Synthesis and bioactivities of novel 2-(thioether/sulfone)-5-pyrazolyl-1,3,4-oxadiazole derivatives

Article abstract of DOI:10.1016/j.cclet.2016.06.055

By introducing the pyrazole moiety into the 5-position of 1,3,4-oxadiazole, a series of novel 2-(thioether/sulfone)-5-pyrazolyl-1,3,4-oxadiazole derivatives were synthesized. Preliminary bioassays suggested that target compounds exhibited appreciable activity against pathogenic bacteria Xanthomonas oryzae pv. oryzae (Xoo) and five phytopathogenic fungi in vitro. Among them, the half-maximal effective concentration (EC₅₀) values of 6c, 7a, 7b and 7c against Xoo were within 16.6 μg/mL and 65.7 μg/mL, which were better than those of commercial agricultural antibacterial bismerthiazol (92.6 μg/mL) and thiodiazole copper (121.8 μg/mL). While compounds 7a, 7b, and 7c exerted comprehensive antifungal activity toward five plant fungi, which were comparable with that of hymexazol. The results demonstrated that this kind of compounds can be further studied and developed as promising antifungal and antibacterial agents.

Full text of DOI:10.1016/j.cclet.2016.06.055

Accepted Manuscript
Title: Synthesis and bioactivities of novel
2-(thioether/sulfone)-5-pyrazolyl-1,3,4-oxadiazole derivatives
Author: Yu-Tao Zheng Teng-Teng Zhang Pei-Yi Wang
Zhi-Bing Wu Lei Zhou Yi-Qiang Ye Xiang Zhou Ming He
Song Yang
PII:
S1001-8417(16)30259-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2016.06.055
CCLET 3809
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Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
28-4-2016
26-6-2016
30-6-2016
Please cite this article as: Yu-Tao Zheng, Teng-Teng Zhang, Pei-Yi Wang, Zhi-Bing
Wu, Lei Zhou, Yi-Qiang Ye, Xiang Zhou, Ming He, Song Yang, Synthesis and
bioactivities of novel 2-(thioether/sulfone)-5-pyrazolyl-1,3,4-oxadiazole derivatives,
Chinese Chemical Letters http://dx.doi.org/10.1016/j.cclet.2016.06.055
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Original article
Synthesis and bioactivities of novel 2-(thioether/sulfone)-5-pyrazolyl-1,3,4-
oxadiazole derivatives
Yu-Tao Zheng a †, Teng-Teng Zhang a †, Pei-Yi Wang a, Zhi-Bing Wu a, Lei Zhou a, Yi-Qiang Ye a, Xiang
Zhou a, Ming He a, Song Yang a,b

a State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key
Laboratory of Green Pesticide and Agricultural Bioengineering, Ministry of Education, Center for R&D of
Fine Chemicals, Guizhou University, Guiyang 550025, China.
b College of Pharmacy, East China University of Science & Technology, Shanghai 550025, China.
 Corresponding author.
E-mail address: jhzx.msm@gmail.com
† These two authors contribute equally to this work.
Graphical Abstract
By introducing the pyrazole moiety into the 5-position of 1,3,4-oxadiazole, a series of novel 2-(thioether/sulfone)-5-pyrazolyl-1,3,4-oxadiazole
derivatives were synthesized. Among them, the EC50 values of 6c, 7a, 7b and 7c against Xanthomonas oryzae pv. oryzae were within 16.6 and
65.7 μg/mL, which were better than those of commercial agricultural antibacterial product bismerthiazol and thiodiazole copper. While
compounds 7a, 7b, and 7c exerted comprehensive antifungal activity toward five plant fungi, which were comparable with that of hymexazol.
ARTICLE INFO
Article history:
Received 28 April 2016
Received in revised form 26 June 2016
Accepted 30 June 2016
Available online
ABSTRACT
By introducing the pyrazole moiety into the 5-position of 1,3,4-oxadiazole, a series of novel 2-(thioether/sulfone)-5-pyrazolyl-1,3,4-oxadiazole
derivatives were synthesized. Preliminary bioassays suggested that target compounds exhibited appreciable activity against pathogenic bacteria
Xanthomonas oryzae pv. oryzae (Xoo) and five phytopathogenic fungi in vitro. Among them, the half-maximal effective concentration (EC50) values of
6c, 7a, 7b and 7c against Xoo were within 16.6 μg/mL and 65.7 μg/mL, which were better than those of commercial agricultural antibacterial
bismerthiazol (92.6 μg/mL) and thiodiazole copper (121.8 μg/mL). While compounds 7a, 7b, and 7c exerted comprehensive antifungal activity toward
five plant fungi, which were comparable with that of hymexazol. The results demonstrated that this kind of compounds can be further studied and
developed as promising antifungal and antibacterial agents.
———

Keywords: Pyrazole 1,3,4-Oxadiazole Synthesis Antibacterial Antifung
1. Introduction
Heterocyclic substructures have been extensively studied for their powerful applications in construction of bioactive compounds [1-
4]. Among them, pyrazole ring as an important functional group has already been used in the development of pharmaceuticals and
agrochemicals due to its derivatives bearing multitudinous bioactivities, including anti-inflammatory, antitumor, herbicidal, insecticidal,
antifungal, and antibacterial activities [5-13]. Furthermore, some pyrazole compounds have already been commercialized as fungicides,
like sedaxane (Syngenta, 2005), isopyrazam (Syngenta, 2006), bixafen (Bayer, 2005), and fluxapyroxad (BASF, 2008) [14-17]. As
another crucial scaffold, 1,3,4-oxadiazole, has exerted promising applications in creating new agrochemicals on account of the diverse
bioactivities of its derivatives [18-21]. In our previous work, we had found a series of new 1,3,4-oxadiazole sulfone compounds
(structure depicted in Fig. 1, lead compound) with high antibacterial/fungicidal bioactivities [22-24]. In order to find new structures
with antibacterial/antifungal bioactivities, the two functional moieties of pyrazole and 1,3,4-oxadiazole were combined into one
molecule by replacing the phenyl group to pyrazole moiety at the 5-position of the lead compound, as shown in Fig. 1. All the title
compounds were bioassayed against pathogenic bacteria Xanthomonas oryzae pv. oryzae (Xoo) and five phytopathogenic fungi.
2. Experimental
All the chemicals were purchased from Aladdin and used as received. The organic solvents were distilled before used. NMR spectra
were obtained by using a JEOL-ECX-500 apparatus. Chemical shifts were reported in parts per million (ppm) down field from TMS
with the solvent resonance as the internal standard. Coupling constants (J) were reported in Hz and referred to apparent peak
multiplications. MS data were recorded on an Agilent ESI-MSD Trap (VL) mass instrument.
2.1 General synthetic procedures for the target compounds (6a-6o) and (7a-7i)
A solution of carbon disulfide (0.015 mol) in ethanol (10 mL) was added dropwise to the mixture of compound 4 (0.01 mol) and
potassium hydroxide (0. 012 mol) in ethanol (40 mL) at room temperature. Then, the reaction mixture was heated under reflux with
stirring for 8 h. After the reaction was completed, ethanol was evaporated to give unpurified intermediate 5. An appropriate
halohydrocarbon (0.01 mol) was added to the solution of unpurified intermediate 5 in water (20 mL) and the mixture was stirred for 1 h
at room temperature. The solid was filtered, purified by column chromatography using a mixture of petroleum ether and ethyl acetate
(10:1) as the eluent, and then the pure target compounds (6a-6o) were obtained.
The compound (6a-6i) (5 mmol) and acetic acid (15 mL) were added to a 50 mL three-neck round-bottom flask equipped with a
magnetic stirrer. The resulting solution was stirred for 10 min when a clear solution was obtained, and then 7% KMnO4 solution (5
mmol) was added dropwise at room temperature and the progress of the reaction was monitored by thin layer chromatography (TLC)
using petroleum ether : ethyl acetate (3:1). After the reaction was completed, 10% NaHSO3 solution was added to deoxidize the
unreacted KMnO4. The resulted solid was filtered, washed with water, from which the pure compounds (7a-7i) can be obtained by
column chromatography using a mixture of petroleum ether and ethyl acetate (15:1) as the eluent.
2.2 In vitro antibacterial bioassay (turbidimeter test)
In our study, all the synthesized target compounds were evaluated for their antibacterial activities against Xoo by the turbidimeter
test in vitro. Dimethylsulfoxide in sterile distilled water served as a blank control, Bismerthiazol and Thiodiazole Copper served as the
positive controls. Approximately 40 μL of solvent NB (1.5 g beef extract, 2.5 g peptone, 0.5 g yeast powder, 5.0 g glucose, and 500
mL distilled water; pH = 7.0–7.2) containing Xoo, incubated on the phase of logarithmic growth, was added to 5 mL of solvent NB
containing the test compounds and positive control. The inoculated test tubes were incubated at 28 ± 1 °C and continuously shaken at
180 rpm for 24-48 h until the bacteria were incubated on the logarithmic growth phase. The growth of the cultures was monitored on a
microplate reader by measuring the optical density at 595 nm (OD595) given by turbiditycorrected values = ODbacterial wilt − ODno bacterial wilt
and the inhibition rate I was calculated by I = (C − T)/C × 100%. C is the corrected turbidity values of bacterial growth on untreated
NB (blank control), and T is the corrected turbidity values of bacterial growth on treated NB. The experiment was repeated three times.
,
3. Results and discussion
The synthesis and structures of (6a-6o), and (7a-7i) are shown in scheme 1. Briefly, ethyltrifluoroacetoacetate (1) was treated with
triethoxymethane to give intermediate (E)-2-trifluoroacetyl-3-ethoxy-2-propenoate (2), followed by the cyclocondensation reaction to
provide an important intermediate ethyl 1-phenyl-5-(trifluoromethyl)-1H-pyrazole-4-carboxylate (3) containing pyrazole group in 82%
yield. Next, the hydrazide 4 was obtained through refluxing 3 in hydrazine hydrate with the yield of 94%. A subsequent reaction with
carbon disulfide in the presence of potassium hydroxide leaded to the formation of the crucial intermediate 5 containing 1,3,4-
oxadiazole. Finally, the corresponding target thioethers (6a-6o) were achieved via thioetherification with halogenated agents in good
yields ranging from 76% to 85%, and subsequently converted into the corresponding sulfones (7a-7i) by oxidizing the related thioether
at room temperature. All the structures were confirmed by 1H NMR, 13C NMR, and MS (detailed information see supplementary data).

In our study, we first evaluated the antibacterial activity of all the title compounds via turbidmeter test [25-27] against pathogenic
bacteria Xanthomonas oryzae pv. oryzae (Xoo), which was considered as one of devastative bacteria against rice in rice-growing
countries. Meanwhile, the commercial agricultural antibacterial bismerthiazol (BT) and thiodiazole copper (TC) were employed for the
comparison of bioactivity in vitro. Preliminary bioassays revealed that most of the target compounds exerted appreciable inhibition
bioactivity against Xoo in the dosage of 200 or 100 μg/mL (Table 1). Among them, compounds 6c, 6e, 6f, 6j, 7a, 7b, and 7c gives the
inhibition rate above 72.3% against Xoo in the dosage of 200 μg/mL, which were better than that of BT (72.1%) and TC (64.2%);
while compounds 6c, 6f, 7a, 7b, and 7c offers better inhibition rate above 66.2% against Xoo than that of BT (53.7%) and TC (43.1%)
in the dosage of 100 μg/mL. The half-maximal effective concentration (EC50) values of 6c, 7a, 7b, and 7c were detected as 47.5, 31.6,
65.7, and 16.6 μg/mL, respectively, which were obviously better than that of commercial bactericides (92.6 or 121.8 μg/mL). Based on
the above results, among all the thioether compounds (6a-6o), the isopropyl group compound (6c) exhibited the best bioactivity against
Xoo than the other groups, while for benzyl thioether compounds, 4-methylbenzyl thioether (6f) gives superior activity than the other
substituted benzyl in the dosage of 200 μg/mL or 100 μg/mL. For sulfone compounds, the antibacterial activity of alkyl sulfone
compounds (such as 7a ~ 7c) was dramatically better than the benzyl derivatives.
The antifungal activity of (6a-6o) and (7a-7i) was examined via the poison plate technique [28] against five phytopathogenic fungi,
Gibberella zeae (G. z.), Fusarium oxysporum (F. o.), Cytospora mandshurica (C. m.), Sclertinia sclerotiorum (S. s.), and Rhizoctonia
solani (R. s.) at the concentrate of 100 μg/mL, Meanwhile, the commercial agricultural antifungal Hymexazol (HM) and Carbendazim
(CB) were employed for the comparison of bioactivity. As shown in table 2, compounds 7a and 7c were observed having
comprehensive antifungal activity with the inhibition rate ranging from 53.8% to 75.5% against the five kinds of fungi, which were
comparable to the commercial fungicide HM. It is worth pointing out that compound 6j exerted good antifungal activity with the
inhibition rate of 86.4% against S. sclerotiorum. In comparison of 6a and 7a, 6b and 7b, 6c and 7c, 6d and 7d, 6f and 7f, the antifungal
activity was improved after oxidizing the thioether into the sulfone, further suggested sulfonyl group as a crucial functional group may
improve the bioactivity of the target compound. It can be seen that compound 7a showed the strongest antifungi activity against the
five phytopathogenic fungi.
3. Conclusion
In summary, a series of 2-(thioether/sulfone)-5-pyrazolyl-1,3,4-oxadiazole derivatives containing both pyrazole moiety and 1,3,4-
oxadiazole moiety were designed and synthesized, and which antibacterial activity and antifungal activity were evaluated via
turbidmeter test or the poison plate technique in vitro. Compounds 6c, 7a, 7b and 7c showed good inhibition effects against Xoo with
the EC50 values ranging from 16.6 μg/mL to 65.7 μg/mL, which were better than those of commercial agricultural antibacterial
bismerthiazol (92.6 μg/mL) and thiediazole copper (121.8 μg/mL). Meanwhile, compounds 7a, 7b, and 7c exerted good antifungal
activities against five plant fungi, which were comparable to that of HM. The results demonstrated that this kind of compounds can be
further studied and developed as promising antifungal and antibacterial agents.
Acknowledgment
We acknowledge the financial support of the Key Technologies R&D Program (No. 2014BAD23B01), National Natural Science
Foundation of China (No. 21372052), the Research Project of Chinese Ministry of Education (No. 213033A, 20135201110005), and
Scientific Research Foundation for the Introduced Talents of Guizhou University (2015-34).
Supplementary data
Experimental characterization data of (6a-6o) and (7a-7i); 1H NMR, 13C NMR and Mass Spectra of 6a, 6d, 7a, and 7d (Figure S1-
S12). This material is available free of charge via the internet at http://www.elsevier.com/.
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Fig. 1. Design strategy of the target compounds.
Scheme 1. Synthetic route of 2-(thioether/sulfone)-5-pyrazolyl-1,3,4-oxadiazole derivatives (6a-6o) and (7a-7i).

Table 1. Inhibition effect of sulfides/sulfones against Xoo.
Inhibition rate (%)
Inhibition rate (%)
Compd.
Compounds
Compd. Regression equation
r
EC50 (μg/mL)
200 µg/mL 100 µg/mL
200 µg/mL 100 µg/mL
6a
6b
6c
6d
6e
6f
55.2 ± 2.8
36.5 ± 2.7
7a
7b
7c
7d
7e
7f
6c
7a
y=1.647x+2.238
y=2.345x+1.482
y=1.258x+2.712
y=3.419x+0.832
0.96
0.96
0.99
0.96
47.5±4.3
31.6±1.2
65.7±3.8
16.6±0.2
98.9 ± 0.3
97.8 ± 0.9
50.3 ± 2.4
93.8 ± 0.3
69.3 ± 3.6
77.5 ± 1.2
84.3 ± 0.8
50.2 ± 2.1
66.2 ± 1.9
53.2 ± 2.2
72.3 ± 1.7
59.8 ± 3.5
68.5 ± 3.0
72.1 ± 0.7
26.7 ± 1.9
83.2 ± 0.3
33.8 ± 1.7
49.1 ± 4.3
66.2 ± 3.9
34.3 ± 3.5
40.1 ± 1.9
38.4 ± 2.8
39.3 ± 1.7
45.8 ± 2.2
43.2 ± 0.7
53.7 ± 1.2
100.0 ± 0.3 100.0 ± 0.2
7b
7c
100.0 ± 0.6
50.8 ± 2.7
67.7 ± 3.6
51.5 ± 4.1
48.3 ± 2.8
57.7 ± 3.3
34.5 ± 3.1
55.6 ± 1.4
59.4 ± 1.8
56.2 ± 1.9
64.2 ± 2.8
96.9 ± 0.5
45.0 ± 3.9
48.0 ± 3.8
40.0 ± 3.6
34.2 ± 4.8
38.4 ± 4.3
12.8 ± 4.9
31.9 ± 2.9
48.7 ± 3.4
40.9 ± 2.1
43.1 ± 3.2
BT
TC
y=1.499x+2.052
y=1.540x +1.788
0.98
0.98
92.6±2.1
121.8±3.6
6g
6h
6i
7g
7h
7i
6j
6m
6n
6o
TC
6k
6l
BT
Table 2. Inhibition effect of sulfides/sulfones at 100 µg/mL against five phytopathogenic fungi.
Inhibition rate (%)
Inhibition rate (%)
C. m.
Compd.
Compd.
G. z.
F. o.
C. m.
S. s.
R. s.
G. z.
F. o.
S. s.
R. s.
6a
6b
6c
6d
6e
6f
7a
7b
7c
53.7±2.8 36.1±1.9 38.7±2.1 57.3±2.9 54.6±2.6
39.1±2.4 30.1±2.6 34.4±1.4 47.1±2.3 46.7±2.1
59.9±1.8 72.4±2.8 70.9±2.4 72.3±2.2 75.5±2.1
45.6±3.1 51.2±1.7 57.8±1.5 62.0±2.3 59.6±3.4
53.8±2.8 70.2±3.2 64.4±2.1 70.1±2.9 69.8±2.4
23.4±1.7 34.2±1.9 25.8±1.8 22.5±2.3 31.7±1.1
44.1±1.9 48.3±2.2 28.0±1.6 25.6±1.4 47.0±2.0
7d
7e
15.4±1.4
17.4±1.7
13.1±1.5
0
0
0
0
10.0±1.7 17.0±2.1
14.3±1.8 14.0±2.4 13.5±1.8
0
0
0
13.6±1.4
0
7f
0
6.1±1.4
0
0
0
13.1±1.9
11.9±1.4
0
41.5±1.5 52.2±2.3 38.9±2.0 36.4±2.2 35.8±2.1
6g
6h
6i
7g
7h
7i
16.6±1.5 16.6±1.6
0
0
0
0
0
0
13.2±1.5
12.0±2.4
0
0
0
0
7.2±1.7
11.7±1.4
8.3±1.7
14.7±1.8 29.0±2.3 17.4±1.8
11.7±1.3 25.8±1.8 23.2±1.7 23.1±2.2 17.2±1.9
6j
6m
6n
6o
CB
23.5±1.8 51.6±2.4 29.6±1.6 86.4±4.9 33.5±1.8
35.3±1.7 26.7±1.7 27.1±1.7 53.8±2.7 50.5±2.1
11.1±1.4
10.9±1.6
9.4±1.6
0
19.0±1.8
7.2±1.7
0
0
0
21.6±2.0
6.0±2.8
14.4±1.5
100
6k
6l
11.7±1.4
0
8.2±1.6
13.3±2.2 12.1±1.7
10.9±1.6 18.5±1.7
100 100
0
HM
72.2±3.6 62.1±2.5 66.1±2.1
100
100