Novel trifluoromethyl sydnone derivatives: Design, synthesis and fungicidal activity

Article abstract of DOI:10.1016/j.bmcl.2021.128114

Crop pathogens reduce the yield and quality of agricultural production. The development of new fungicides will help to sustain this protection and overcome fungicide resistance. Sydnone is a kind of mesoionic, which has a wide range of biological activiti

Full text of DOI:10.1016/j.bmcl.2021.128114

Bioorg. Med. Chem. Lett. 44 (2021) 128114
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Novel trifluoromethyl sydnone derivatives: Design, synthesis and
fungicidal activity
,
,c,*
Shaoqing Dua, Xueping Hub, Xusheng Shaoa *, Xuhong Qiana
a Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
b Hangzhou Institute of Innovative Medicine, College of Pharmaceutical Sciences, Zhejiang University, Hangzhou, Zhejiang 310058, China
c School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200241, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Crop pathogens reduce the yield and quality of agricultural production. The development of new fungicides will
help to sustain this protection and overcome fungicide resistance. Sydnone is a kind of mesoionic, which has a
wide range of biological activities. The application of sydnones in agriculture is less, and the study of these
compounds will lead to the discovery of new active compounds. In this study, we designed and synthesized a
series of noval sydnone mesoionic derivatives by active substructure splicing. All compounds were characterized
using 1H and 13C NMR spectroscopy. Among them, trifluoromethyl compound D17 showed good bioactivity
against Pseudoperonospora cubensis (EC50 = 49 mg Lꢀ 1) in vivo, the activity was similar to that of the control
Kresoxim-methyl (EC50 = 44 mg Lꢀ 1). However, the target of these compounds should not only be tyrosinase,
and the mode of action needs to be further studied. In addition, the structure–activity relationship indicated that
the trifluoromethyl group was more beneficial for antifungal activity. This is the first report that fluorine-
containing N(3)-benzyl sydnone compounds have good fungicidal activity. These results will provide a basis
for the development of sydnone mesoionic as new lead fungicidal agents.
Sydnone
Antifungal
Mesoionic
Trifluoromethyl
Fungal diseases on crops have pose a serious threat to global food
security. It has indicated that the global crop losses caused by crop pests
and pathogens to wheat, rice, corn, potatoes and soybeans are range
between 17% and 30%1. The use of fungicides is an important means to
ensure agricultural production. But prolonged and excessive use of
fungicides have caused serious resistance problems. Therefore, there is
an urgent need to development of new fungicides with high efficiency,
novel action mechanism and high safety.
selecting different patterns of electron density by selecting different
mesoionic systems7. Sydnone8 (Fig. 1) is a kind of mesoionic9, which has
a wide range of biological activities10, such as antibacterial11, antimi-
crobial12,13, anti-inflammatory14,15, antitumor16,17, antioxidant activ-
ities18, antidiabetic19, analgesic20 and insecticidal21 activities. But the
fungicidal activity of sydnone is rarely reported. Through the modifi-
cation of this kind of compound, excellent lead fungicide with high
bioactivity and new mode of action is expected to be obtained.
Tyrosinase is a key enzyme in the synthesis of melanin2, which is
widely found in animals, plants, and microorganisms3,4. Melanin5 is not
only related to the pathogenic ability of fungi, but also enhances the
fungal resistance to environmental stress (pH, temperature, radiation,
poison, natural enemies, etc.). Therefore, the inhibition of tyrosinase can
control the growth of fungi. In 2018, Lopes et al.6 reported that piperonal
1,3,4-thiadiazolium-2-phenylamines mesoionic derivatives exhibited
non-competitive tyrosinase inhibitory activity, and PMI-5 (Fig. 1) was
the most active compound.
Triflumezopyrim22 (Fig. 1) is the first commercialized mesoionic
pesticide. With the advent of triflumezopyrim, research on mesoionic
compounds has received impetus. This series of compounds possess high
insecticidal activity against brown planthopper and lepidoptera23. The
activity of these compounds is considerably affected by two substituents
on the mesoionic ring24. Novel pyrido[1,2-
α]pyrimidinone mesoionic
compounds containing vanillin moieties also showed excellent activity
against Xanthomonas oryzae pv. Oryzae25. Mesoionic compounds can be
used to design new insecticides or fungicides by modifying their
substituents.
The potential value as biologically active substances of mesoionic
compounds is found in their relatively small size and the possibility of
Here, encouraged by the structure of pyrido[1,2-α]pyrimidinone
* Corresponding authors at: Shanghai Key Laboratory of Chemical Biology, School of Pharmacy, East China University of Science and Technology, 130 Meilong
Road, Shanghai 200237, China.
E-mail addresses: shaoxusheng@ecust.edu.cn (X. Shao), xhqian@ecust.edu.cn (X. Qian).
https://doi.org/10.1016/j.bmcl.2021.128114
Received 26 February 2021; Received in revised form 23 April 2021; Accepted 15 May 2021
Available online 17 May 2021
0960-894X/© 2021 Elsevier Ltd. All rights reserved.

S. Du et al.
Bioorganic & Medicinal Chemistry Letters 44 (2021) 128114
Results and discussion
Chemistry
The synthesis of sydnones D1–D24 were shown in Fig. 3. Initially,
intermediate c was obtained via reaction of compounds a and b,
following which it was transformed into the corresponding target
compound D via a one-pot reaction.
The first step is substitution reaction, which is more likely to occur
when the substituents of amines contain stronger electron withdrawing
groups. When the substituents of amines are more electron donating, the
reaction time should be prolonged or the reaction solvent should be
changed to ethyl acetate to increase the reaction temperature. The one-
pot reaction involved nitritation of the secondary imine group and
condensation reaction. The keys to the reaction are low temperature and
long reaction time. The reaction works well in a nitrogen atmosphere.
When isoamyl nitrite and trifluoroacetic anhydride are used, the reac-
tion time is relatively short, and a good yield is obtained.
Fig. 1. The chemical structures of sydnone, triflumezopyrim, and PMI-5.
mesoionic and 1,3,4-thiadiazole-2-phenylamine mesoionic derivatives,
sydnones were used as the lead structure in which the substituted benzyl
group was introduced at the N(3) position and the substituted phenyl
group at the C(4) position (Fig. 2). In addition, compounds containing
trifluoromethyl are widely used in bioactive molecules because of their
high electronegativity and bioavailability. Therefore, trifluoromethyl
was introduced into the benzene ring in this article. In short, a series of
novel 3-benzyl-4-phenyl-sydnones were designed and synthesized.
Subsequently, antifungal activities were screened against Pseudoper-
onospora cubensis (CDM), Colletotrichum orbiculare (CA), Puccinia sorghi
(CSR), Blumeria graminis (WPM), Pyricularia grisea (RB), Botrytis cinerea
Pers (CGM), and Sphaerotheca cucurbitae (CPM).
Antifungal bioactivities of compounds D1–D24 and analysis of structure-
activity relationships (SAR)
The antifungal activities of compounds D1–D24 are shown in Ta-
bles 1, 2 and 3. Kresoxim-methyl (KSM) and azoxystrobin (AZS) were
selected as positive controls at 400 mg Lꢀ 1, while pyrisoxazole (PSA)
and tricyclazole (TCA) were selected as positive controls at 25 mg Lꢀ 1
using the spore germination method.
Fig. 2. Design of novel sydnone mesoionic compounds D1-D24.
Fig. 3. General route of synthesis of the target compounds D1–D24. Reagents: (i) ether, reflux; (ii) isoamyl nitrite, THF, TFAA, r.t.
2

S. Du et al.
Bioorganic & Medicinal Chemistry Letters 44 (2021) 128114
Table 1
In vitro antifungal activities of the target compounds D1–D24
.
NO.
R1
Ph
R2
inhibition rate(%, 25 mg Lꢀ 1
RB
)
CGM
50
D1
0
D2
Ph
0
0
D3
D4
Ph
Ph
0
0
50
50
D5
D6
D7
Ph
Ph
Ph
80
0
0
0
0
0
D8
Ph
0
0
D9
Ph
Ph
Ph
0
0
0
0
0
0
D10
D11
D12
D13
Ph
Ph
Ph
0
0
0
50
D14
D15
D16
D17
D18
D19
Ph
0
0
0
60
0
50
0
0
0
0
50
0
D20
0
0
(continued on next page)
3

S. Du et al.
Bioorganic & Medicinal Chemistry Letters 44 (2021) 128114
Table 1 (continued)
NO.
R1
R2
inhibition rate(%, 25 mg Lꢀ 1
RB
)
CGM
0
D21
50
D22
D23
D24
H
H
H
0
0
0
0
0
0
PSA
TCA
–
–
–
–
/
100
/
100
RB, Phyricularia grisea; CGM, Botrytis cinerea Pers; PSA, Pyrisoxazole; TCA, Tricyclazole; /, not measured.
Table 2
In vivo antifungal activities and tyrosinase inhibitory activities of the target
Table 3
compounds D1–D24.
EC50 Values of the Target Compounds D16, D17 and D19.
NO.
NO.
EC50 (mg Lꢀ 1
CPM
)
inhibition rate(%)
400 mg Lꢀ 1
50 mg Lꢀ 1
tyrosinase
WPM
CDM
CDM
CA
WPM
CSR
D16
D19
TDM
D17
KSM
138
147
1.86
/
564
224
1.20
/
/
/
D1
50
0
0
20
0
0
47
61
14
48
52
40
28
33
49
40
59
60
18
55
47
28
44
33
54
53
52
52
20
23
/
/
D2
0
0
49
44
D3
0
0
0
0
/
/
D4
10
0
0
20
0
0
D5
0
0
CDM, Pseudoperonospora cubensis; WPM, Blumeria graminis; CPM, Erysiphe
cichoracearum; TDM, Triadimenol; KSM, Kresoxim-methyl; /, not measured.
Inhibition of Mushroom Tyrosinase
D6
0
0
0
0
D7
0
0
0
0
D8
0
0
0
0
D9
0
0
0
0
pesticides, a halogen atom was introduced to the benzene ring on R1 in
the design of D15–D24.
D10
D11
D12
D13
D14
D15
D16
D17
D18
D19
D20
D21
D22
D23
D24
KSM
AZS
KA
0
0
0
0
0
0
0
0
0
0
0
0
Therefore, we designed and synthesized compounds D15–D24 with
trifluoromethyl substituted benzene ring and chloro-substituted ben-
zene ring. Some of these compounds displayed good antifungal activ-
ities. For example, compounds D17 (90%) and D18 (90%) displayed
identical antifungal activities against CDM. In addition, compounds D16
(95%), D17 (90%), D19 (90%), and D22 (85%) displayed antifungal
activities against WPM.
20
0
0
20
0
0
0
0
0
0
0
0
20
90
90
0
0
95
90
80
90
80
80
85
60
0
0
70
0
0
0
0
0
0
20
0
50
0
0
To analyze SARs of the target compounds D1–D24, high bioactivity
compounds D16, D17, and D19 were selected for further studies and
their EC50 values are listed in Table 3.
0
0
0
0
80
0
0
0
0
The EC50 of D16 and D19 for WPM were inferior (D16, 564 mg Lꢀ 1
;
100
/
/
/
/
D19, 224 mg Lꢀ 1) to that for CPM (D16, 138 mg Lꢀ 1; D19, 147 mg Lꢀ 1).
Compared to D16, D19 and D22, possessed the same R2 substituents, but
not the same R1 substituents. Results showed that 2-trifluoromethylben-
zene at R2 significantly improved its anti-powdery mildew activity. The
inhibitory activities of compounds against powdery mildew were lower
than that of the control, indicating that further improvement was
required.
100
/
100
/
100
/
/
/
92
CDM, Pseudoperonospora cubensis; CA, Colletotrichum orbiculare; CSR, Puccinia
sorghi; WPM, Blumeria graminis; KSM, Kresoxim-methyl; AZS, Azoxystrobin; KA,
Kojic acid; /, not measured.
In the spore germination experiments (25 mg Lꢀ 1), compound D5
(80%) showed good antifungal activity against RB. The introduction of a
chlorine atom at the 4-position of N(3) aryl was conducive for the ac-
tivity. However, the activity of the compound was low at 10 mg Lꢀ 1, and
hence, no further studies have been conducted.
Compound D17, in which R1 and R2 both contain 3-trifluoromethyl-
benzene ring, showed good antifungal activity against CDM (EC50 = 49
mg Lꢀ 1). Compounds D17, D20, and D23 have the same R2 substituents,
but not the same R1 substituents. The antifungal activities of compounds
D20 and D23 against CDM were low. Therefore, the presence of the 3-
trifluoromethylbenzene ring at the R1 position considerably affected
CDM inhibition. The inhibitory activity of D17 was slightly lower than
that of the control KSM (EC50 = 44 mg Lꢀ 1), such compounds have the
value of continuing research.
As shown in Table 2, compounds D1–D14 showed low in vivo anti-
fungal activities against the tested fungi (400 mg Lꢀ 1). But the retention
of trifluoromethyl substituted benzene ring on R2 could increase the
activity. Based on the structure of commercial pyrido[1,2-α]pyr-
imidinone mesoionic compounds and advantages of fluorinated
4

S. Du et al.
Bioorganic & Medicinal Chemistry Letters 44 (2021) 128114
Compounds D17 and D18 showed considerable activity against CDM
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plementary data to this article can be found online at https://doi.org/10
.1016/j.bmcl.2021.128114.
at 400 mg Lꢀ 1; however, at lower concentration of 100 mg Lꢀ 1, D18 lost
its activity.
A previous report showed that 1,3,4-thiadiazolium-2-phenylamine
mesoionic compounds inhibited tyrosinase, which is a key enzyme of
melanin synthesis, and catalyzes the hydroxylation of L-tyrosine to L-
DOPA and the oxidation of L-DOPA to dopaquinone. Therefore, we
tested the inhibitory activity of sydnone compounds on tyrosine using
Kojic Acid (KA) as the positive control (Table 2).
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Acknowledgments
We thank Shenyang Research Institute of chemical industry for the
test of fungicidal activity. This work was financially supported by Key
Project of the Shanghai Science and Technology Committee
(No.18DZ1112703), the National Natural Science Foundation of China
(No.31830076) and the Shenzhen Science and Technology Program (No.
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Appendix A. Supplementary data
Supplementary material that may be helpful in the review process
5