Synthesis and antibacterial activity of pyridinium-tailored 2,5-substituted-1,3,4-oxadiazole thioether/sulfoxide/sulfone derivatives

Article abstract of DOI:10.1016/j.bmcl.2016.01.029

By introducing the pyridinium group into 2,5-substituted-1,3,4-oxadiazole, a series of pyridinium-tailored 2,5-substituted-1,3,4-oxadiazole thioether/sulfoxide/sulfone derivatives were obtained, and their antibacterial activities were evaluated via turbidimeter test in vitro. The bioassays reveal that most of the target compounds exhibit better inhibition activities against pathogen Xanthomonas oryzae pv. oryzae, Ralstonia solanacearum, and Xanthomonas axonopodis pv. citri than positive controls bismerthiazol (CK₁) or thiodiazole copper (CK₂). Among them, I-8, I-10, I-12, II-10, II-12, III-10, and III-12 exert excellent inhibition activities against the three pathogenic bacteria with the half-maximal effective concentration (EC₅₀) values ranging from 0.54 to 12.14 μg/mL. Our results demonstrate that pyridinium-tailored 1,3,4-oxadiazole thioether/sulfoxide/sulfone derivatives can serve as potential alternative bactericides for the management of plant bacterial diseases.

Full text of DOI:10.1016/j.bmcl.2016.01.029

Accepted Manuscript
Synthesis and antibacterial activity of pyridinium-tailored 2,5-substituted-1,3,4-
oxadiazole thioether/sulfoxide/sulfone derivatives
Pei-Yi Wang, Lei Zhou, Jian Zhou, Zhi-Bing Wu, Wei Xue, Bao-An Song, Song
Yang
PII:
S0960-894X(16)30029-4
DOI:
http://dx.doi.org/10.1016/j.bmcl.2016.01.029
BMCL 23494
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
4 November 2015
2 January 2016
12 January 2016
Please cite this article as: Wang, P-Y., Zhou, L., Zhou, J., Wu, Z-B., Xue, W., Song, B-A., Yang, S., Synthesis and
antibacterial activity of pyridinium-tailored 2,5-substituted-1,3,4-oxadiazole thioether/sulfoxide/sulfone
derivatives, Bioorganic & Medicinal Chemistry Letters (2016), doi: http://dx.doi.org/10.1016/j.bmcl.2016.01.029
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Bioorganic & Medicinal Chemistry Letters
Synthesis and antibacterial activity of pyridinium-tailored 2,5-substituted-1,3,4-
oxadiazole thioether/sulfoxide/sulfone derivatives
Pei-Yi Wang†, Lei Zhou†, Jian Zhou, Zhi-Bing Wu, Wei Xue, Bao-An Song*, Song Yang*
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 of Guizhou University, Guiyang, 550025, China.
Corresponding Author Emails: jhzx.msm@gmail.com, songbaoan22@yahoo.com.
† These two authors contribute equally to this work.
ARTICLE INFO
ABSTRACT
Article history:
Received
Revised
Accepted
Available online
By introducing the pyridinium group into 2,5-substituted-1,3,4-oxadiazole, a series of
pyridinium-tailored 2,5-substituted-1,3,4-oxadiazole thioether/sulfoxide/sulfone derivatives
were obtained, and their antibacterial activities were evaluated via turbidimeter test in vitro. The
bioassays reveal that most of the target compounds exhibit better inhibition activities against
pathogen Xanthomonas oryzae pv. oryzae, Ralstonia solanacearum, and Xanthomonas
axonopodis pv. citri than positive controls bismerthiazol (CK1) or thiodiazole copper (CK2).
Among them, I-8, I-10, I-12, II-10, II-12, III-10, and III-12 exert excellent inhibition activities
against the three pathogenic bacteria with the half-maximal effective concentration (EC50)
values ranging from 0.54 ~ 12.14 µg/mL. Our results demonstrate that pyridinium-tailored 1,3,4-
oxadiazole thioether/sulfoxide/sulfone derivatives can serve as potential alternative bactericides
for the management of plant bacterial diseases.
Keywords:
1,3,4-Oxadiazole
Pyridinium
Amphiphile
Synthesis
Antibacterial
2015 Elsevier Ltd. All rights reserved.
Pathogenic bacteria, one of the most serious threats to the
agricultural crops and economic crops, can cause many kinds of
plant diseases, such as rice bacterial leaf blight, tobacco bacterial
wilt, and citrus bacterial canker.1-3 The three plant diseases are
mainly caused by Gram-negative pathogenic bacteria
Xanthomonas oryzae pv. oryzae (Xoo), Ralstonia solanacearum
(R. solanacearum), and Xanthomonas axonopodis pv. citri (Xac),
respectively, and have led to enormous economic losses each
year.4-6 As one of the control measures toward these plant
diseases, chemical control, has become a powerful and useful
tool in the past decades.7-8 However, even though a few
commercial bactericides have been developed, their poor
efficiency, high phytotoxicity, high residue level, negative impact
toward the environment, or pathogen resistance are still a major
global concern. For example, bismerthiazol, one of the major
bactericides for the control of rice bacterial leaf blight, only
shows the control efficiency of 25.49% at a high dosage of 200
µg/mL. Furthermore, its application has already resulted in the
appearance of bismerthiazol-resistant strains of Xoo in Anhui
Province, China.9-10 Therefore, the exploration and development
of high-efficient and safe bactericides to prevent and control
plant bacterial diseases still remains an arduous task in pesticide
science.
various substituted piperazines and piperidines exhibiting
noteworthy bioactivities against two types of Gram-positive
bacteria and six types of Gram-negative bacteria.16 Garudachari
and co-workers have evaluated the antibacterial activity of a
series of N-alkyl-7-(trifluoromethyl) quinolin-4-amine derivatives
containing 1,3,4-oxadiazole moiety, and found that the target
compounds exerted significant antibacterial activities against
Mycobacterium smegmatis and Pseudomonas aeruginosa.17
Furthermore, our previous work have demonstrated that 1,3,4-
oxadiazole thioether/sulfoxide/sulfone derivatives exhibiting
good control efficiency toward plant bacterial diseases, and have
led to two antibacterial candidates in commercialization stage.7-
9,18-20
Obviously, heterocyclic compounds containing 1,3,4-
oxadiazole are the promising agents in the development of novel
high-efficient bactericide.
As another interesting candidate in the development of novel
bactericide, pyridinium-functionalized amphiphiles, were found
having potent biocidal activities against Gram-negative and
Gram-positive bacteria by disrupting the innate defense system of
bacterial membrane.21-24 For example, Kahriman and co-workers
introduced pyridinium moiety bearing different alkyl tails into 1-
alkyl-4-oxo-1,4-dihydroquinoline and found that the amphiphile
bearing a hexyl tail exhibited the highest bioactivities against
Gram-negative (Pseudomonas aeruginosa) and Gram-positive
(Staphylococcus aureus) bacteria.25 Brahmachari and co-workers
have evaluated a series of compounds containing a pyridinium
unit as the polar head group and different length of alkyl chain as
the hydrophobic part, and found that amphiphiles with
In the exploration of novel bactericide, heterocyclic systems
containing 1,3,4-oxadiazole have attracted great attentions due to
their potent biological activities against bacteria and fungi.11-15
For example, Patel and co-workers reported a series of 3-(5-
sulfanyl-1,3,4-oxadiazol-2-yl)-2H-chromen-2-ones
bearing
1

Figure 1. Design route for the target molecules.
pentadecyl or heptadecyl exhibited excellent bioactivities against
against Xoo than CK1 except I-4. And their EC50 reaches the
minimum value 0.54 µg/mL (I-12) with the increase in the alkyl
length, indicating that suitable hydrophobicity of the amphiphile
is favorable for the bioactivities of the target compounds.
Meanwhile, I-n also show better antibacterial activity against R.
solanacearum and Xac than CK2. EC50 values of I-8, I-10, and I-
12 against R. solanacearum are 4.03, 0.94, and 0.75 µg/mL,
respectively; EC50 values of I-n against Xac range from 1.62 ~
6.98 µg/mL. In our previous work, the EC50 value of 2-
two types of Gram-positive (Bacillus subtilis and Micrococcus
luteus) and Gram-negative (Escherichia coli and Klebsiella
aerogenes) bacteria.26 Apparently, pyridinium group can be used
as a functional tool to enhance the bioactivity of the target
compounds due to its positive charge can facilitate the interaction
with anionic cell components and consequently conduce to the
disruption of the innate defense system of bacterial membranes.
Inspired by those reports, it was proposed that appreciable
bioactivity may be achieved through embedding the pyridinium
moiety into heterocyclic systems containing 1,3,4-oxadiazole
moiety. As shown in Figure 1, a series of 2,5-substituted-1,3,4-
oxadiazole thioether/sulfoxide/sulfone derivatives were designed,
with the hydrophobic heterocyclic tails and the hydrophilic
pyridinium cations linked by different length of alkyl chains. Till
now, few studies focused on the usage of this kind of
amphiphiles in growth suppression of plant pathogenic bacteria.
(methylsulfonyl)-5-(2,4-dichlorophenyl)-1,3,4-oxadiazole
was
found to be 16.55 µg/mL against R. solanacearum,8 indicating
that high-efficient target molecules were successfully obtained as
we proposed. In comparison of the bioactivities of I-8 and I-8Br
(without pyridinium group), the antibacterial activities are
significantly enhanced after introducing the pyridinium group
into 2,5-substituted-1,3,4-oxadiazole thioether, suggesting that
appreciable bioactivity can be achieved through introducing the
pyridinium group into heterocyclic systems. Meanwhile, this
study also offers a strategy for us to achieve high-efficient
bactericide via manipulating the length of the alkyl chains.
The synthesis and structure of pyridinium-functionalized 2,5-
substituted-1,3,4-oxadiazole thioether (I-n) are shown in Scheme
1. Briefly, the starting material 2,4-dichlorobenzoic acid was
treated by three steps including esterification, hydrazidation, and
cyclization to give the crucial intermediate 5-(2,4-
dichlorophenyl)-1,3,4-oxadiazole-2-thiol,7-9 which then reacted
with dibromo-substituted alkanes to afford bromide-tailored
intermediates (I-nBr). Finally, these bromide-tailored
intermediates were incubated in pyridine at 50 °C to afford the
target compounds I-n (n = 4, 6, 8, 10, 12). All the structures were
characterized by 1H NMR, 13C NMR, and HRMS (detailed
information see supplementary data).
Turbidimeter test27-28 was carried out to evaluate the
antibacterial activities of I-n against Xoo, R. solanacearum, and
Xac in vitro. And the commercial agricultural antibacterial
bismerthiazol (CK1) was selected as the positive control for Xoo,
as well as thiodiazole copper (CK2) for R. solanacearum and Xac.
As shown in Table 1, I-n exhibit better antibacterial activities
Given the above results that pyridinium tailored 2,5-
substituted-1,3,4-oxadiazole
thioether
exhibit
excellent
antibacterial activities, we wonder if the oxidation of thioether
into sulfoxide/sulfone will enhance the bioactivities of the final
products. Therefore, 2,5-substituted-1,3,4-oxadiazole sulfoxide/
sulfone (II-n and III-n) were designed and synthesized. As
shown in Scheme 1, the intermediate bromide-tailored thioether
(I-nBr) was treated with a mixture of H2O2 and Na2WO4·2H2O to
give bromide-modified sulfoxide/sulfone [II-nBr (n = 8, 10, 12)
or III-nBr (n = 4, 6, 8, 10, 12), characterization data see
supplementary data], and consequently reacted with pyridine at
50 oC to give the comparative compounds II-n (n = 8, 10, 12) and
III-n (n = 4, 6, 8, 10, 12). It can be seen clearly from Figure 2
that the EC50 valves of I-n are lower than those of II-n or III-n,
revealing that the bioactivity of target molecules was decreased
during the oxidization of thioether to sulfoxide/sulfone.
Scheme 1. Synthetic route of pyridinium-tailored 2,5-substituted-1,3,4-oxadiazole thioether/sulfoxide/sulfone derivatives.
2

Table 1. Antibacterial activity of the [I-n, II-n, III-n (n = 4, 6, 8, 10, 12); I-8Br, II-8Br, and III-8Br] against pathogen Xoo, R.
solanacearum, and Xac in vitro.
Xoo
R. solanacearum
Xac
Compounds
EC50
(µg/mL)
>80
EC50
EC50
Regression equation
r
Regression equation
r
Regression equation
r
(µg/mL)
(µg/mL)
I-4
/
/
y = 5.973x-5.639
y = 8.423x-4.739
y = 3.610x+2.813
y = 9.480x+5.263
1.00
0.94
0.97
0.92
60.41±6.00
14.33±0.76
4.03±0.44
0.94±0.08
y = 1.893x+3.403
y = 1.174x+4.334
y = 2.072x+4.171
y = 3.337x+4.011
0.95
1.00
0.96
0.98
6.98±0.79
3.70±0.36
2.51±0.03
1.98±0.12
I-6
y = 4.751x-2.212
y = 4.783x+0.875
y = 10.495x+4.661
0.96
0.96
0.97
0.97
32.97±1.12
7.28±0.41
1.08±0.15
0.54±0.03
I-8
I-10
I-12
II-8
y = 3.454x+5.923
y = 1.280x+2.924
y = 9.293x+6.179
y = 3.985x-0.177
1.00
0.96
0.75±0.03
19.90±1.32
y = 4.041x+4.158
y = 2.141x+2.195
0.99
0.94
1.62±0.20
20.40±1.59
0.99 41.84±10.27
II-10
II-12
III-4
III-6
III-8
III-10
III-12
y = 2.266x+2.543
y = 5.262x+3.226
y = 0.635x+3.859
y = 1.103x+3.216
y = 1.080x+3.517
y = 4.752x+0.856
y = 6.198x+2.626
0.97
0.95
0.99
1.00
0.95
0.96
0.95
12.14±1.54
2.17±0.12
62.62±9.63
41.50±6.31
23.58±3.48
7.45±0.31
2.42±0.12
y = 3.597x+3.270
y = 10.229x+4.856
/
0.94
0.91
/
3.03±0.25
1.03±0.03
>80
y = 3.200x+2.804
y = 13.373x+2.058
y = 2.604x+0.727
y = 4.809x-2.411
y = 19.887x-21.261
y = 12.733x-6.670
y = 18.598x-2.675
0.98
0.99
0.99
0.94
1.00
0.91
1.00
4.86±1.19
1.66±0.23
43.75±3.67
34.76±4.20
20.92±2.83
8.25±0.09
2.59±0.29
/
/
>80
y = 5.993x-4.390
y = 3.851x+3.043
y = 10.627x+4.939
0.96
0.95
0.95
36.89±4.86
3.22±0.33
1.01±0.15
I-8Br
II-8Br
/
/
/
/
>80
>80
/
/
/
/
>80
>80
/
/
/
/
>80
>80
III-8Br
/
/
>80
/
/
/
>80
/
/
/
>80
CK1
CK2
y = 1.499x+2.052
/
0.98
/
92.61+2.15
/
/
/
/
/
y = 1.031x+2.942
0.99 216.70±5.12
y = 2.153x+0.938
0.98
77.04±1.96
previous work.8 Among them, EC50 values of II-10, II-12, III-10,
and III-12 range from 1.01 ~ 12.14 µg/mL against Xoo, R.
solanacearum, and Xac, which can be promising bactericides
against plant bacterial diseases. It is also clear that the
bioactivities of sulfoxide/sulfone derivatives are improved with
the increase in the alkyl length. In comparison of the bioactivities
of II-8 and II-8Br, III-8 and III-8Br, the antibacterial activities
are also dramatically enhanced after introducing the pyridinium
group into 2,5-substituted-1,3,4-oxadiazole sulfoxide/sulfone,
further suggesting that appreciable bioactivity can be achieved
through introducing pyridinium group into heterocyclic systems.
In summary, a variety of novel pyridinium-tailored 2,5-
substituted-1,3,4-oxadiazole
thioether/sulfoxide/sulfone
derivatives were obtained via embedding the pyridinium group
into 2,5-substituted-1,3,4-oxadiazole, and their antibacterial
activities were examined by the turbidimeter test against
pathogen Xoo, R. solanacearum, and Xac in vitro, respectively.
The results reveal that most of the target compounds exhibit
better inhibition activities against these three bacteria than
bismerthiazol (CK1) or thiodiazole copper (CK2). Furthermore,
I-8, I-10, I-12, II-10, II-12, III-10, and III-12 exert excellent
inhibition activities against the three pathogenic bacteria with
EC50 values ranging from 0.54 ~ 12.14 µg/mL. The structure-
activity relationships reveal that the bioactivities of the target
compounds are enhanced with the increase in the alkyl length,
and the bioactivities of thioether derivatives are better than that
of sulfoxide/sulfone derivatives. Given the above results,
pyridinium-tailored 1,3,4-oxadiazole thioether/sulfoxide/sulfone
derivatives can serve as potential bactericides against plant
bacterial diseases.
Figure 2. The comparison of EC50 of I-n, II-n, III-n against (a) Xoo, (b) R.
solanacearum, and (c) Xac in vitro.
Acknowledgments
We acknowledge the financial support of the National Natural
Science Foundation of China (21372052, 21202025), Key
Technologies R&D Program (2014BAD23B01), the Research
Project of Chinese Ministry of Education (213033A,
20135201110005), Guizhou Province S&T Program ([2012]6012
Meanwhile, the bioactivity of sulfoxide derivatives against R.
solanacearum and Xac are better than that of sulfone derivatives.
However, the bioactivities of sulfoxide/sulfone derivatives
against the three kinds of bacteria pathogens are still better than
those of the two positive controls (Table 1) and the
sulfoxide/sulfone derivatives without pyridinium tailor in our
3

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4

Graphical Abstract
Synthesis and antibacterial activity of
pyridinium-tailored 2,5-substituted-1,3,4-
oxadiazole thioether/sulfoxide/sulfone
derivatives
Leave this area blank for abstract info.
Pei-Yi Wang†, Lei Zhou†, Jian Zhou, Zhi-Bing Wu, Wei Xue, Bao-An Song*, Song Yang*
3