Design, synthesis, and antibacterial activity against rice bacterial leaf blight and leaf streak of 2,5-substituted-1,3,4-oxadiazole/thiadiazole sulfone derivative

Article abstract of DOI:10.1016/j.bmcl.2014.02.060

A series of 2,5-substituted-1,3,4-oxadiazole/thiadiazole sulfone derivatives were synthesized and evaluated for their antibacterial activities against rice bacterial leaf blight and leaf streak caused by Xanthomonas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicolaby via the turbidimeter test in vitro. Antibacterial bioassay results indicated that most compounds demonstrated good inhibitory effect antibacterial bioactivities against rice bacterial leaf blight and leaf streak. Among the title compounds, compound 6c demonstrated the best inhibitory effect against rice bacterial leaf blight and leaf streak with half-maximal effective concentration (EC₅₀) values of 1.07 and 7.14 μg/mL, respectively, which were even better than those of commercial agents such as Bismerthiazol and Thiediazole Copper. In vivo antibacterial activities tests at greenhouse conditions demonstrated that the controlling effect of compounds 6c (43.5%) and 6g (42.4%) against rice bacterial leaf blight were better than those of Bismerthiazol (25.5%) and Thiediazole Copper (37.5%).

Full text of DOI:10.1016/j.bmcl.2014.02.060

Bioorganic & Medicinal Chemistry Letters 24 (2014) 1677–1680
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis, and antibacterial activity against rice bacterial leaf
blight and leaf streak of 2,5-substituted-1,3,4-oxadiazole/thiadiazole
sulfone derivative
Pei Li, Li Shi, Xia Yang, Lei Yang, Xue-Wen Chen, Fang Wu, Qing-Cai Shi, Wei-Ming Xu, Ming He,
⇑
De-Yu Hu, Bao-An Song
State Key Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key Laboratory of Green Pesticide and Agricultural Bioengineering,
Ministry of Education, Center for Research and Development of Fine Chemicals, Guizhou University, Guiyang 550025, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 2,5-substituted-1,3,4-oxadiazole/thiadiazole sulfone derivatives were synthesized and evalu-
ated for their antibacterial activities against rice bacterial leaf blight and leaf streak caused by Xanthomo-
nas oryzae pv. oryzae and Xanthomonas oryzae pv. oryzicolaby via the turbidimeter test in vitro.
Antibacterial bioassay results indicated that most compounds demonstrated good inhibitory effect anti-
bacterial bioactivities against rice bacterial leaf blight and leaf streak. Among the title compounds, com-
pound 6c demonstrated the best inhibitory effect against rice bacterial leaf blight and leaf streak with
Received 14 January 2014
Revised 18 February 2014
Accepted 21 February 2014
Available online 3 March 2014
Keywords:
Synthesis
Antibacterial activity
Leaf blight and leaf streak
1,3,4-Oxadiazole/thiadiazole
Sulfone
half-maximal effective concentration (EC50) values of 1.07 and 7.14 lg/mL, respectively, which were even
better than those of commercial agents such as Bismerthiazol and Thiediazole Copper. In vivo antibacte-
rial activities tests at greenhouse conditions demonstrated that the controlling effect of compounds 6c
(43.5%) and 6g (42.4%) against rice bacterial leaf blight were better than those of Bismerthiazol (25.5%)
and Thiediazole Copper (37.5%).
Ó 2014 Elsevier Ltd. All rights reserved.
Rice is one of the most important staple crops worldwide.
Unfortunately, its production is constrained by several bacterial
diseases. Rice bacterial leaf blight and leaf streak caused by patho-
gens such as Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas
oryzae pv. oryzicola (Xoc) are the most important and well-known
bacterial diseases of rice in rice-growing countries.1–4 The infection
caused by these two diseases at the maximum tillering stage re-
sults in blighting of leaves. A yield loss of at least 10% caused by
these two diseases has been reported in the past 30 years.5,6 Cur-
rently, only a few control methods are available for bacterial rice
diseases, including chemical and biological methods and the use
of resistant cultivars and lines.7 Therefore, searching for new anti-
bacterial agents remains a daunting task in pesticide science.
1,3,4-Oxadiazole/thiadiazole, a privileged structure, represents
a key motif in heterocyclic chemistry and is important in
medicinal chemistry because of its ability to exhibit a wide
range of pharmacological activities. Recently, derivatives of
1,3,4-oxadiazole/thiadiazole have been reported for their anti-
bacterial,8,9 antifungal,10–14 inflammatory,15 antianxiety,16 and
antitubercular activities.17 Meanwhile, sulfone derivatives are also
known to exhibit a wide spectrum of biological activities because
the sulfone group is an important core found in many biologically
active compounds with a wide range of biological activities
including antibacterial,8,9 antifungal,18 insecticidal,19 antiviral,20
herbicidal,21 anticancer,22 anti-HIV-1,23 antihepatitis,24 antitumor,25
and anti-inflammatory properties.26
In 2005, Guimaraes, and co-workers performed molecular mod-
eling and pharmacokinetic studies to demonstrate that the intro-
duction of a 1,3,4-oxadiazole ring to the inhibitors can change
their polarity, flexibility, and metabolic stability. Moreover, the
1,3,4-oxadiazole scaffold can also function as acceptors of hydro-
gen bonds formation, which enables its use as an isosteric
R1
O
O
Abbreviations: Xoo, Xanthomonas oryzae pv. oryzae; Xoc, Xanthomonas oryzae pv.
S
oryzicolaby; EC50,
half-maximal effective concentration; 1H NMR, 1H nuclear
R2
magnetic resonance; 13C NMR, 13C nuclear magnetic resonance; IR, infrared
spectroscopy; SAR, structure–activity relationship.
N
O
N
⇑
Corresponding author. Tel.: +86 (851)362 0521; fax: +86 (851)362 2211.
E-mail address: songbaoan22@yahoo.com (B.-A. Song).
Figure 1. Compounds that were reported against tobacco bacterial wilt and tomato
bacterial wilt.
http://dx.doi.org/10.1016/j.bmcl.2014.02.060
0960-894X/Ó 2014 Elsevier Ltd. All rights reserved.

1678
P. Li et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1677–1680
substituent for amide or ester groups.27 In addition, in our previous
study, we have reported a series of sulfone derivatives containing
the 1,3,4-oxadiazole moiety (Fig. 1), which showed potent antibac-
terial activities against tobacco bacterial wilt and tomato bacterial
wilt.8,9 Based on these findings, we aim to introduce a benzyl
fragment, which could enhance the flexibility of the molecular
backbone to combine with the receptor protein molecular of path-
ogenic bacteria, and a sulfone fragment to the 1,3,4-oxadiazole/
thiadiazole skeleton to build a novel family of bioactive molecules.
In this letter, we reported the synthesis and characterization of
several sulfone derivatives containing the 1,3,4-oxadiazole/thiadi-
azole moiety, and bioassay results demonstrated that several of the
title compounds exhibited the best inhibitory effect against rice
bacterial leaf blight and leaf streak caused by pathogens Xoo and
Xoc. In order to develop highly active and readily available bacteria
inhibitors, the structure–activity relationship (SAR) derived from
antibacterial activities of compounds was also discussed. To the
best of our knowledge, this is the first report of sulfone derivatives
containing the 1,3,4-oxadiazole/thiadiazole moiety with potent
controlling effect against rice bacterial leaf blight and leaf streak.
Compounds 6a to 60h were synthesized, as shown in Scheme 1,
based on previously described methods.8–10 Using phenylacetic
acid as the starting material, compounds 6a to 60h were
synthesized in five steps including esterification, hydrazidation,
cyclization, thioetherification and oxidation. The physical charac-
teristics, IR, 1H NMR, 13C NMR and elemental analysis data for all
synthesized compounds are reported in the Supplementary data,
and the representative data for 6c are shown below.
2-(Methyl sulfonyl)-5-(4-fluorobenzyl)-1,3,4-oxadiazole (6c):
white solid; mp 139–140 °C; yield 65.3%; 1H NMR (500 MHz,
DMSO-d6, ppm) d: 7.39–7.20 (m, 4H, Ar-H), 4.59 (s, 2H, Ar-CH2-),
3.59 (s, 3H, –CH3); 13C NMR (125 MHz, DMSO-d6, ppm) d: 175.52,
169.89, 136.32, 132.71, 131.47, 129.40, 43.49, 34.89; IR (KBr,
cmꢀ1) v: 3021, 2932, 1593, 1554, 1341, 1155; Anal. Calcd for C10H9-
FN2O3S: C, 46.85; H, 3.55; N, 10.95. Found: C, 46.87; H, 3.54; N,
10.93.
In this study, the inhibitory effect of the synthesized series of
2,5-substituted-1,3,4-oxadiazole/thiadiazole sulfone derivatives
was evaluated for their antibacterial activities in vitro against rice
bacterial leaf blight and leaf streak via the turbidimeter test.8,28 For
comparison, the activities of Bismerthiazol and Thiediazole Copper
were evaluated at the same conditions. The results of the prelimin-
ary bioassays, as indicated in Tables 1 and 2, showed that com-
pounds 6a, 6b, 6c, 6d, 6e, 6f, 6g, 6h and 60a exhibited significant
inhibition effects against rice bacterial leaf blight and leaf streak,
and a control efficacy of 100% was observed at 200 and 100 lg/
mL, which were even better than those of Bismerthiazol and Thie-
diazole Copper.
The half-maximal effective concentration (EC50) values of the
synthesized compounds as well as for Bismerthiazol and Thiediaz-
ole Copper were presented in Tables 1 and 2. Table 1 showed that
compounds 6a, 6b, 6c, 6d, 6e, 6f, 6g and 6h inhibited rice bacterial
leaf blight in vitro with the EC50 values of 2.93, 5.78, 1.07, 19.61,
12.23, 25.28, 1.96 and 4.72 lg/mL, respectively. Meanwhile, Table 2
indicated that compounds 6a, 6b, 6c, 6d, 6e, 6f, 6g and 6h inhibited
rice bacterial leaf streak with the EC50 values of 10.08, 16.62, 7.14,
13.37, 17.68, 33.88, 8.45 and 25.61 lg/mL, respectively. Among the
title compounds, 6c demonstrated the best inhibitory effect against
rice bacterial leaf blight and leaf streak with EC50 values of 1.07 and
7.14 lg/mL, respectively, which were even better than those of Bis-
merthiazol and Thiediazole Copper.
To determine the effect of antibacterial potency in vivo, the
antibacterial bioassay of compounds 6c and 6g against rice bacte-
rial leaf blight were performed via the leaf-cutting method29
at greenhouse conditions at a concentration of 200 lg/mL. Bis-
merthiazol and Thiediazole Copper, the most successful registered
bactericides for plant in China, were used as positive control sam-
ples. The results, as listed in Table 3, indicated that compounds 6c
and 6g have potent controlling effect against rice bacterial leaf
blight at 43.5% and 42.4%, respectively, which were better than
those of Bismerthiazol (25.5%) and Thiediazole Copper (37.5%).
The data were statistically analyzed via ANOVA (least significant
difference), and the results showed that no significant differences
(p > 0.05) exist between each of the commercial agents and com-
pounds 6c and 6g 15 days after spraying.
As an extension of this approach, the synthesis and structure–
activity relationships were deduced on the basis of the activity val-
ues in Tables 1 and 2. Three main conclusions were drawn. First,
the presence of the –H or –F groups at 4-position and the –Cl group
at 2,4-positions of benzyl in the corresponding compounds pre-
sented good antibacterial activities against rice bacterial leaf blight
and leaf streak. The activities of the compounds followed the order
6c > 6e, 6a > 6e, 6g > 6e and 60c > 60e, 60a > 60e, 60g > 60e (Tables 1
and 2). Second, compared with the same substituent on benzyl,
methyl was replaced with ethyl at the R2 substituent group caused
a decrease in the activity against rice bacterial leaf blight and leaf
streak in the order 6a > 6b, 6c > 6d and 60a > 60b, 60c > 60d (Tables 1
and 2). Third, compared with the same substituent on benzyl and
the R2 substituent group, the activities of 1,3,4-oxadiazole sulfone
derivatives were superior to those of 1,3,4-thiadiazole sulfone
Scheme 1. Synthetic route of 6a–6h and 60a–60h.

P. Li et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1677–1680
1679
Table 1
Inhibition effect of the compounds against rice bacterial leaf blight
No.
Compounds
R1
Inhibition (%)
200 ( g/mL)
Toxic regression equation
r
EC50 (lg/mL)
R2
l
100 (lg/mL)
6a
6b
6c
6d
6e
6f
6g
H
H
4-F
4-F
4-Cl
4-Cl
2,4-2Cl
2,4-2Cl
H
H
4-F
4-F
4-Cl
4-Cl
2,4-2Cl
2,4-2Cl
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
100 0.45
100 0.18
100 0.16
100 0.65
100 0.22
100 0.78
100 0.58
100 0.66
100 0.21
100 0.43
100 0.46
87 1.22
91 0.46
75 1.11
100 1.64
56 3.78
72 0.65
64 2.76
100 0.32
100 0.24
100 0.81
100 0.82
100 0.34
100 0.54
100 0.26
100 0.57
100 0.16
86 1.78
80 2.48
67 3.34
65 2.48
58 1.87
100 2.83
43 4.55
54 1.23
43 3.15
y = 1.558x + 4.410
y = 1.679x + 3.721
y = 1.409x + 4.958
y = 1.327x + 3.285
y = 2.045x + 2.776
y = 1.773x + 2.513
y = 1.593x + 4.535
y = 2.002x + 3.650
y = 1.733x + 2.196
y = 2.351x + 1.177
y = 1.708x + 2.270
y = 2.346x + 0.677
y = 2.470x + 0.468
y = 1.925x + 1.268
y = 2.225x + 1.518
y = 1.451x + 1.892
y = 1.499x + 2.052
y = 1.540x + 1.788
0.98
0.96
0.97
0.99
0.99
0.99
0.99
0.99
0.99
0.98
0.97
0.99
0.98
0.99
0.99
0.99
0.98
0.98
2.39 0.56
5.78 0.66
1.07 0.68
19.61 1.98
12.23 1.45
25.28 2.24
1.96 0.99
6h
4.72 1.10
60a
41.50 4.45
42.28 3.87
39.66 2.91
69.62 5.21
68.36 4.48
86.84 5.18
36.72 4.67
138.67 6.87
92.61 2.15
121.82 3.59
60b
60c
60d
60e
60f
60g
60h
–CH2CH3
Bismerthiazol
Thiediazole Copper
Table 2
Inhibition effect of the compounds against rice bacterial leaf streak
No.
Compounds
R1
Inhibition (%)
200 ( g/mL)
Toxic regression equation
r
EC50 (lg/mL)
R2
l
100 (lg/mL)
6a
6b
6c
6d
6e
6f
6g
H
H
4-F
4-F
4-Cl
4-Cl
2,4-2Cl
2,4-2Cl
H
H
4-F
4-F
4-Cl
4-Cl
2,4-2Cl
2,4-2Cl
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
–CH2CH3
–CH3
100 0.12
100 0.54
100 0.98
100 0.26
100 0.67
100 0.34
100 0.55
100 0.43
100 0.32
92 0.77
88 1.22
70 6.32
80 0.46
68 3.45
100 2.32
52 4.11
57 5.56
35 4.31
100 0.34
100 0.67
100 0.21
100 0.28
100 0.98
100 0.65
100 0.78
100 0.31
100 0.21
86 1.43
71 3.76
61 2.98
52 2.48
49 2.48
86 2.32
31 2.65
35 6.76
15 2.11
y = 1.750x + 3.244
y = 2.213x + 2.299
y = 2.107x + 3.201
y = 1.830x + 2.939
y = 1.818x + 2.732
y = 1.300x + 3.011
y = 1.837x + 3.297
y = 1.506x + 2.879
y = 1.429x + 2.838
y = 2.549x + 0.706
y = 2.222x + 1.059
y = 1.801x + 1.526
y = 2.076x + 0.972
y = 1.802x + 1.268
y = 2.293x + 1.419
y = 1.368x + 1.827
y = 1.349x + 2.058
y = 2.313x ꢀ 0.623
0.97
0.99
0.99
0.98
0.97
0.96
0.99
0.97
0.99
0.98
0.98
0.99
0.99
0.99
0.99
0.99
0.97
0.99
10.08 1.22
16.62 2.78
7.14 0.97
13.37 1.59
17.68 2.87
33.88 3.56
8.45 1.65
25.61 2.54
32.58 2.12
48.37 5.25
59.38 2.79
84.90 5.19
87.15 4.81
117.77 7.54
36.45 2.75
208.66 7.49
151.66 5.98
269.80 7.11
6h
60a
60b
60c
60d
60e
60f
60g
60h
–CH2CH3
Bismerthiazol
Thiediazole Copper
Table 3
Inhibition effect of testing compounds against rice bacterial leaf blight at greenhouse conditions at 200
lg/mL
Compounds
15 Days after spraying
Morbidity (%)
Disease index (%)
Control efficiencyc (%)
6c
6g
100
100
100
100
0.0
52.0
53.0
68.5
57.5
0.0
43.5 3.4c
42.4 2.1c
25.5 1.7b
37.5 3.3a
100.0 0.0c
/
Bismerthiazol
Thiediazole Copper
CK1a
CK2b
100.0
92.0
a
b
c
CK1: blank control sample.
CK2: negative control sample.
Statistical analysis was conducted via the ANOVA method at a condition of equal variances assumed (p > 0.05) and equal variances not assumed (p < 0.05). Different
lowercase letters indicate the values of inhibition and EC50 with significant difference among different treatment groups at p < 0.05.
derivatives and followed the order 6a > 60a and 6c > 60c (Tables 1
and 2).
In conclusion, we synthesized a series of 2,5-substituted-1,3,4-
oxadiazole/thiadiazole sulfone derivatives, evaluated their antibac-
terial activities against rice bacterial leaf blight and leaf streak via
the turbidimeter test in vitro, and assayed their controlling effect
against rice bacterial leaf blight based on the leaf-cutting method
at greenhouse conditions in vivo. Antibacterial bioassay results
indicated that most compounds demonstrated excellent inhibitory
effect antibacterial bioactivities against rice bacterial leaf blight
and leaf streak. The antibacterial tests showed that when the
presence of the –H or –F groups at 4-position and the –Cl group
at 2,4-positions of benzyl and methyl at the R2 substituent group
were attached to the 2,5-position of oxadiazole/thiadiazole, the

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P. Li et al. / Bioorg. Med. Chem. Lett. 24 (2014) 1677–1680
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In vivo antibacterial activities tests at greenhouse conditions dem-
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