolated yield.
No reaction.
c
TiCl with Brønsted acids such as H SO conc. HCl, TsOH and HAc under the same
4
2
4,
condition dramatically decreased the yield (Table 1, entries 3–6), demonstrating that
TiCl is highly efficient for this reaction. Regarding the effects of different catalysts, a
4
series of Lewis acids were extensively investigated. The screening study indicated that
Lewis acids including ZnCl , CaCl and Cu(OAc) were ineffective in promoting this
2
2
2
reaction, while FeCl afforded the desired product in a lower yield along with some
3
unknown impurities (Table 1, entries 7–11). Encouraged by this result, then the stoi-
chiometric of the catalyst on the reaction system was explored. Carrying out the reac-
tion with 0.5 equiv. of TiCl afforded the product 2a in 34% yield (entry 12). A
4
constant increase in the yield of 2a was observed with the increase in catalyst loading
(
entries 2 and 13). There was a slight increase in the yield using 3 equiv. of TiCl (entry
4
1
4), which led us to select 2 equiv. of catalyst for this transformation. Subsequently,
ꢂ
upon varying temperatures of the reaction between 110 and 90 C, the conversion
declined, 110 C was proved to the optimal temperature for the reaction. Finally, the
reaction was found to be efficient with 2 equiv. of catalyst in DMA at 110 C, affording
ꢂ
ꢂ
a yield of the product with 85% within 1.5 h.
With the optimized conditions in hand, we next set out to investigate the generality
of this reaction. The synthesis of 1,3,4-oxadiazoles was demonstrated to be tolerant to
various substituted hydrazides, such as benzohydrazides, acetohydrazide and butyrohy-
drazide, as listed in Table 2. The benzohydrazides bearing electron-donating and -with-
drawing groups were both proven to be compatible to this reaction. Benzohydrazides
with electron-donating groups, such as CH and OCH , generated the corresponding
3
3
coupling products in better yields than those of electron-withdrawing groups (Table 2,

4
L. ZHANG ET AL.
a
Table 2. Substrate scope with respect to substituted hydrazides.
Entry
1
Substrate
Reactant/Solvent
Product
Yield
DMA
85%
b
1a
2a
2
3
4
5
DMA
DMA
DMA
DMA
90%
84%
77%
78%
1
b
2b
1c
2c
2d
2e
1
d
1
e
6
7
DMA
DMA
66%
73%
1
f
2f
1
g
2g
8
9
DMA
77%
80%
1
h
1
2h
DMBA
i
2i
(continued)

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Entry
Substrate
Reactant/Solvent
Product
Yield
10
11
12
DMF
82%
79%
77%
1
a
2j
DMF
DMF
1
c
2k
1
d
2l
1
3
DMPA
DMBA
70%
75%
1
f
2m
1
4
5
c
1
d
2n
1
74%
1
e
2
o
a
ꢂ
Experiments were performed with substituted hydrazides (0.3 g) and DMA derivates (3 mL) at 110 C for 1.5 h,
isolated yield.
b
c
2a can also be obtained from acetohydrazide and DMBA.
ꢂ
Experiments were performed with 4-methylbenzohydrazide (5 mmol) and DMBA (3.7 g, 25 mmol) at 110 C for 1.5 h,
isolated yield.
entries 2b, 2c versus 2d–2g), probably owing to the fact that the electron-withdrawing
groups disfavor the formation of reaction intermediate. Besides, the results also indi-
cated that the position of the substituents on the benzene moiety had little effect
(Table 2, 2e and 2g) on the reaction outcome. In addition, butyrohydrazide was well-
tolerated in this reaction and generated the product 2i in 80% yield. It is noteworthy to
mention that the present reaction conditions were also suitable for the reaction of het-
erocyclic substrate and the nicotinohydrazide delivered to the corresponding product 2h
in good yield.
On the other hand, in order to explore the effects of different substituted amides
on this reaction, some DMA derivatives were also investigated, including DMF, N,N-
dimethylpropionamide (DMPA) and N,N-dimethylbenzamide (DMBA). As shown in
Table 2, it appears that this method was compatible for a wide range of DMA derivatives. In
general, the treatment of 1a with DMF and DMPA produced the corresponding products in
moderate to good yields (Table 2, 2j–2m). Notably, sterically hindered DMBA could also be
amenable to the reaction and afforded the corresponding product 2n in 75% yield.
Moreover, for that with long-chain alkyl group amide, the reaction preceded smoothly as
well resulted in 2o with 74% yield.

6
L. ZHANG ET AL.
a
Table 3. Substrate scope with respect to substituted thionyl hydrazides.
Entry
1
Substrate
Reactant/Solvent
Product
Yield
DMA
88%
3
a
a
4a
4b
2
3
4
DMF
DMPA
DMBA
85%
80%
81%
3
3a
4c
3a
4d
5
6
7
DMF
DMA
65%
72%
70%
3
b
4e
3
b
4f
DMPA
3
b
4g
a
ꢂ
Experiments were performed with (0.3 g) and DMA (3 mL) at 110 C for 1.5 h, isolated yield.
Furthermore, the developed reaction was also successfully applied to thionyl hydra-
zides. The corresponding cyclization products 4a–4d generated from benzothiohydra-
zide and DMA, DMF, DMPA or DMBA were successfully obtained under the standard
conditions, respectively (Table 3, entries 1–4). However, for the thiosemicarbazide sub-
strate containing a free amino group, no desired product was observed. Instead, N-acyl
1,3,4-thiadiazol-2-amines (4e–4g, Table 3, entries 5–7) were furnished in slightly lower
yields. This may be attributed to the further transamidation of the primary amine after
the 1,3,4-thiadiazol-2-amines were generated. Interestingly, to the best of
knowledge, there are no methodologies reported for the one-pot synthesis of N-acyl
1,3,4-thiadiazol-2-amines.

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Scheme 2. Gram scale synthesis of 2-methyl-5-phenyl-1,3,4-oxadiazole.
Considering the synthetic usefulness of this reaction, a scale-up experiment was con-
ducted. To our delight, when the reaction was scaled up to 73 mmol (1a, 10 Gram
scale), the desired product 2a was isolated in 82% yield via the optimized reaction con-
ditions (Scheme 2). This reaction was thus demonstrated to be effective and scalable.
Conclusion
In summary, we have developed a convenient approach to synthesize 2,5-disubstituted
1
,3,4-oxadiazoles and 1,3,4-thiadiazoles from hydrazides and DMA derivatives utilizing
TiCl as the dehydrating and cyclization agent. This novel procedure is efficient in
4
yielding 1,3,4-oxadiazoles and 1,3,4-thiadiazoles, and thus have potential application to
the preparation of such bioactive compounds.
Experimental
Hydrazides, thionyl hydrazides and other reagents were commercially available and
obtained from Adamas, tansoole or Macklin, and were used without further purification
unless otherwise noted. Melting points were determined on X-4 microscopic melting
1
point apparatus and were uncorrected. H NMR and 13C HNMR spectra were obtained
using a Bruker-600 MHz spectrometer with DMSO-d or CDCl as solutions and TMS
6
3
as the internal standard. Chemical shifts are reported in ppm and coupling constants (J)
in Hz. HRMS spectra were obtained with an Aglient 6210 Triple Quad LC–MS instru-
ment. All reactions were monitored by TLC with GF254 silica gel coated plates (petrol-
eum ether/ethyl acetate ¼ 3:1). Flash column chromatography was carried out using
2
00–300 mesh silica gel. The identity of the known compounds was established by the
1
comparison of their H and 13C NMR peaks with the authentic values.
General procedure for the synthesis of 1,3,4-oxadiazole derivatives (2a–2o)
To a 10 mL three-necked round bottle was charged with benzoylhydrazine (0.30 g,
2
.2 mmol), TiCl (0.83 g, 4.4 mmol) and DMA (3 mL, 32.1 mmol). The resulting solution
4
ꢂ
was warmed to 110 C and stirred at this temperature for 1.5 h. When the reaction was
completed, 50 mL water was added and the resulting mixture was extracted with 50 mL
ethyl acetate three times. The combined organic layer was successively washed with
H O (50 mL) and then brine (50 mL), then dried over anhydrous Na SO , filtered and
2
2
4
concentrated under reduced pressure. The residue was purified by column chromatog-
raphy on silica gel with petroleum ether and ethyl acetate to give the title products. 2-

8
L. ZHANG ET AL.
ꢂ
1
Methyl-5-phenyl-1,3,4-oxadiazole (2a) white solid. Yield, 85%. M.p. 65–67 C H NMR
;
(
600 MHz, DMSO-d ) (d, ppm): 8.02–7.93 (m, 2H), 7.45–7.35 (m, 3H), 2.59 (s, 3H).;
6
1
3C NMR (150 MHz, DMSO-d ) (d, ppm): 164.4, 164.4, 132.2, 129.8, 126.7, 124.0, 11.1.
6
þ
HRMS (ESI): m/z [MþNa] Calcd for C H N ONa: 183.0529; Found: 183.0524.
9
8 2
General procedure for the synthesis of 1,3,4-thiadiazole derivatives (4a–4g)
To a 10 mL three-necked round bottle was charged with benzothiohydrazide (0.30 g,
2
.0 mmol), TiCl (0.75 g, 4.0 mmol) and DMA (3 mL, 32.1 mmol). The resulting solution
4
ꢂ
was warmed to 110 C and stirred at this temperature for 1.5 h. When the reaction was
completed, 50 mL water was added and the resulting mixture was extracted with 50 mL
ethyl acetate three times. The combined organic layer was successively washed with
H O (50 mL) and then brine (50 mL), then dried over anhydrous Na SO , filtered and
2
2
4
concentrated under reduced pressure. The residue was purified by column chromatog-
raphy on silica gel with petroleum ether and ethyl acetate to give the title products. 2-
ꢂ
1
Methyl-5-phenyl-1,3,4-thiadiazole (4a). White solid (88%); M.p. 100–102 C; H NMR
600 MHz, CDCl ) (d, ppm): 7.97–7.86 (m, 2H), 7.52–7.40 (m, 3H), 2.78 (s, 3H); 13C
(
3
NMR (150 MHz, CDCl ) (d, ppm): 168.9, 164.9, 131.0, 130.1, 129.1, 127.9, 15.9. HRMS
3
þ
(
ESI): m/z [MþH] Calcd for C H N S: 177.0481; Found: 177.0483.
9
9 2
Spectral data for the synthesized compounds can be found via the
Supplementary Content’.
‘
Funding
We appreciate the finical support from the National Natural Science Foundation of China [No.
2
1907013] and the Scientific and Technological Research Program of Chongqing Municipal
Education Commission (KJQN20190043).
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