Inhibition of tobacco bacterial wilt with sulfone derivatives containing an 1,3,4-oxadiazole moiety

Article abstract of DOI:10.1021/jf203772d

A series of new sulfone compounds containing the 1,3,4-oxadiazole moiety were designed and synthesized. Their structures were identified by ¹H and ¹³C nuclear magnetic resonance and elemental analyses. Antibacterial bioassays indicated that most compounds exhibited promising in vitro antibacterial bioactivities against tobacco bacterial wilt at 200 μg/mL. The relationship between structure and antibacterial activity was also discussed. Among the title compounds, 5′c, 5′h, 5′i, and 5′j could inhibit mycelia growth of Ralstonia solanacearum in vitro by approximately 50% (EC₅₀) at 39.8, 60.3, 47.9, and 32.1 μg/mL, respectively. Among them, compound 5′j was identified as the most promising candidate due to its stronger effect than that of Kocide 3000 [Cu(OH)₂] within the same concentration range. Field trials demonstrated that the control effect of compound 5′j against tobacco bacterial wilt was better than that of the commercial bactericide Saisentong. For the first time, the present work demonstrated that sulfone derivatives containing 1,3,4-oxadiazole can be used to develop potential bactericides for plants.

Full text of DOI:10.1021/jf203772d

Article
pubs.acs.org/JAFC
Inhibition of Tobacco Bacterial Wilt with Sulfone Derivatives
Containing an 1,3,4-Oxadiazole Moiety
Wei-Ming Xu,† Fei-Fei Han,† Ming He, De-Yu Hu, Jiang He, Song Yang,*,‡ and 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, Guizhou University, Huaxi District, Guiyang 550025, China
S
* Supporting Information
ABSTRACT: A series of new sulfone compounds containing the 1,3,4-oxadiazole moiety were designed and synthesized. Their
1
structures were identified by H and 13C nuclear magnetic resonance and elemental analyses. Antibacterial bioassays indicated
that most compounds exhibited promising in vitro antibacterial bioactivities against tobacco bacterial wilt at 200 μg/mL. The
relationship between structure and antibacterial activity was also discussed. Among the title compounds, 5′c, 5′h, 5′i, and 5′j
could inhibit mycelia growth of Ralstonia solanacearum in vitro by approximately 50% (EC50) at 39.8, 60.3, 47.9, and 32.1 μg/mL,
respectively. Among them, compound 5′j was identified as the most promising candidate due to its stronger effect than that of
Kocide 3000 [Cu(OH)2] within the same concentration range. Field trials demonstrated that the control effect of compound 5′j
against tobacco bacterial wilt was better than that of the commercial bactericide Saisentong. For the first time, the present work
demonstrated that sulfone derivatives containing 1,3,4-oxadiazole can be used to develop potential bactericides for plants.
KEYWORDS: antibacterial activity, sulfone derivatives, 1, 3, 4-oxadiazole moiety, tobacco bacterial wilt
yl)-1,3,4-oxadiazole] and IIa [2-(benzylsulfinyl)-5-(3,4,5-trime-
thoxyphenyl)-1,3,4-oxadiazole] possess high antifungal activ-
ities against 10 kinds of fungi [effective dose for 50% inhibition
(EC50) = 19.9−93.3 μg/mL]. Structure−activity relationship
(SAR) analyses have suggested that 2-(methylsulfonyl)-1,3,4-
oxadiazole moiety is crucial for the potent antifungal activities.
Controlling of plant bacterial diseases has long been a
challenging mission in the agricultural sector. The constantly
increasing threats caused by tobacco bacterial wilt (Ralstonia
solanacearum), tomato bacterial wilt (Pseudomonas solanacea-
rum), and Xanthomonas oryzae have become a matter of great
concern throughout the world. The application of traditional
pesticides has not proved very effective, and at the same time
high residue level or negative impact on the environment was
caused. Copper formulation, a commercial bactericide, is a type
of inorganic pesticide which can enhance resistance in host
tobacco and tomato plant. Despite being useful in the
treatment of plants affected by tobacco bacterial wilt, the use
of copper formulation for field trial is largely limited due to its
phytotoxicity, strong alkali, and low mobility. Therefore, the
search for new antibacterial agents still remains a daunting task
in pesticide science.18
INTRODUCTION
■
Sulfone derivatives are an important class of bioactive
compounds that have a wide spectrum of activities. These
compounds have varied activities as antibacterial,1 insecticidal,2
antifungal,3 herbicidal,4 antihepatitis,5 antitumor,6 anti-inflam-
matory,7 anticancer,8 anti-HIV-1,9 and antitubercular agents.10
There is evidence that the key feature of these compounds is a
five- or six-membered heterocycle attached to the sulfone.11
Modifications on these heterocycles have been performed. For
example, 2-((4-chlorobenzyl)sulfonyl)-5-(methylsulfonyl)-
1,3,4-thiadiazole has been prepared by Joachim et al.12 This
compound exhibits good inhibitory activity against Plasmopara
viticola at 10 mg/kg concentration. On the other hand, 2,4-
dibromo-5-methyl-1-((2-methyl-5-nitrophenyl)sulfonyl)-1H-
imidazole has been reported by Assmann et al.13 This
compound exhibits strong activity against Phytophthora infestans
and P. viticola at 50 g/ha dosage. Moreover, 2-(5-ethyl-1-
methyl-1H-pyrazol-3-yl)-5-(methylsulfonyl)-1,3,4-oxadiazole
has been prepared by Yuan et al.14 This compound exhibits
medium inhibitory activity against Phoma asparagi. Tthe first
commercialized fungicide of this structure type was Oxy-
carboxin, developed by the U.S. Uniroyal Co. in 1966. In the
past few decades, a large number of other fungicides containing
sulfone (e.g., Tolylfluanid, Dichlofluanid, Cyazofamid, Amisul-
brom, and Oryzaemate) with potent bioactivities have been
introduced in the market.15 With growing applications
mentioned above, the research on the synthesis and bioactivity
of sulfone derivatives have attracted extensive attentions from
chemists and biologists in recent years. In our previous work we
had demonstrated antifungal activities of a series of novel
sulfone derivatives of 2-sulfonyl-5-(3,4,5-trimethoxyphenyl)-
1,3,4-oxadiazole (I)16 and 2-sulfinyl-5-(3,4,5-trimethoxyphen-
yl)-1,3,4-oxadiazole (II).17 Further in vitro bioassays have
disclosed that Ia [2-(methylsulfonyl)-5-(3,4,5-trimethoxyphen-
To aid the development of high activity, proper alkali, high
mobility, and readily available plant bacteria inhibitors, we
developed herein a series of new 2-(methyl/ethylsulfonyl)-
1,3,4-oxadiazole sulfone derivatives and evaluated them for
their antibacterial activities. The title compounds displayed
different antibacterial activities at 200 μg/mL; 5′c, 5′h, 5′i, and
5′j could inhibit mycelia growth of R. solanacearum in vitro by
Received: September 16, 2011
Revised: December 28, 2011
Accepted: December 29, 2011
Published: December 29, 2011
© 2011 American Chemical Society
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Scheme 1. Synthetic Route of 5a−5′j
2913, 1605, 1557, 1357, 1153. Anal. Calcd for C10H9FN2O3S: C,
46.87; H, 3.54; N, 10.93. Found: C, 46.62; H, 3.81; N, 10.59.
approximately 50% (EC50) at 39.8, 60.3, 47.9, and 32.1 μg/mL.
The field trials demonstrated that the control effect of
compound 5′j was better than that of the commercial
bactericide Saisentong, a kind of organocopper formulation.
The present work demonstrates that sulfone derivatives
containing 1,3,4-oxadiazole can be used to develop potential
agrochemicals. To the best of our knowledge, this is the first
report on the antibacterial activity of 2-(methyl/ethyl sulfonyl)-
1,3,4-oxadiazole derivatives.
In Vitro Antibacterial Bioassay. The antibacterial activities of
some of the synthesized compounds were tested against tobacco
bacterial wilt by the turbidimeter test.20 The compounds were tested at
concentrations of 500 and 200 μg/mL. Acetone in sterile distilled
water served as the blank control, whereas Kocide 3000 was the
positive control. About 40 μL of solvent NB (3 g of beef extract, 5 g of
peptone, 1 g of yeast powder, 10 g of glucose, 18 g of agar, 1 L of
distilled water, pH 7.0−7.2) containing tobacco bacterial wilt was
added to 5 mL of solvent NA (3 g of beef extract, 5 g of peptone, 1 g
of yeast powder, 10 g of glucose, 1 L of distilled water, pH 7.0−7.2)
containing the test compounds and Kocide 3000. The inoculated test
tubes were incubated at 30 1 °C and continuously shaken at 180
rpm for 2 d. The growth of the cultures was monitored on a
spectrophotometer by measuring the optical density at 600 nm
MATERIALS AND METHODS
■
1
Instruments. H and 13C NMR (solvent CDCl3 or acetone-d6 or
DMSO-d6) spectral analyses were performed on a JEOL-ECX 500
NMR spectrometer at room temperature using TMS as an internal
standard. The IR spectra were obtained from a KBr pellet using a
SHIMADZU-IR Prestige-21 spectrometer. The melting points were
determined on an XT-4 digital microscope (Beijing Tech Instrument
Co.) Analytical thin-layer chromatography was performed on silica gel
GF254 (400 mesh). Column chromatographic operations were
performed on silica gel (100−200 or 200−300 mesh, Qingdao
Haiyang Chemical Co.). A Sephadex LH-20 column chromatographic
instrument (Beijing Huideyi Tech Instrument Co.) and an XAD-7HP
macroporous resin (Rohm and Haas Co.) were employed for the
extraction and purification of chemical composition. Tobacco seeds
(Yun 85) were kindly provided by Guizhou Institute of Tobacco.
Fertilized soil (Baltisches substrate, HAWITA BALTIC) was bought
from Hawita Gruppe GmbH for plant cultivation.
(OD600) given by turbiditycorrected values = ODbacterial wilt − ODno bacterial wilt
,
I (%) = (Ctur − Ttur)/Ctur × 100. Ctur represents the corrected turbidity
values of bacterial growth on untreated NA (blank control), and Ttur
means the corrected turbidity values of bacterial growth on treated
NA. I denotes the inhibition rate.
Some of the title compounds were tested against tobacco bacterial
wilt under different concentrations (200, 100, 50, 25, 12.5, and 0 μg/
mL). EC50 values were statistically estimated by probit analysis with
the probit package of the SPSS 17.0 software using a personal
computer. The average EC50 was computed from at least three
separate analyses for growth inhibition using the basic EC50 program
version SPSS 17.0.
In Vivo Antibacterial Activities Tests Under Greenhouse
Conditions. Seedling plates were covered with thus prepared soil.
Three to four uniform tobacco seeds were sown in each spot of the
seedling plate. The seedling plates were then placed in sterilized water
for floating cultivation at 25 °C. After 30 d, the tobacco plants (with
about three leaves) were carefully transplanted to pots containing 500
g of soil, one plant for each pot. After 10 d, the uniformly grown
tobacco plants will have four to six leaves and are ready to test. Four
compounds were used in this study, compounds 5′c, 5′j, commercially
available Cu(OH)2 (Kocide 3000, Dupont, 46.1% WDG), and
Saisentong. All compounds were dissolved in DMSO at 50 mg/mL
and then diluted with deionized water containing 0.1% Tween-20 to
the final concentration of 500 μg/mL. The Cu(OH)2 WDG was
diluted directly with deionized water to 500 μg/mL.
General Synthetic Procedures for Title Compounds. Com-
pounds 5a−5′j were synthesized according to known methods,19 as
shown in Scheme 1. The starting materials were substituted benzoic
acid, fatty acids, and heterocyclic acid. 2-Thiol-5-substituted-1,3,4-
oxadiazole was synthesized in three steps via esterification,
hydrazidation, and cyclization. The oxadiazole analogs were then
converted into their corresponding thioether derivatives 4 by
thioetherification. Finally, the target sulfones 5 were obtained via
1
oxidation of the thioether. The physical characteristics, IR, H NMR,
13C NMR, and elemental analysis data for all the synthesized
compounds are reported in the experimental protocols. The data for
5′j is shown below, while data for the others can be found in the
Supporting Information.
TTC media was inoculated with tobacco bacterial wilt and cultured
for 24−48 h. Single colonies with strong virulence were then picked
out and transferred into 100 mL of NB culture medium and were then
incubated for 24 h, at 30 °C, 180 rpm to give the bacterial stock
suspension. Then 10 mL of stock suspension was added into 1 L of
NB culture media and was cultured for a further 24 h to give the
2-(Ethylsulfonyl)-5-(4-fluorophenyl)-1,3,4-oxadiazole (5′j).
Yield 80.3% of a white solid; mp 127−129 °C; 1H NMR (500
MHz, CDCl3) δ 8.15−7.75 (m, 4H, ArH), 3.53(q, J = 7.45, 2H, CH2),
1.47 (t, J = 7.45, 3H, CH3); 13C NMR (125 MHz, CDCl3) δ 167.4,
163.3, 135.3, 130.6, 131.3, 129.6, 50.3, 6.7; IR (KBr, cm−1) ν 3038,
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a
Table 1. Inhibition Effect and Toxicity of the Testing Compounds (500, 200 μg/mL) against Tobacco Bacterial Wilt
inhibition (%)
compd
500 μg/mL
20.1 0.8c
200 μg/mL
toxic regression equation
r
EC50(μg/mL)
5a
0.0 0.3 g
17.0 0.7 cd
6.0 2.4efg
2.8 0.3f
5′a
46.0 0.4a
10.1 1.0de
9.3 0.4e
5b
5′b
5′c
100.0 2. 9a
12.0 1.1de
52.0 1.5a
31.0 1.5b
31.4 0.3ab
14.2 0d
100.0 2.7a
11.1 1.3def
46.7 0.8b
19.2 0.5c
18.7 2.1cde
9.9 0.4e
y = 3.1363x − 0.0223
0.8883
39.8 4.9a
5d
5′d
5e
5′e
5f
5′g
39.0 0.4ab
100.0 2.6a
100.0 3.1a
100.0 12.2abcd
100.0 10.0a
15.0 0.3d
92.3 4.4a
100.0 1.9ag
100.0 4.9a
100.0 8.7ab
100.0 5.7a
5′h
y = 3.8065x − 1.7789
y = 1.4616x + 2.5443
y = 1.8065x + 2.2789
0.9912
0.9583
0.8843
60.3 2.8c
47.9 3.3b
32.1 2.3c
5′i
5′j
Saisentong
Kocide 3000
Cu(OH)2
y = 4.8739x − 3.1000
0.9792
45.9 6.6bc
a
The statistic analysis was conducted by ANOVA method at the condition of equal variances assumed (p > 0.05) and equal variances not assumed (p
< 0.05). The different lowercase letters indicate the values of inhibition and EC50 with significantly difference among different treatment groups at p
< 0.05.
bacteria working suspension ready for infection.21,22 Testing
RESULTS AND DISCUSSION
■
compound solution was poured into the tobacco root soil, 40 mL
The syntheses of 5a−5′j are shown in Scheme 1, the electron-
for each root. After 24 h, the tobacco roots were intentionally damaged
rich (methyl/ethyl)thio moiety 4 can be oxidized to sulfone by
by cutting two or three times with knives. Then the prepared bacteria
a variety of agents, such as m-CPBA27 and H2O2.28 However,
working suspension was poured into the root soil, 20 mL for each root.
the C−S bond between methyl/ethyl and sulfonyl can be easily
Deionized water without compound or bacteria was set as blank
broken. In the present experiments, the methyl/ethylthio
control, while bacteria treatment without compound treatment was set
moiety 4 was oxidized with potassium permanganate to yield
the corresponding methyl/ethylsulfonyl 5. The oxidation
reaction was very fast and completed in 10 min at room
as negative control.23,24 Three replicates were carried out for each
treatment. The disease infection rates of the tobacco plants were
investigated at 7, 14, and 21 days and then the plants were treated with
testing compounds solutions. The plant disease infection rate,
temperature. The C−S bond between methyl/ethyl and
sulfonyl could break if the reaction time was prolonged. As
infection index, etc. were recorded according to the “Protocols of
1
Disease Investigation and Classification”.25
indicated by H NMR, the methyl absportion peaks shown at
3.5−2.8 ppm disappeared, in favor of the broad peaks between
9.5 and 11 ppm.
Field Trials of 5′j against Tobacco Bacterial Wilt. Field trials of
5′j against tobacco bacterial wilt were conducted in Guizhou Province,
China, in May 2011. The effect of the natural infection of R.
solanacearum was studied in a field with tobacco having suffered
tobacco bacterial wilt for 1 year. Sterile distilled water served as the
control, whereas the commercial bactericide Saisentong (C5H4N6S4Cu,
5,5′-(methylenediimino)bis[1,3,4-thiadiazole-2(3H)-thione] copper
salt; China Zhejiang Dongfeng Chem. Ind. Co., Ltd.) was the positive
control. The experimental design area of the plot was 30 m2. The field
was flooded twice, and then compounds were sprayed on the foliage of
the tobaccos. For each treatment, three replicates were conducted. The
tobacco bacterial wilt disease index system was previously introduced
as follows: 0 = healthy, 1 = one leaf wilting, 2 = two or three leaves
wilting, 3 = two or three healthy leaves and all others wilting, and 4 =
all leaves wilting. The tobacco bacterial wilt index values were
determined as follow: disease index = (4W + 3X + 2Y + Y)/4A, where
W, X, and Y are equal to the corresponding sum of the disease
classification of plants with symptoms after 7, 14, or 21 d. A is equal to
the sum of the total investigated plants. The inhibiting effects of the
field trials were calculated by the formula I (%) = (CK − PT1)/CK ×
100, where CK represents the disease index of fungi growth on an
untreated field, PT1 represents the disease index of fungi on a treated
field after treatment. I represents the inhibition rate.26 SPSS 17.0 was
used for the statistical analyses of the data of inhibition. ANOVA (least
significant difference method) was performed to analyze the
differences among the treatment groups.
In Vitro Antibacterial Activities of Oxadiazole
Sulfones. R. solanacearum infects over 50 families of plants,
including vegetables, ornamentals, crops, fruits, and woody
perennial plants. Host plants of global importance include
tomato, eggplant, pepper, tobacco, ginger, and groundnut. The
inhibition effects of oxadiazole methyl or ethyl sulfone
compounds on tobacco bacterial wilt were studied. The results
of the preliminary bioassays were compared with those of
DuPont Kocide 3000 fungicide/bactericide; as indicated in
Table 1, the compounds had low to satisfactory antibacterial
activities against the tested bacteria. It was found that
compounds 5′c, 5′h, 5′i, and 5′j exhibited significant inhibition
effects against tobacco bacterial wilt, showing a control efficacy
of almost 100% at 500 or 200 μg/mL, which was equal to that
of the most active fungicide Kocide 3000, with control efficacy
= 100% at 200 μg/mL. The variances among the different
substitutes on phenyl greatly affected the antibacterial activities
of the compounds against tobacco bacterial wilt. For example,
when R = 3-fluorophenyl or 4-fluorophenyl, there was an
apparent increase in antibacterial activity. On the other hand,
when R was introduced to the other substituted phenyls, the
corresponding compounds inhibited tobacco bacterial wilt by
0%−46.7% at 200 μg/mL. Further toxicity bioassays disclosed
that 5′c, 5′h, 5′i, and 5′j remarkably inhibited tobacco bacterial
1038
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a
Table 2. Inhibition Effect of Testing Compounds against Tobacco Bacterial Wilt under Greenhouse Conditions at 500 μg/mL
7 days after spraying
14 days after spraying
21 days after spraying
disease index
morbidity disease index
control
morbidity disease index
control
control
compd
5′c
(%)
(%)
efficiency (%)
(%)
(%)
efficiency (%)
morbidity (%)
(%)
efficiency (%)
60.0
13.3
6.7
60.0
13.3
6.7
34.6 2.2c
85.5 0.7b
92.7 1.6a
41.9 1.4c
100.0 0.0a
−
100.0
60.0
73.3
100.0
0.0
86.7
40.0
40.0
95.0
0.0
12.0 1.0c
59.3 2.0b
59.3 2.7b
3.4 0.5d
100.0 0.0a
−
100.0
100.0
100.0
100.0
0.0
91.7
61.7
78.3
100.0
0.0
8.3 1.5c
38.3 0.7b
21.7 0.5b
0.0 0.0d
100.0 0.0ad
−
5′j
Saisentong
Cu(OH)2
50.3
0.0
53.3
0.0
b
CK1
b
CK2
100.0
91.7
100.0
98.3
100.0
100.0
a
The statistical analysis was conducted by ANOVA method at the condition of equal variances assumed (p > 0.05) and equal variances not assumed
(p < 0.05). The different lowercase letters indicate the values of inhibition and EC50 with significantly difference among different treatment groups at
b
p < 0.05. CK1: blank control; CK2: negative control.
Table 3. Results of Field Trials of 5′j against Tobacco Bacterial Wilt
7 d after spraying
14 d after spraying
21 d after spraying
a
c
compd
dosage (g ai/ha)
index values
inhibition (%)
index values
inhibition (%)
index values
inhibition (%)
Saisentong
540
405
3.8
3.4
9.8
61.2 12.7a
65.3 8.0a
3.9
4.2
71.1 6.4a
68.8 8.7a
6.4
5.5
67.3 10.5a
71.9 11.2a
5′j
b
CK
13.5
19.6
a
b
c
ai means active ingredient. CK means untreated blank control. The statistical analysis was conducted by ANOVA method at the 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 significantly difference among different treatment group at p < 0.05.
wilt, exhibiting EC50 values of 39.8, 60.3, 47.9, and 32.1 μg/mL,
respectively. These compounds possessed good activities
against the tested bacteria and were even superior to the
commercial agent Kocide 3000 (EC50 = 45.9 μg/mL). This
finding suggested that 5′c, 5′h, 5′i, and 5′j may be promising
lead structures for the further discovery of new antibacterial
agents.
In Vivo Antibacterial Activities of Some Compounds
under Greenhouse Conditions. All CK2 (positive control)
were seriously infected in 1 week after the bacteria suspension
treatment, indicating that this R. solanacearum strain has strong
virulence. At the same time, CK1 (negative control) was not
infected throughout the whole month, indicating that tobacco
plants were healthy in the whole growth period and were not
infected by other pathogens.
It can be seen that the controlling effect of compounds 5′c
was 34.6%, which was lower than that of the positive control
Cu(OH)2, 41.9%. The other compounds 5′j and Saisentong
were much more potent against tobacco bacterial wilt with
efficacies of 85.5% and 92.7%, respectively, which were much
higher than that of Cu(OH)2. The controlling effects at 14 d
after the second treatment of compounds 5′c, 5′j, and
Saisentong were all prominent, while Cu(OH)2 was almost
inactive. It can be seen from Table 2 that at 21 d after the third
treatment, both tested compounds and Saisentong still
exhibited certain controlling effect, while Cu(OH)2 was totally
inactive. In general, among the tested compounds, the effects of
compounds 5′j and Saisentong were much better than the
others, and compound 5′j was persistently and stably effective
against tobacco bacterial wilt.
Field Trials of 5′j against Tobacco Bacterial Wilt. Field
trials of 5′j against tobacco bacterial wilt were carried out, and
the results are summarized in Table 3. The inhibition rates of
5′j at 405 g of active ingredient/ha WP dosage were 65.3%,
68.8%, and 71.9% after 7, 14, and 21 d, respectively. The effect
of 5′j was better than that of the commercial bactericide
Saisentong; furthermore, compound 5′j was safe for the
tobacco, similar to the commercial fungicide, and no visible
injury was observed on the aerial parts of the tobacco. The data
was statistically analyzed by ANOVA (least significant differ-
ence), and the results showed no significant differences (p >
0.05) between Saisentong and 5′j at 7 d, 14 d, and 21 d after
spraying. Comparing the same treatment at different time, the
results showed no differences between any 7 d, 14 d, and 21 d
after spraying (p > 0.05) for 5′j and Saisentong.
SAR Analysis of the Antibacterial Activities against
Tobacco Bacterial Wilt. As summarized in Table 1, some of
the compounds showed promising potency against tobacco
bacterial wilt. To examine the SAR, different substituent groups
were introduced into the 1,3,4-oxadiazole ring. When R was
fixed and R′ = methyl or ethyl, no significant activity changes
were observed. When R was phenyl or 3- or 4-fluoro
substituted phenyl, the corresponding target compounds had
excellent activities, and some title compounds showed good
activities comparable with that of Saisentong. On the basis of
the activity values indicated in Table 1, the relationships of the
antibacterial activities with the different phenyls (type, position,
and number of substituents) were deduced. Three main
conclusions were drawn. First, at the same substituted position
of the phenyl, the electron-withdrawing group was superior to
the electron-donating group in terms of antibacterial activity.
For example, inhibition of 5′d (R = 3,4-difluorophenyl) on
tobacco bacterial wilt was 52.0% and 46.7% at 500 μg/mL,
whereas 5′b (R = 3,4-dimethoxyphenyl) was only 9.3% and
2.8% under the same condition. EC50 of 5′j (R = 4-
fluorophenyl) on tobacco bacterial wilt was 32.1 μg/mL, and
compound 5′j showed 100% inhibition against tobacco bacterial
wilt at 500 μg/mL, changing the fluoro group into a H or CH3
atom, EC50 of 5′c (R = phenyl) was 39.9 μg/mL and inhibition
of 5′g (R = 4-methylphenyl) was 39.0% at 500 μg/mL. Second,
compared with the same substituents on phenyl, the
monosubstituents had higher activities than the polysubstitu-
ents. For example, inhibiton of 5′j (R = 4-fluorophenyl) on
tobacco bacterial wilt was 100% against tobacco bacterial wilt at
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500 μg/mL, whereas EC50 of 5′h (R = 2,4-difluorophenyl) was
60.3 μg/mL and 5e (R = 3,4-difluorophenyl) had 31.0%
efficacy at 500 μg/mL. Third and last, comparing compounds
with the same substituents on the phenyl, those with
substituents at the 3- or 4-position always had higher inhibition
rates for tobacco bacterial wilt. For example, EC50 of 5′i (R = 3-
fluorophenyl) was 47.9 μg/mL and EC50 of 5′j (4-
fluorophenyl) on tobacco bacterial wilt was 32.1 μg/mL.
In summary, a series of new sulfone compounds containing
the 1,3,4-oxadiazole moiety were designed and synthesized on
the basis of the lead compound Ia. The title compounds 5′c,
5′h, 5′i, and 5′j exhibited favorable activity against tobacco
bacterial wilts in vitro as well as in vivo compared to the
commercial fungicides/bactericides Kocide 3000 and Saisen-
tong. The antibacterial tests showed that when the electron
withdrawing group at the 3- or 4-position of phenyl was
attached to the 5-position of oxadiazole, the corresponding
compounds presented good antibacterial activities. The field
trials demonstrated that the control effect of compound 5′j was
better than that of the commercial bactericide Saisentong. To
our knowledge, this is the first report of sulfone compounds
containing 1,3,4-oxadiazole moieties with potent controlling
effect against tobacco bacterial wilt on tobacco plants. Further
field studies on the biological efficacies, crop safety, and
toxicities of these compounds as bactericide candidates need to
be conducted.
ASSOCIATED CONTENT
■
S
* Supporting Information
The characterizations of 5a−5′j. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Tel: +86(851)362-0521. Fax: +86(851)362-2211. E-mail:
songbaoan22@yahoo.com; fcc.syang@gzu.edu.cn.
■
Present Address
‡State Key Laboratory Breeding Base of Green Pesticide and
Agricultural Bioengineering, Key Laboratory of Green Pesticide
and Agricultural Bioengineering, Ministry of Education,
Guizhou University, Huaxi District, Guiyang 550025, China.
Author Contributions
†These authors have contributed equally to the manuscript.
Funding
We gratefully acknowledge the financial support of the National
Key Program for Basic Research (No. 2010CB 126105), Key
Technologies R&D Program (Nos. 2011BAE06B05), and
Special Fund for Agro-scientific Research in the Public Interest
(No.201203022)
ABRREVIATIONS USED
■
R. solanacearum, Ralstonia solanacearum; EC50, 50% effective
1
1
concentration; H NMR, H nuclear magnetic resonance; 13C
NMR, 13C nuclear magnetic resonance
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