Design and Synthesis of Novel 4-Hydroxyl-3-(2-phenoxyacetyl)-pyran-2-one Derivatives for Use as Herbicides and Evaluation of Their Mode of Action

Article abstract of DOI:10.1021/acs.jafc.9b03109

In order to develop a novel herbicide containing the β-triketone motif, a series of 4-hydroxyl-3-(2-phenoxyacetyl)-pyran-2-one derivatives were designed and synthesized. The bioassay results showed that compound II15 had good pre-emergent herbicidal activity even at a dosage of 187.5 g ha^-1. Moreover, compound II15 showed a broader spectrum of weed control when compared with a commercial herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), and displayed good crop safety to Triticum aestivum L. and Zea mays Linn. when applied at 375 g ha^-1 under pre-emergence conditions, which indicated its great potential as a herbicide. More importantly, studying the molecular mode of action of compound II15 revealed that the novel triketone structure is a proherbicide of its corresponding phenoxyacetic acid auxin herbicide, which has a herbicidal mechanism similar to that of 2,4-D. The present work indicates that the 4-hydroxyl-3-(2-phenoxyacetyl)-pyran-2-one motif may be a potential lead structure for further development of novel auxin-type herbicides.

Full text of DOI:10.1021/acs.jafc.9b03109

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Functional Structure/Activity Relationships
Design and synthesis of novel 4-hydroxyl-3-(2-phenoxyacetyl)-pyran-2-
one derivatives for use as herbicides and evaluation of their mode of action
Kang Lei, Pan Li, Xuefang Yang, Shiben Wang, Xuekun
Wang, Xuewen Hua, Bin Sun, Lusha Ji, and Xiaohua Xu
J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.9b03109 • Publication Date (Web): 27 Aug 2019
Downloaded from pubs.acs.org on August 27, 2019
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Design and synthesis of novel 4-hydroxyl-3-(2-phenoxyacetyl)-pyran-2-one
derivatives for use as herbicides and evaluation of their mode of action
Kang Lei,* a, b,† Pan Li,a, † Xue-Fang Yang,c Shi-Ben Wang,a Xue-Kun Wang,a
Xue-Wen Hua,a, b Bin Sun,a, b Lu-Sha Ji,a Xiao-Hua Xu* b
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a School of Pharmacy, Liaocheng University, Liaocheng, China
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b State Key Laboratory of Elemento-Organic Chemistry, Institute of Elemento-Organic Chemistry,
Nankai University, Tianjin, China
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c College of agriculture, Shanxi Agricultural University, Shanxi, China
†
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Authors contributed equally to this work
*
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Correspondence to: Kang Lei, E-mail: leikang@lcu.edu.cn; Xiao-Hua Xu, E-mail:
xiaohuaxu@nankai.edu.cn.
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ABSTRACT
In order to develop a novel herbicide containing the β-triketone motif, a series of
4-hydroxyl-3-(2-phenoxyacetyl)-pyran-2-one derivatives were designed and
synthesized. Bioassay results showed that compound II15 had good pre-emergent
herbicidal activity even at a dosage of 187.5 g ha-1. Moreover, compound II15
showed a broader spectrum of weed control when compared with a commercial
herbicide 2,4-dichlorophenoxyacetic acid (2,4-D), and displayed good crop safety to
Triticum aestivum L. and Zea mays Linn. when applied at 375 g ha-1 under
pre-emergence conditions, which indicated its great potential as a herbicide. More
importantly, studying the molecular mode of action of compound II15 revealed that
the novel triketone structure is a proherbicide of its corresponding phenoxyacetic acid
auxin herbicide, which has a herbicidal mechanism similar to that of 2,4-D. The
present work indicates that the 4-hydroxyl-3-(2-phenoxyacetyl)-pyran-2-one motif
may be a potential lead structure for further development of novel auxin-type
herbicides.
Keywords: 4-hydroxyl-3-(2-phenoxyacetyl)-pyran-2-one, β-triketone, synthesis,
herbicidal activity, auxin herbicide
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INTRODUCTION
β-Triketones are known as potential agrochemicals because they exhibit several
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8, 9
biological activities, including herbicidal 1-6, antibacterial , antifungal
and
insecticidal activity 10. Among the various β-triketones reported to date, pyran-based
diketone lactones have been extensively investigated, and many pyran-based diketone
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lactones (Fig. 1) are reported owing to their effective role as antiviral agents
,
free-radical scavengers 12, and acidichromic colorants 13, 14. However, relatively little
is known about the application of pyran-based diketone lactones in pesticide
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formulation
.
In
our
previous
work,
it
was
shown
that
3-benzoyl-4-hydroxylcoumarin derivatives (Fig. 1), which belong to the family of
pyran-based diketone lactones, display herbicidal activity and fungicidal activity 16, 17
.
Therefore, this encouraged us to continue to explore the biological activity of
pyran-based diketone lactones in pesticide.
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Figure 1. Structures of various pyran-based diketone lactones.
Phenoxyacetic acid and its derivatives are key intermediates in the synthesis of
biologically active compounds. Over the last few decades, many
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phenoxyacetyl-containing derivatives have been successfully synthesized, and these
18-22
derivatives display a wide range of biological activity in drug chemistry
.
Moreover, phenoxyacetic acid and its derivatives have also been reported to exhibit
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herbicidal 23, insecticidal , and fungicidal activity 25. Based on the above-mentioned
facts, we envisaged that introducing phenoxyacetyl at the 3rd-position of
4-hydroxylcoumarin and 4-hydroxy-6-methyl-2H-pyran-2-one can help construct a
variety of 4-hydroxyl-3-(2-phenoxyacetyl)-pyran-2-one derivatives (Fig. 2,
compounds I and II), which should possess some interesting biological activity.
Therefore, as a continuation of our work on synthesizing and evaluating the biological
activity of pyran-based diketone lactones in pesticide formulation, 21
3-(2-phenoxyacetyl)-4-hydroxylcoumarin derivatives (Fig. 2, compound I) and 21
4-hydroxy-6-methyl-3-(2-phenoxyacetyl)-2H-pyran-2-one derivatives (Fig. 2,
compound II) were synthesized, and their herbicidal activities were evaluated to
develop a novel herbicide containing β-triketone motif. To the best of our knowledge,
this
is
the
first
report
on
the
herbicidal
activity
of
4-hydroxyl-3-(2-phenoxyacetyl)-pyran-2-one derivatives.
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Figure 2. Design strategy used to prepare the target compounds.
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MATERIALS AND METHODS
Chemical synthesis procedures
The synthetic pathway used to prepare the target compounds is outlined in Scheme
1. The yields were not optimized.
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Scheme 1. Synthetic route of preparing target compounds I1-I21 and II1-II21.
General procedure for the synthesis of aryloxyacetyl chloride derivatives 4
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The key intermediate aryloxyacetyl chloride derivatives 4 were synthesized
following a reported procedure 5. The appropriate phenol 1 (20 mmol) and potassium
carbonate (40 mmol) were added successively to N, N-dimethylformamide (100 mL)
in a 250 mL flask with stirring, and the mixture was heated up to 50 °C for 1 h.
Methyl chloroacetate (20 mmol) was added to the mixture and the suspension was
stirred at 50 °C for 12 h. Then, the mixture was cooled to room temperature, poured
into water, and stirred for another 0.5 h. To the solution was added 200 mL of ethyl
acetate, the ethyl acetate layer was separated and concentrated by rotary evaporation.
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Methanol (50 mL) and water (5 mL) were added to the resulting residue, and then
lithium hydroxide (100 mmol) was added to the solution with stirring. The solution
was heated up to 50 °C for 12 h. After completion of the reaction, 30 mL of methanol
was removed by rotary evaporation. The resulting solution was cooled to room
temperature and acidified by hydrochloric acid solution (6 M) to pH = 1. The
resulting solid was collected by filtration, washed with water, and dried in a vacuum
to afford aryloxyacetic acids 3. Subsequently, aryloxyacetic acids 3 (10 mmol) were
dissolved in dichloromethane (20 mL) in a 50 mL flask, oxalyl chloride (20 mmol)
and N, N-dimethylformamide (one drop) were added. The mixture was stirred at room
temperature for 12 h. After completion of the reaction, dichloromethane was removed
by rotary evaporation to provide aryloxyacetyl chloride derivatives 4, which used in
the next step without purification.
General procedure for the synthesis of 3-(2-phenoxyacetyl)-4-hydroxylpyran-2-one
derivatives (compound I1-I21 and compound II1-II21)
3-(2-phenoxyacetyl)-4-hydroxylpyran-2-one derivatives were prepared following a
16, 17,
reported method
26. 4-Hydroxylcoumarin 5 (5 mmol) and 20 mL of
dichloromethane were added to a 100 mL flask. Aryloxyacetyl chloride derivatives 4
was added to the solution at 0 °C, the solution was stirred for another 30 min, then,
triethylamine (10 mmol) and 4-dimethylaminopyridine (0.5 mmol) were added
successively to the mixture, and the solution was stirred for 12 h. After completion of
the reaction, aqueous hydrochloric acid solution (1 M, 20 mL) was added. The
dichloromethane layer was separated and washed with H2O, saturated sodium chloride
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solution, dried by anhydrous sodium sulfate, concentrated by rotary evaporation, and
the residue was scratched from ethyl acetate and petroleum ether (1/1 by volume) to
give desired product 6, which used in the next step without further purification.
A solution of compound 6 (2 mmol) in dichloromethane (20 mL) was added to
triethylamine (3 mmol), 18-crown-6 (0.2 mmol) and potassium cyanide (1 mmol).
The mixture was stirred at room temperature for 72 h. The mixture was poured into
water and extracted with dichloromethane. The combined organic phase was dried
with anhydrous sodium sulfate, filtered and removed by rotary evaporation. The
residue was scratched from ethyl acetate and ethanol (1/1 by volume) to give target
compounds I1-I21.
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Compounds II1-II21 were synthesized by the similar procedure to compounds
I1-I21. The data of 1H NMR, 13C NMR, and HRMS of all target compounds is given
in the Supporting Information.
X-ray diffraction of compounds I9 and II14
Compound I9 was recrystallized from a mixture of dichloromethane and methanol
(1/1 by volume) to afford a suitable single crystal. Compound II14 was recrystallized
from dichloromethane to afford a suitable single crystal. Crystallographic data for
compounds I9 and II14 had been deposited with the Cambridge Crystallographic Data
Centre as supplementary publications with the deposition numbers 1834640 and
1874641, respectively. The detail data can be obtained free of charge from
http://www.ccdc.cam.ac.uk/.
Herbicidal activity evaluation
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Based on the reported procedure 26-29, herbicidal activity was evaluated with three
replicates per treatment. Mesotrione and 2,4-D were selected as positive control. The
preliminary herbicidal activity of target compounds was determined with Brassica
campestris root test and Echinochloa crusgalli cup test. Further herbicidal activity
study of target compounds against four species representative of monocotyledonous
and dicotyledonous plants (Brassica campestris, Amaranthus retroflexus,
Echinochloa crusgalli, Digitaria sanguinalis) was performed in the greenhouse.
Compound II15 was selected to study the herbicidal spectra. The procedure is given
in the Supporting Information.
Crop selectivity
5
Based on the reported procedure , the crop selectivity of compound II15 was
evaluated with three replicates per treatment. Three representative crops, namely,
Triticum aestivum L., Zea mays Linn. and Gossypium spp, were selected for crop
selectivity studies in the greenhouse. The procedure is given in the Supporting
Information.
Phenotypic study of Arabidopsis thaliana
Arabidopsis thaliana employed in this study is in the Columbia (Col-0) background.
Surface-sterilized seeds sown onto Murashige and Skoog (MS) plates at 4°C for 3
days under the darkness to favor vernalization and then grown at 22 °C for 2 days
under LD (16h light/8h darkness). Subsequently, plants were transferred to 1/2MS
plates supplemented with compound II15 (from 1 µM to 1.0 nM), 2,4-D (from 1 µM
to 1.0 nM), 2-(2-chloro-4-fluorophenoxy) acetic acid (from 1 µM to 0.1 µM),
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4-hydroxy-6-methyl-pyran-2-one (1 µM) and mesotrione (1 µM), respectively. The
solution of compound II15, 2,4-D, 2-(2-chloro-4-fluorophenoxy) acetic acid,
4-hydroxy-6-methyl-pyran-2-one and mesotrione was prepared in 0.1% MeOH and
diluted in the 1/2MS to obtain the tested concentrations, respectively. Solvent (0.1%
MeOH) was added also to control plates. Plants were grown on vertically oriented
plates at 22 °C for 7 days under LD (16h light/8h darkness) for phenotypic
investigation and root length measurement.
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Quantitative real-time-PCR
Surface-sterilized seeds were sown onto MS plates. The plant was grown on plates
for two weeks from day of sowing under LD (16h light/8h darkness) at 22 °C.
Subsequently, the plant was carefully placed in a petri dish with filter paper covered.
The solution of compound II15 (10 μM), 2,4-D (10 μM), and mesotrione (10 μM) was
added to the petri dish, respectively. Control plants were treated with distilled water.
The leaves of seedlings were collected for RNA extraction after 0 h, 12 h, and 24 h,
respectively. For quantitative real-time PCR (qRT-PCR), total RNAs were extracted
from plant tissues using Trizol reagent (TaKaRa, Japan) according to the
manufacturer’s instructions. Reverse transcription reactions were performed with the
PrimeScript RT reagent kit with gDNA Eraser (Takara, Japan). qRT-PCR reactions
were performed on real-time thermal cycling system (Bio-Rad, USA), and the
SYBR-Green Ex Taq II kit (TaKaRa, Japan) was used for detecting gene expression
abundances. The gene expression levels were normalized with the reference gene
ACTIN-2. Primers used for PCR are given in the Supporting Information (Table S1).
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The degradation study of compound II15 in Arabidopsis thaliana
Surface-sterilized seeds were sown onto MS plates. Plants were grown on plates at
22 °C for 14 days under LD (16 h light/8 h dark). Subsequently, plants were carefully
placed in a petri dish with filter paper covered. The solution of compound II15 (50
μM) and 2-(2-chloro-4-fluorophenoxy) acetic acid (50 μM) was added to the petri dish,
respectively. After 15 min of incubation, the treated plants were transferred to the MS
plates. The leaves of seedlings were collected for extraction after 0 h, 1 h, 1.5 h, 2 h,
and 2.5 h, respectively. Methanol was used as extraction fluid. The ingredients of
extraction were analyzed by HPLC on a Shimadzu HPLC system using a 5 μm C18
column (150 × 4.6 mm; Agilent Zorbax) and 3-indolebutyric acid (IBA) was used as
the internal standard. A linear gradient with increasing methanol (solvent A) against
double-distilled H2O (solvent B) at a flow rate of 1 mL min–1 over 35 min was used.
Both solutions contained 0.1 % phosphoric acid. HPLC conditions were as follows: 0
min, 30 % A; 20 min, 70 % A; 22 min, 30 % A; 35 min, stop. The results are
presented at 230 nm.
Statistical analysis
The values shown in each table are mean values ±SD of at least three repeated
experiments. SPSS 22.0 (SPSS, Chicago, IL) was used as the statistical software
program.
RESULTS AND DISCUSSION
Chemistry
As depicted in Scheme 1, the target compounds can be prepared via a five-step
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synthetic route using several phenols as the starting material. In the presence of
potassium carbonate as base, phenols 1 reacted with methyl chloroacetate, and the
corresponding methyl aryloxyacetates 2 were produced. Hydrolysis of methyl
aryloxyacetates 2 with lithium hydroxide as a base gave aryloxyacetyl acids 3.
Subsequently, aryloxyacetyl acids 3 were transformed into their corresponding
aryloxyacetyl chloride derivatives 4 using oxalyl chloride. Reacting compound 5 with
the aryloxyacetyl chloride derivatives 4 in the presence of 4-dimethylaminopyridine
and trimethylamine gave intermediate 6, which was treated with potassium cyanide
and 18-crown-6 at room temperature to give target compounds I1-I21 in 41– 61%
yield.
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Compounds II1-II21 were synthesized using a procedure similar to that used for
compounds I1-I21. The structures of all the target compounds were identified using
1H and 13C NMR spectroscopy, and HRMS. Furthermore, the structures of compound
I9 and compound II14 were confirmed using X-ray diffraction analysis (CCDC
1834640 and 1874641; Fig. 3).
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Figure 3. X-ray crystal structures of compounds I9 (right) and II14 (left).
In vitro herbicidal activity
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The herbicidal activity of the target compounds was preliminarily determined using
a Brassica campestris root test and Echinochloa crusgalli cup test. Mesotrione and
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2,4-D were selected as positive control samples. As shown in Fig. 4A, the target
compounds, such as I2, I4, I12, I15-I17, I20, I21, II2, II4, II12, and II15-II19,
exhibited excellent herbicidal activity against the dicotyledonous plant Brassica
campestris with inhibition >90% even at a dosage of 10 μg mL−1. However, only
some of the compound II, such as II12 and II15-II20, exhibited >70% control against
Echinochloa crusgalli at a dosage of 100 μg mL−1 (Fig. 4B). These preliminary results
indicate that the target compounds have good herbicidal activity and exhibit better
herbicidal activity against dicotyledonous plants.
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Figure 4. Effects (% inhibition) of compounds I1-I21 and II1-II21 on mustard root
and barnyardgrass seedling growth; (A) dosage = 10 µg mL-1; (B) dosage = 100 µg
mL-1; The error bars represent the standard deviation (s. d.) from three biological
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replicates.
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Molecular mode of action of the target compounds
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In order to explore the molecular mode of action of target compounds, compound
II15 was selected to study the herbicidal mechanism with Arabidopsis thaliana as a
model plant. As shown in Fig. 5A, Arabidopsis thaliana treated with mesotrione at a
dosage of 1.0 µM developed bleaching symptoms, whereas Arabidopsis thaliana
treated
with
2,4-D,
2-(2-chloro-4-fluorophenoxy)
acetic
acid,
4-hydroxy-6-methyl-pyran-2-one or compound II15 did not develop any bleaching
symptoms, indicating these compounds have a herbicidal mechanism different to that
of mesotrione. Subsequently, the phenotypes of Arabidopsis thaliana in response to a
range of compound II15 concentrations were investigated. It was found that
compound II15 displays significant inhibitory activity against Arabidopsis thaliana
root growth with IC50 value of 26.42 nM, which comparable with commercial
herbicide 2,4-D (IC50=13.55 nM) (Inhibition curve is given in Supporting Information,
Fig. S1). Interestingly, Arabidopsis thaliana treated with compound II15 at a lower
concentration (such as 0.1 and 0.3 µM) exhibited obviously inhibited root elongation
and induced in lateral root hairs formation, which consistent with the observation that
plants exposed to 2,4-D or 2-(2-chloro-4-fluorophenoxy) acetic acid (Fig. 5B). These
preliminary phenotypes indicate that compound II15 may be an auxin-type compound
and has a herbicidal mechanism similar to that of 2,4-D.
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Figure 5. Photographs illustrating seedling and root phenotype of Arabidopsis
thaliana
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To further verify whether compound II15 is an auxin-type compound, the effect of
compound II15 on the expression levels of auxin related genes was investigated. The
auxin induced gene (IAA5), auxin regulated gene (GH3.3) and auxin transport gene
(AUX1) were selected for test, since these genes play an important role in auxin early
response and transport 30-37. As shown in Fig. 6, the trend in the change observed for
the tested genes induced by mesotrione was different to that of 2,4-D. The expression
levels of IAA5, GH3.3 and AUX1 were down-regulated after Arabidopsis thaliana
treated with mesotrione. Since mesotrione is not an auxin analogue, the
downregulation of these genes may be induced by its damage to the plant. To our
delight, upon increasing the treatment time with compound II15, the expression levels
of IAA5, GH3.3 and AUX1 was up-regulation, and the change trend was consistent
with those observed for 2,4-D, which confirmed the auxin-type property of compound
II15.
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Figure 6. Effect of compound II15, mesotrione, and 2,4-D on the expression levels of
IAA5, GH3.3 and AUX1; The error bars represent the standard deviation (s. d.) from
three biological replicates.
To explore whether the auxin-type property of compound II15 are due to its
degradation products, the degradation of compound II15 in Arabidopsis thaliana was
investigated. 2-(2-Chloro-4-fluorophenoxy) acetic acid was selected as a control. It
was found that 2-(2-chloro-4-fluorophenoxy) acetic acid and compound II15 were
gradually degraded in the plant (Fig. 7A and 7B). Interestingly, with the degradation
of compound II15, a new peak was observed at 19 min, which is same as the peak
time observed for 2-(2-chloro-4-fluorophenoxy) acetic acid (Fig. 7B). LC–MS
analysis showed that there are two dominant ions m/z 311.17 (M-H+) and m/z 203.14
(M-H+) in negative ionization mode, which corresponds to the molecular weight of
compound II15 (M = 312.02) and 2-(2-chloro-4-fluorophenoxy) acetic acid (M =
204.00) (Fig. 7C). This result indicates that compound II15 is degraded to
2-(2-chloro-4-fluorophenoxy) acetic acid in the plant. It is worth noting that, after 2.5
h, 2-(2-chloro-4-fluorophenoxy) acetic acid was completely degraded, while
compound
II15
undergone
continuous
degradation
to
release
2-(2-chloro-4-fluorophenoxy) acetic acid, indicating compound II15 will prolong the
lifetime of phenoxycarboxylic acid in the plant and exert a sustained herbicidal effect.
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Figure 7. Degradation analysis of 2-(2-chloro-4-fluorophenoxy) acetic acid and
compound II15 in Arabidopsis thaliana
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Herbicidal activity in greenhouse tests and SARs study
Based on the above preliminary bioassay results, the herbicidal activity of all target
compounds was further tested on four species representative of monocotyledonous
and dicotyledonous plants at a dosage of 1500 g ha-1 located in a greenhouse. 2,4-D
was selected as a positive control. As shown in Figure 8, most of the target
compounds displayed stronger herbicidal activity against dicotyledonous plants when
compared to monocotyledonous plants, which was consistent with the preliminary in
vitro results. Moreover, in most cases, the target compounds had stronger
pre-emergent herbicidal activity than post-emergent herbicidal activity against all the
weeds tested. This may be attributed to the target compounds being easily absorbed or
degraded in the soil when compared to on the surface of the plant. Furthermore, we
were encouraged to observe that some of target compound II, such as II15 and II20,
exhibited 400% sum inhibition against all weeds tested under pre-emergence
conditions, which equal to the herbicidal activity of 2,4-D. These bioassay results
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indicate that compounds II15 and II20 are worthy of further study.
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Figure 8. Effects (% inhibition) of compounds I1-I21 and II1-II21 on loss of plant
weight at a dosage of 1500 g ha-1 in greenhouse testing; Pre: pre-emergence; Post:
post-emergence; BC: Brassica campestris; AR: Amaranthus retroflexus; EC:
Echinochloa crusgalli; DS: Digitaria sanguinalis.
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Subsequently, compounds II15 and II20 were tested against all weeds to further
evaluated their activity using a dose reduction with serial two-fold dilutions. As
shown in Table 1, the herbicidal activity of target compounds II15 and II20 became
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321
322
323
progressively lower against the weeds tested at a dosage of 750, 375, and 187.5 g ha-1.
In addition, upon decreasing the dosage, the herbicidal activity of compounds II15
and II20 under post-emergence conditions decreased faster than that observed under
pre-emergence conditions, implying these compounds exhibit enhanced herbicidal
activity under pre-emergence conditions than under post-emergence conditions.
Compounds II15 and II20 exhibit good herbicidal activity against the weeds tested at
a dosage of 750 g ha-1 under pre-emergence conditions, which is comparable with the
activity of 2,4-D. However, compound II20 displayed lower herbicidal activity than
did 2,4-D, when the dosage was reduced to 187.5 g ha-1. To our delight, compound
II15 still displayed good pre-emergent herbicidal activity against Brassica campestris
and Digitaria sanguinalis with 82.7 and 61.8% inhibition at a dosage of 187.5 g ha-1,
respectively, which is higher than the herbicidal activity observed for 2,4-D.
Meanwhile, compound II15 displays good pre-emergent herbicidal activity against
Amaranthus retroflexus with 83.5% inhibition at a dosage of 187.5 g ha-1, which is
comparable with the activity observed for 2,4-D. These promising results indicate that
compound II15 has good herbicidal activity under pre-emergence conditions and may
be serve as a lead compound for further optimization.
Table 1. Effects (% inhibition) of compounds II15 and II20 on loss of plant weight at
a dosage of 1500 g ha-1 in greenhouse testing a
Brassica
Amaranthus
retroflexus
Echinochloa
crusgalli
Digitaria
Rate (g
ha-1)
campestris
sanguinalis
Comp.
Pre
Post
Pre
Post
Pre
Post
Pre
Post
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750
375
100
100
41.5±1.2
27.9±2.3
75.0±1.1
47.8±1.2
19.1±2.7
100
100
83.3±0.9
75.9±1.6
51.9±0.8
78.2±1.8
59.1±1.6
30.5±1.7
100
77.0±1.2
62.4±2.0
23.8±2.5
91.8±1.8
72.4±3.7
0
34.7±1.8
8.9±1.0
0
98.3±0.9
94.7±0.7
61.8±1.2
72.3±2.1
60.3±1.2
30.2±1.3
100
31.2±0.9
15.9±2.1
0
95.5±1.4
82.7±1.8
100
95.4±0.9
83.5±2.2
100
II15
187.5
750
29.7±1.3
9.8±3.1
0
40.4±1.7
34.7±0.7
0
375
91.4±0.6
52.0±1.5
100
95.5±0.7
56.1±3.2
100
II20
187.5
750
99.3±0.7
77.1±2.1
54.6±1.4
51.1±1.8
28.3±2.3
13.8±2.1
52.8±2.5
15.9±1.7
0
375
94.3±1.2
47.7±4.6
100
98.5±1.5
94.4±1.7
100
61.0±2.5
20.0±1.7
2,4-D
187.5
100
100
324
a Each value represents the mean ±SD of three experiments
325
326
327
328
329
330
331
332
333
334
335
Based on the bioassay data obtained from the greenhouse tests at a dosage of 1500
g ha-1 under pre-emergence conditions, the SARs were preliminarily investigated
performing parallel activity contrast studies between compounds I12 and I15-I21, and
compounds II12, and II15-II21 against Brassica campestris, Amaranthus retroflexus,
Echinochloa crusgalli, and Digitaria sanguinalis. The results shown in Fig. 9 indicate
that series II always displayed higher herbicidal activity against the weeds tested
when compared to series I. The possible reason for this observation is that the plants
have different absorption and/or metabolic capacity toward series I and II. The
preliminary SARs indicate that removing the benzene ring from the
4-hydroxylcoumarin fragment in series I was beneficial toward improving the
herbicidal activity.
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337
Figure 9. Parallel activity contrast studies between compounds I12 and I15-I21 and
compounds II12 and II15-II21 against Brassica campestris (A), Amaranthus
retroflexus (B), Echinochloa crusgalli (C), and Digitaria sanguinalis (D). The
inhibition activity was tested at 1500 g ha-1 and expressed in ordinate.
338
339
340
341
342
343
344
345
346
347
348
Analyzing the pre-emergent herbicidal activity against Brassica campestris among
compounds II1-II10 reveals that the compounds without a substituent or with an
electron-donating group (compounds II1 and II5-II10) generally had lower activities
than did the compounds bearing an electron-withdrawing group (compounds II2 and
II3). For compounds II11-II14 with two chlorine substituents on the benzene ring, the
herbicidal activity against Brassica campestris could be placed in the following order:
II12 (2-Cl-4-Cl) > II11 (2-Cl-3-Cl) > II13 (2-Cl-5-Cl) > II14 (3-Cl-5-Cl), which
demonstrates that the 2,4-disubstitution pattern is the most active and the
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3,5-disubstitution pattern is the least active. These findings indicate that the
electron-withdrawing group introduced into benzene ring in compound II is beneficial
toward improving the herbicidal activity, and the position of the substituent on the
benzene ring is very important for herbicidal activity, with the 2,4-disubstitution
pattern confirmed as the optimal orientation.
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
Among the 2,4-disubstituted analogues (compounds II12 and II15-II21),
compounds bearing 2-Cl-4-F, 2-Me-4-F, and 2-Br-4-F substituents on the benzene
ring display higher herbicidal activity against Brassica campestris than do those with
other types of substituents at the same positions. For example, the results of obtained
for the herbicidal activity of compounds II12 and II15-II19 show that their herbicidal
activity can be placed in the following order: II15 (2-Cl-4-F) > II12 (2-Cl-4-Cl) >
II16 (2-Cl-4-Br); II17 (2-Me-4-F) > II18 (2-Me-4-Cl) > II19 (2-Me-4-Br). These
findings indicate that a F atom introduced at the 4-position on the benzene ring of
compound II is beneficial to improving the herbicidal activity against Brassica
campestris.
Herbicidal spectrum and crop safety of compound II15
To evaluate whether compound II15 has the potential to be developed as a
herbicide, we tested its herbicidal spectra and crop safety at a dosage of 375 g ha-1.
The herbicidal spectrum experiments show that compound II15 displays >80%
inhibition against 9 of the 12 tested weeds at 375 g ha-1 (Fig. 10). It was worth noting
that compound II15 displays >80% inhibition against Ixeris denticulate, Eleusine
indica, and Puccinellia distans, whereas 2,4-D only displays <50% inhibition to these
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372
373
374
375
376
377
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380
weeds. This finding indicates that compound II15 has a broader spectrum of weed
control than does the commercial herbicide 2,4-D at a dosage of 375 g ha-1 under
pre-emergence conditions. Subsequently, three representative crops, Triticum
aestivum L., Zea mays Linn. and Gossypium spp, were selected for further crop
selectivity studies (Table 2). The results showed that Triticum aestivum L. and Zea
mays Linn. displayed a high tolerance toward compound II15 at a dosage of 375 g
ha-1, but compound II15 was not selective for Gossypium spp (51.3% injury). These
promising results indicate that compound II15 has the potential to be developed as a
pre-emergence herbicide for weed control in Triticum aestivum L. and Zea mays Linn.
fields.
381
382
Figure 10. Herbicidal spectrum testing of compounds II15 under pre-emergence
condition at the dosage of 375 g ha-1.
383
384
385
Table 2. Pre-emergence crop selectivity of compound II15 at the dosage of 375 g ha-1
(injury inhibition) a
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% injury
Comp.
Triticum aestivum L.
Zea mays Linn.
Gossypium spp
II15
0
0
0
0
51.3±2.6
0
2, 4-D
a Each value represents the mean ±SD of three experiments
386
In summary, a series of 4-hydroxyl-3-(2-phenoxyacetyl)-pyran-2-one derivatives
have been designed using molecular hybridization between pyran-based diketone
lactones and aryloxyacetic acid moieties. Forty-two 4-hydroxyl-3-(2-phenoxyacetyl)-
pyran-2-one derivatives were prepared in moderate to good yield. Our bioassay results
showed that compound II15 displayed good pre-emergent herbicidal activity, even at
a dosage of 187.5 g ha-1. Our herbicidal spectrum study revealed that compound II15
had a broader spectrum of weed control than did the commercial herbicide 2,4-D, and
displayed good crop safety against Triticum aestivum L. and Zea mays Linn. at a
dosage of 375 g ha-1, which indicated its great potential as a herbicide for weed
control in Triticum aestivum L. and Zea mays Linn. fields. By investigating the
phenotypes of Arabidopsis thaliana, detecting the effect on auxin response genes and
studying of degradation of compound II15, it was found that compound II15 is
metabolized to form an corresponding aryloxy acetic acid in the plant and has a
herbicidal mechanism similar to that of 2,4-D, which indicates compound II15 may
be a potential lead structure for the further development of novel auxin-type
herbicides. It is worth mentioning that, in our current work, the compounds obtained
by molecular hybridization prolong the lifetime of herbicides in the plant, which may
23
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exert a sustained herbicidal effect. This is a step forward as less herbicide need be
used, which would have a very important meaning in practical applications. Further
studies on the structural optimization of compound II15 are ongoing in our laboratory.
ACKNOWLEDGEMENTS
We are very grateful to the National Natural Science Foundation of China (Grant
No. 31701827 and 81803360), the Natural Science Foundation of Shandong Province
(Grant No. ZR2017BH037 and ZR2017BC053) and The National Key Research and
Development Program of China 2016YFD0200503.
SUPPORTING INFORMATION
Supporting information may be found in the online version of this article.
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