Natural products as sources of new fungicides (V): Design and synthesis of acetophenone derivatives against phytopathogenic fungi in vitro and in vivo

Article abstract of DOI:10.1016/j.bmcl.2018.07.031

A series of acetophenone derivatives (10a–10i, 11, 12a–12g, 13a–13g, 14a–14d and 15a–15l) were designed, synthesized and evaluated for antifungal activities in vitro and in vivo. The antifungal activities of 53 compounds were tested against several plant pathogens, and their structure–activity relationship was summarized. Compounds 10a–10f displayed better antifungal effects than two reference fungicides. Interestingly, the most potent compound 10d exhibited antifungal properties against Cytospora sp., Botrytis cinerea, Magnaporthe grisea, with IC₅₀ values of 6.0–22.6 μg/mL, especially Cytospora sp. (IC₅₀ = 6.0 μg/mL). In the in vivo antifungal assays, 10d displayed the significant protective efficacy of 55.3% to Botrytis cinerea and 73.1% to Cytospora sp. The findings indicated that 10d may act as a potential pesticide lead compound that merits further investigation.

Full text of DOI:10.1016/j.bmcl.2018.07.031

Accepted Manuscript
Natural products as sources of new fungicides (V): Design and synthesis of
acetophenone derivatives against phytopathogenic fungi in vitro and in vivo
Wen-Jia Dan, Thi-Mai-Luong Tuong, Da-Cheng Wang, Ding Li, An-Ling
Zhang, Jin-Ming Gao
PII:
S0960-894X(18)30609-7
https://doi.org/10.1016/j.bmcl.2018.07.031
BMCL 25962
DOI:
Reference:
Bioorganic & Medicinal Chemistry Letters
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Accepted Date:
6 January 2018
12 July 2018
18 July 2018
Please cite this article as: Dan, W-J., Tuong, T-M., Wang, D-C., Li, D., Zhang, A-L., Gao, J-M., Natural products
as sources of new fungicides (V): Design and synthesis of acetophenone derivatives against phytopathogenic fungi
in vitro and in vivo, Bioorganic & Medicinal Chemistry Letters (2018), doi: https://doi.org/10.1016/j.bmcl.
2018.07.031
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Natural products as sources of new fungicides (V): Design and synthesis of
acetophenone derivatives against phytopathogenic fungi in vitro and in vivo
Wen-Jia Dan†, Thi-Mai-Luong Tuong†, Da-Cheng Wang, Ding Li, An-Ling Zhang*,
Jin-Ming Gao*
Shaanxi Key Laboratory of Natural Products & Chemical Biology, College of
Chemistry & Pharmacy, Northwest A&F University, 22 Xinong Road, Yangling
712100, Shaanxi, China.
*Corresponding authors
Tel./fax: +86 29 87092515.
E-mail address: jinminggao@nwsuaf.edu.cn
†These authors contributed equally to this work.
1

ABSTRACT
A series of acetophenone derivatives (10a–10i, 11, 12a–12g, 13a–13g, 14a–14d and
15a–15l) were designed, synthesized and evaluated for antifungal activities in vitro
and in vivo. The antifungal activities of 53 compounds were tested against several
plant pathogens, and their structure–activity relationship was summarized.
Compounds 10a–10f displayed better antifungal effects than two reference fungicides.
Interestingly, the most potent compound 10d exhibited antifungal properties against
Cytospora sp., Botrytis cinerea, Magnaporthe grisea, with IC50 values of 6.0 to 22.6
µg/mL, especially Cytospora sp. (IC50 = 6.0 µg/mL). In the in vivo antifungal assays,
10d displayed the significant protective efficacy of 55.3% to Botrytis cinerea and
73.1% to Cytospora sp. The findings indicated that 10d may act as a potential
pesticide lead compound that merits further investigation.
Keywords: Natural products; acetophenone derivatives; antifungal activity;
structure–activity relationship
2

Pathogenic fungi, hard to control, are considered a great threat for farming and food
production including livestock, poultry and crop.1,2 Accordingly, >80% of the crops
areas in the United States are treated with fungicides every year, and the benefit of
fungicide use is estimated to boost farm income by nearly $13 billion annually. And in
China, fungicides used as crop protection have also occupied a larger share of
agricultural resources.3 However, the emergence of fungicide-resistant strains has
brought great challenges to normal production and life, which prompt us to develop
new fungicides. Natural products with low mammalian toxicity, easy decomposition,
friendly to environment, specific to targeted species, and unique mode of action, are
an important source of antifungal agents.4 2,4-Dihydroxy-5-methylacetophenone (1),
isolated from the cultures of the fungus Polyporus picipes (Figure 1), has been
reported as a good antifungal agent against several phytopathogens in our patent.5
Xanthoxylin (2), isolated from a medicinal plant Melicope borbonica, also exhibited
good activities against Penicillium expansum.6
Figure 1. Design of the acetophenone derivatives (10−14).
3

In our previous work, we assessed the antifungal activity of several series of
acetophenone analogues (3–5) (Figure 1).5,7,8 The SAR studies mainly focused on (1)
alkyl or benzyl acetophenone ether derivatives on the acetophenone nuleus, which
concluded that the coexistence of m-OH and p-lipophilic alkyl substituents were
beneficial; (2) chlorine, alkyl or aromatic substitutions on α-position of ketone
carbonyl, which illustrated that chlorine-substitution, the longer chain or multi-side
chain are favorable. Interestingly, phenolic hydroxyl, which could form
intramolecular hydrogen bond with ketone carbonyl groups, exhibited positive or
negative efficiency in antifungal activity. To obtain higher antifungal acetophenone
agents, some acetophenone analogues, especially bromoacetophenones (10), were
synthesized. Considering the role of nitrogen-containing heterocycles (6 and 7) in
9,10
antifungal agents,
we further synthesized their N-heterocyclic derivatives (12, 13)
based on bromoacetophenones. With the formation of B ring (8)11, some
chromen-4-one analogues (14) were also prepared (Figure 1). Inspired by these results,
herein we report synthesis of several series of acetophenone derivatives (10a–10i, 11,
12a–12g, 13a–13g and 14a–14d) and their anti-phytopathogenic fungal activity in
vitro and in vivo.
4

S
NH2
c
N
R3
12a 12g
O
Br
R2
a
b
N
N
10a 10i
O
R4
d
Me
R1
13a 13g
OH
O
O
9a 9l
Me
11
R1
R1
R2
R3
R4
No.
No.
No.
No.
No.
9a
9b
9c
9d
9e
9f
4-F
9h 2-OH
4-F
12a 4-F
13a 4-F
10a
10b
9i
12b
13b
4-Cl
2,6-OH-4-OMe
2,4-OMe-6-OH
2,6-OH
4-Cl
4-Cl
4-Cl
9j
12c
13c
4-Br
10c 4-Br
10d 4-Me
4-Br
4-Br
9k
9l
12d
13d
4-Me
4-OMe
4-OMe
4-Me
12e
12f
13e
4-OMe
10e
10f
2-OH-4-OMe
4-OMe
4-SO2Me
2-F
4-SO2Me
13f
4-SO2Me
2-F
2-F
4-SO2Me
2-OH
9g
12g
13g
10g
10h
10i
2-OH
2-OH
2-OH-4-OMe
Scheme 1. Syhthesis of acetophenone derivatives 10a–10i, 11, 12a–12g and 13a–13g. Reagents
o
and conditions (a) p-TSA, NBS, MeCN, 80 C, 40%–89%; (b) NaH, EtOAc, THF, r.t., 25 min,
o
72%; (c) thiourea, I2, 100 C, DMF, 18–24 h, 57%–89%; (d) 2-aminopyridine, NaHCO3, EtOH,
reflux, 2 h, 51%–83%.
Synthetic routes to bromoacetophenones (10a−10i) and N-heterocyclic derivatives
(12a−12g and 13a−13g) are depicted in Scheme 1.12–15 Bromoacetophenones were
prepared from the corresponding commercial acetophenones by stirring the NBS with
o
p-TSA in CH3CN at 80 C. Subsequently, compounds 12a–12g were obtained in the
presence of thiourea. Condensations of bromoacetophenones with 2-aminopyridine in
the presence of sodium bicarbonate formed the N-heterocyclic derivatives 13a–13g.
Treatment of compound 9h with NaH in ethyl acetate and THF gave compound 11.
Chromen-4-one analogues (14a–14d) were obtained from the corresponding
O-hydroxyacetophenones as depicted in Scheme 2.16
5

Scheme 2. Synthesis of chromen-4-one analogues 14a–14d. Reagents and conditions (e) NaH,
HCOOEt/THF, aq. HCl, 56%–82%.
Preliminary screening of all compounds was performed for their in vitro antifungal
activities against seven phytopathogenic fungi, Fusarium solani, Cytospora sp.,
Colletotrichum gloeosporioides, Botrytis cinerea, Alternaria solani, Fusarium
graminearum and Magnaporthe grisea by using the mycelium growth rate method
with thiophanate-methyl and hymexazol as the positive controls.17 The results were
listed in Table S1. To further evaluate the inhibitory potencies of the synthesized
compounds, the half maximal inhibitory concentration (IC50) values of the compounds
with high inhibition rate (>70%) were determined. As listed in Table 1, eight of the
tested compounds presented different fungicidal activity against the seven
phytopathogens. Especially, some synthetic compounds 10a−10f had more potent
antifungal spectrum than the positive controls. Specifically, compound 10a, with a
fluorine atom at C-4’, had antifungal activities against a broad panel of fungal species,
F. solani, Cytospora sp., C. gloeosporioides, A. solani, F. graminearum and M. grisea,
with IC50 values in range from 7.4 to 22.8 µg/mL. Interestingly, antifungal potency of
10c with a 4’-bromine atom was 5.5−fold of 10a against Alternaria solani, while 10b
with a 4’-chlorine atom was inactive, indicating the bromine substituent is the most
sensitive to this pathogen. Compound 10a presented the strongest inhibitory activities
against C. gloeosporioides (IC50 = 10.9 µg/mL), and both 10a and 10c exerted the
6

stronger inhibition of F. graminearum growth with their IC50 values of 7.4 and 6.2
µg/mL, respectively, than the positive controls (Table 1), while 10b had no effect. In
addition, compounds 10a−10c inhibited the growth of the rice blast fungus
Magnaporthe grisea, a causal agent of the prevalent rice blast disease, with IC50
values of 11.3, 19.9, and 46.4 µg/mL, respectively, whose potencies were much more
potent than these two positive drugs, indicating 10a was the highest growth inhibitor.
In case of compounds 10a−10f, 12d and 13b, possessing a 4’-methyl substituent,
displayed potent antifungal activities against two fungal pathogens, Cytospora sp. and
Magnaporthe grisea, with IC50 values of 6.0 and 10.7 µg/mL, respectively, which
gave much higher activity than the two control fungicides, but had a weak effect on
Botrytis cinerea (IC50 = 22.6 µg/mL). These results indicated that 4’-methyl
substituent seems to be the most sensitive to Cytospora sp. and M. grisea.
Table 1. Antifungal activity of compounds 10a–10f, 12d and 13b.
IC50 (µg/mL)
Compd.
Fusarium
solani
Cytospora Colletotrichum
sp. gloeosporioides cinerea
12.6 1.2
Botrytis
Alternaria Fusarium
Magnaporthe
solani
graminearum grisea
>100
>100
10a
10b
10c
10d
10e
10f
22.8 0.6
27.2 0.4
25.8 0.9
45.7 0.3
34.4 0.4
77.8 1.9
31.6 0.8
>100
10.9 0.3
>100
48.7 0.9
>100
7.4 0.4
>100
11.3 0.9
19.9 0.8
46.4 2.2
10.7 0.9
35.2 0.6
61.9 0.9
>100
18.3 0.3
34.7 0.7
6.0 0.7
9.5 0.5
26.5 0.6
35.0 0.4
>100
25.6 0.4
>100
73.4 0.6
22.6 0.4
43.3 3.2
71.7 0.8
>100
8.8 0.6
42.7 0.3
21.4 2.6
25.0 0.4
>100
6.2 1.2
57.7 0.9
25.3 2.3
37.6 0.8
47.7 1.4
>100
46.3 0.6
33.3 0.6
>100
12d
13b
Th.a
Hy.a
>100
>100
>100
38.4 1.2
>100
>100
>100
32.8 0.6
>100
23.3 0.7
>100
18.0 0.4
>100
92.6 1.2
17.6 0.4
14.0 0.6
71.3 0.7
96.9 0.8
aTh. = Thiophanate-methyl, Hy. = Hymexazol.
Among the acetophenone derivatives, some compounds possessing α-bromo
substitution presented distinct inhibition potency against all tested fungi, whereas the
N-heterocyclic derivatives and chromen-4-one analogues were found to reduce the
activity of the parent compounds, except for 12b and 13d. These findings suggested
that the carbonyl of acetophenones was the most crucial for higher inhibitory effects
and the cyclization of acyl chain was not favorable for improving the activity (i.e.,
7

12a−12g, 13a−13g, and 14a−14d). In comparison with ortho-fluoro substituted
derivatives of α-bromoacetophenones, the para-substituted derivatives had more
potent inhibition against tested fungi (10a vs 10g) since the inhibition rate of the
ortho-fluoro derivative 10g at 50 µg/mL was less than 60% (Table S1).
As shown in Table 1, compounds 10a, 10c and 10d exhibited good antifungal activity
against different fungi. Specifically, compound 10d showed a peak IC50 value of 6.0
µg/mL against Cytospora sp.. So we chose 10d and Cytospora sp. as the templates to
further explore the effect of acetophenone derivatives bearing various substituents at
the alpha position of the carbonyl group. We synthesized 12 compounds derived from
10d and evaluated their antifungal activity against the sensitive strain Cytospora sp.
(Scheme S1 and Table S2). Under the test concentration (50 µg/mL), α-halogen
substituted derivatives exhibited higher antifungal potential than others (excepting for
α-fluoro substituent, the inhibition rates of 10d, 15c and 15b over 90%); 15f, 15h and
15d showed moderate potency (with their inhibition rates over 50%); 15j, 15a, 15i,
15l, 15g, 15e and 15k exhibited low inhibitory to Cytospora sp. (as their inhibition
rates lower than 50%), correspondingly. Further comparison of IC50 values, 10d (Br,
6.0 µg/mL) > 15c (I, 13.9 µg/mL) > 15b (Cl, 15.4 µg/mL), confirmed the importance
of bromine atom.
Figure 2. SAR of acetophenone derivatives.
Based on these results, it could be concluded that the structure-antifungal activity
relationship was shown in Figure 2: 1) the α-position of acetophenone substituted with
8

bromine played a greater role for improving the antifungal activities; 2) for all
α-bromine acetophenones, C-4’ substituted acetophenones exhibited superior
antifungal activity over C-2’ substituted acetophenones; 3) considering the activity of
N-heterocyclic derivatives, the carbonyl of acetophenone is a determinant; 4) the
B-ring moiety is not effective.
Table 2. Calculated molecular properties of potential compounds.
Log Pa
2.36
2.88
3.01
2.25
1.07
2.31
2.20
3.65
TPSAb
17.07
17.07
17.07
26.30
51.21
17.07
48.15
17.31
nONc
nONNHd
MWe
Compd.
1
1
1
2
3
1
3
2
0
0
0
0
0
0
2
0
217.04
233.49
277.94
213.07
229.07
277.13
210.68
208.26
10a
10b
10c
10d
10e
10f
12d
13b
b
c
aLog P = octanol–water partition coefficient, TPSA = topologic polar surface area, nON = number of hydrogen
bond acceptors, dnONNH = number of hydrogen bond donors, eMW = molecular weight.
In order to further verify the “fungicide-likeness” of these compounds, their molecular
physicochemical properties, including octanol-water partition coefficient, topologic
polar surface area, the number of hydrogen bond acceptors, number of hydrogen bond
donors and molecular weight, were calculated by using Molinspiration tool (as shown
in Table 2).21 For all preferred compounds, the Log P values are lower than 5, number
of hydrogen bond acceptors are lower than 5 and number of hydrogen bond donors
are lower than 2. Their physicochemical properties are corresponding to a screening
rule of fungicides, which indicate their potential for candidate lead fungicides.22
Furthermore, the in vivo fungicidal activity (protective effect) of the more promising
compound 10d was tested against B. cinerea on tomato (Figure S1A) and Cytospora
sp. on apple (Figure S1B) by utilizing reported method with a slight modification.18-20
To our delight, compound 10d exhibited excellent preventative effects against B.
cinerea and Cytospora sp., with the protective efficacy of 55.3% and 73.1%,
respectively. This result demonstrates the potential of the bromoacetophenone
9

analogues as novel candidates for protecting crop or stored agricultural products.
In conclusion, 53 compounds were prepared and their inhibition activities were
evaluated against several plant-pathogenic fungi, and their structure–activity
relationships were briefly summarized. Most of the compounds 10a−10f had a good
broad antifungal spectrum and good “fungicide-like” properties. Notably, the in vivo
antifungal assays using B. cinerea and Cytospora sp. have demonstrated the potential
of bromoacetophenone analogues as novel candidate lead pesticides.
Acknowledgements
This work was supported by grants from the Program of Unified Planning Innovation
Engineering of Science & Technology in Shaanxi Province (No. 2015KTCQ02-14),
the Natural Science Foundation of Shaanxi Province (2014JZ2-001) and the National
Natural Science Foundation of China (No. 21502152, 21542016, 30971882).
Supplementary data
Supplementary data associated with this article can be found in the online version.
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13

Highlights
ꢀ
A series of acetophenone derivatives were synthesized and evaluated for
antifungal activity.
ꢀ
ꢀ
The structure–activity relationships were summarized.
Compound 10d exhibited excellent antifungal activity in vitro and in vivo.
14