Design, synthesis and fungicidal activity of N-substituted benzoyl-1,2,3,4-tetrahydroquinolyl-1-carboxamide

Article abstract of DOI:10.1016/j.bmcl.2016.03.085

To find a new lead compound with high biological activity, a series of N-substituted benzoyl-1,2,3,4-tetrahydroquinolyl-1-carboxamide were designed using linking active substructures method. The target compounds were synthesized from substituted benzoic acid by four steps and their structures were confirmed by ¹H NMR, IR spectrum and elemental analysis. The in vitro bioassay results indicated that some target compounds exhibited excellent fungicidal activities, and the position of the substituents played an important role in fungicidal activities. Especially, compound 5n, exhibited better fungicidal activities than the commercial fungicide flutolanil against two tested fungi Valsa Mali and Sclerotinia sclerotiorum, with EC₅₀ values of 3.44 and 2.63 mg/L, respectively. And it also displayed good in vivo fungicidal activity against S. sclerotiorum with the EC₅₀ value of 29.52 mg/L.

Full text of DOI:10.1016/j.bmcl.2016.03.085

Bioorganic & Medicinal Chemistry Letters 26 (2016) 2544–2546
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design, synthesis and fungicidal activity of N-substituted
benzoyl-1,2,3,4-tetrahydroquinolyl-1-carboxamide
Peng Lei a, Yan Xu a, Juan Du a, Xin-Ling Yang a, Hui-Zhu Yuan b, Gao-Fei Xu a, Yun Ling a,
⇑
a Department of Applied Chemistry, College of Science, China Agricultural University, Beijing 100193, China
b Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, China
a r t i c l e i n f o
a b s t r a c t
Article history:
To find a new lead compound with high biological activity, a series of N-substituted benzoyl-1,2,3,4-
tetrahydroquinolyl-1-carboxamide were designed using linking active substructures method. The target
compounds were synthesized from substituted benzoic acid by four steps and their structures were con-
firmed by 1H NMR, IR spectrum and elemental analysis. The in vitro bioassay results indicated that some
target compounds exhibited excellent fungicidal activities, and the position of the substituents played an
important role in fungicidal activities. Especially, compound 5n, exhibited better fungicidal activities
than the commercial fungicide flutolanil against two tested fungi Valsa mali and Sclerotinia sclerotiorum,
with EC50 values of 3.44 and 2.63 mg/L, respectively. And it also displayed good in vivo fungicidal activity
against S. sclerotiorum with the EC50 value of 29.52 mg/L.
Received 3 November 2015
Revised 3 March 2016
Accepted 25 March 2016
Available online 25 March 2016
Keywords:
1,2,3,4-Tetrahydroquinoline
Carboxamide
Synthesis
Fungicidal activities
Ó 2016 Published by Elsevier Ltd.
Using natural products as ideal lead compounds is one of the
most effective methods to develop new pesticides with novel
structures and mechanisms.1,2 1,2,3,4-tetrahydroquinoline (THQ)
derivatives, widely existing in natural products,3,4 have been stud-
ied extensively because of their good bioactivities, such as antitu-
mor activity,5–7 antimalarial activity,8 fungal chitin synthase
inhibitory activity.9,10 For instance, aspernigerin (Fig. 1), a novel
alkaloid consisting of two THQs structures isolated from the
extract of a culture of Aspergillus niger IFB-E003 in 2006, has been
shown to be cytotoxic to the tumor cell lines and is a potential
medicine or lead compound.11 In our previous studies, we also
found that aspernigerin exhibited certain fungicidal activities,
insecticidal activities and herbicidal activities.12 Angustureine,
galipeine, cuspareine and galipinine (Fig. 1), isolated from the
Galipea officinalis Hancock, the bark of the South American angos-
tura tree, were reported to have good in vitro antimalarial activity,
antifungal activities. Especially, compounds I-01 and I-02 dis-
played excellent inhibition activities of 92.5% and 91.6% against
Helminthosporium maydis, respectively, as compared to the com-
mercial fungicide carbendazim.21
Introducing active groups of natural products is an effective
method to develop new eco-friendly agrochemicals. As above men-
tioned, THQ is an important and active group widely existing in
natural products. To find a new lead compound with high biologi-
cal activity, we introduced THQ into the benzamide moiety,
designed and synthesized a series of N-substituted benzoyl-
1,2,3,4-tetrahydroquinolyl-1-carboxamide compounds, and the
design idea is shown in Scheme 1. Their activities were evaluated
in this Letter.
O
with IC50 values of 0.09–38
lg/mL on the chloroquine-resistant
N
N
strains.13,14
N
N
O
N
Benzamide moiety is an important functional group in both
medicinal chemistry and agrochemicals.15–17 Flutolanil, mepronil,
benodanil, mebenil, fluopyram, and fluopicolide (Fig. 2) are known
as the commercial fungicides.18,19 Researchers have developed an
interest in this field because of their excellent bioactivities until
today.20 For instance, a series of 2-(acetoxy)-benzamide deriva-
tives (Fig. 2), reported by Xiaorong Tang in 2014, exhibited good
Aspernigerin
Angustureine
R
O
O
N
N
OCH3
Galipeine: R = OH
Cuspareine: R = OCH3
Galipinine
⇑
Corresponding author. Tel./fax: +86 10 62732223.
E-mail address: lyun@cau.edu.cn (Y. Ling).
Figure 1. The structures of natural products containing THQs.
http://dx.doi.org/10.1016/j.bmcl.2016.03.085
0960-894X/Ó 2016 Published by Elsevier Ltd.

P. Lei et al. / Bioorg. Med. Chem. Lett. 26 (2016) 2544–2546
2545
Cl
O
Cl
O
R2
N
N
H
R1
H
N
Cl
CF3
Flutolanil: R1 = 2-CF3, R2 = 3-OCH(CH3)2
Mepronil: R1 = 2-CH3, R2 = 3-OCH(CH3)2
Benodanil: R1 = 2-I, R2 = H
Fluopicolide
R
Mebenil: R1 = 2-CH3, R2 = H
O
Cl
CF3
N
H
CF3
O
O
N
H
N
O
Fluopyram
I-01: R = H
I-02: R = Cl
Figure 3. The X-ray crystal structure of 5r.
Figure 2. The structures of compounds containing benzamide.
mycelium growth rate method.23 Flutolanil (Fig. 2), a commercial
benzamide fungicide, and pyraclostrobin were assessed under
the same conditions as positive controls. The bioassay results
(Table 1) indicated that about half of the target compounds
showed great fungicidal activities against both two tested fungi
with inhibition rate of more than 80%. For V. mali, there are nine
compounds (5b, 5c, 5e, 5h, 5i, 5l, 5m, 5n and 5q) displayed equal
or higher activities than the commercial fungicide flutolanil
(81.5%) at 50 mg/L, especially compound 5n showed the best activ-
ity (94.8%) of all target compounds. Structure–activity analysis
revealed that the position of the substituent R played an important
role in fungicidal activities. Meta-position and para-position of the
benzene ring exhibited equivalent fungicidal activities, however,
better than the ortho-position of the benzene ring. Meta-position
and para-position were beneficial to increase their fungicidal activ-
ities, compound 5b (R = 3-F, 81.2%) and 5c (R = 4-F, 83.6%) dis-
played the similar activities. However ortho-position compound
5a (R = 2-F, 65.8%) showed lower activity than 5b and 5c. The same
rule could also be found when R was other substituent (5e, 5f > 5d;
5h, 5i > 5g; 5k, 5l > 5j). For S. sclerotiorum, two compounds (5n
95.4%, and 5r 98.1%) showed higher activities than the commercial
fungicide flutolanil (95.0%) at 50 mg/L. Generally, the fungicidal
activities, when the substituents R were two chlorines, were better
than the single chlorine substituent. The activities of 5p, 5q, 5r, 5s
were 91.2%, 93.9%, 98.1% and 93.9% at 50 mg/L, respectively, which
were higher than compounds 5d, 5e and 5f (79.1%, 88.1% and
87.0%, respectively).
These compounds with inhibitory activities over 80% were cho-
sen to determine EC50 values. The results were shown in Table 1.
Three compounds (5h, 5l and 5n) against V. mali, and two com-
pounds (5n and 5p) against S. sclerotiorum, showed lower EC50 val-
ues than the commercial fungicide flutolanil. Especially, compound
5n (R = 4-t-Bu) exhibited the best activity among all the target
compounds against both fungi V. mali and S. sclerotiorum, with
EC50 values of 3.44 and 2.63 mg/L, respectively. Unfortunately, all
the compounds showed weaker activities than the other commer-
cial fungicide pyraclostrobin. However, they had the advantages of
simple structures and easy synthesis. The most promising com-
pound 5n was chosen to be evaluated its in vivo fungicidal activity
against S. sclerotiorum using the reported method.24 As shown in
H
N
O
O
O
R2
N
H
N
N
H
R
R1
Target compounds
Scheme 1. The design idea of the target compounds.
Fungicide
Scheme 2 is the synthetic method of compounds 5a–5t. The tar-
get compounds were synthesized from substituted benzoic acid in
four steps and their structures were confirmed by 1H NMR, IR spec-
tra and elemental analysis (Supporting information).22 In order to
determine the stereo structure of the target compounds, a single
crystal of the target compound 5r (R = 2,5-Cl2) was prepared. The
single crystal was obtained by slow evaporation of a solution of
compound 5r in ethyl acetate at room temperature. As shown in
ꢀ
Figure 3, the crystal data for 5r: triclinic, space group P1,
a = 7.9904(7) Å, b = 9.6230(8) Å, c = 10.8411(10) Å,
b = 70.970(8)°,
= 87.890(7)°, V = 781.62(12) Å3, Z = 2, T = 180.01
(10) K, (MoK
) = 0.426 mmÀ1, Dcalcd = 1.484 g/mm3, 5269 reflec-
tions measured (6.2 6 2 6 50.042), 2770 unique (Rint = 0.0371)
which were used in all calculations. The final R1 was 0.0422
(I > 2 (I)) and wR2 was 0.1052. Crystallographic data have been
a = 82.689(7)°,
c
l
a
H
r
deposited with the Cambridge Crystallographic Data Centre, and
the deposition number was CCDC 1450032.
Compound 5n (R = 4-t-Bu), exhibited good activities, was taken
as an example to analyze the 1H NMR spectra data. A single peak at
d 1.32 ppm was due to the protons of the C(CH3)3. Two protons of
the 3-position of THQ were observed at d 1.99–2.06 ppm as multi-
ple peaks. The chemical shifts as triple peaks were observed at d
2.80 ppm with J = 6.6 Hz and 3.84 ppm with J = 6.5 Hz due to the
protons of 4-position and 2-position of THQ, respectively. Eight
protons of the benzene ring were showed at d 7.12–7.66 ppm. A
single peak at d 8.37 ppm was due to the proton of the NH.
The in vitro fungicidal activities of all the target compounds
have been determined against two pathogenic fungi, Valsa mali
and Sclerotinia sclerotiorum, at a concentration of 50 mg/L, using
O
O
N
N
H
R
R
R
R
a
c
b
d
R
COOH
COCl
CONCO
CONH2
1
2
3
4
5
5a: R = 2-F 5d: R = 2-Cl 5g: R = 2-Br 5j: R = 2-CH3
5b: R = 3-F 5e: R = 3-Cl 5h: R = 3-Br 5k: R = 3-CH3 5n: R = 4-t-Bu 5q: R = 2,4-Cl2 5t: R = 2,6-F2
5c: R = 4-F 5f: R = 4-Cl 5i : R = 4-Br 5l : R = 4-CH3 5o: R = 4-NO2 5 r: R = 2,5-Cl2
5m: R = 4-CF3 5p: R = 2,3-Cl2 5s: R = 2,6-Cl2
Scheme 2. Synthetic route of the target compounds. Reagents and conditions: (a) SOCl2, toluene, reflux, 3 h; (b) aqua ammoniae, 0–10 °C, 1 h; (c) (COCl)2, 1,2-dichloroethane,
reflux, 6 h; (d) 1,2,3,4-tetrahydroquinoline, toluene, reflux, 3 h.

2546
P. Lei et al. / Bioorg. Med. Chem. Lett. 26 (2016) 2544–2546
Table 1
Acknowledgments
The in vitro antifungal activities of 5a–5t against Valsa mali and Sclerotinia
sclerotiorum
We acknowledge the financial support from the National Natu-
ral Science Foundation of China (No. 21272266 and No. 21472236).
We also give special thanks to Prof. Baoju Li and Mr. Xuewen Xie
for their kind help on the in vivo bioassay for compound 5n against
S. sclerotiorum.
Compd.
R.
V. mali
S. sclerotiorum
Inhibition
EC50
(mg/L)
Inhibition
EC50
(mg/L)
ratea (%)
ratea (%)
b
5a
2-F
65.8
81.2
83.6
51.5
82.3
79.3
60.8
87.5
86.8
38.1
78.9
87.2
83.1
94.8
70.0
78.9
81.5
77.3
50.0
43.5
81.5
98.6
NT
78.2
74.0
57.9
79.1
88.1
87.0
83.9
86.6
88.1
67.1
87.0
75.1
79.2
95.4
69.4
91.2
93.9
98.1
93.9
63.7
95.0
100
NT
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
5q
3-F
4-F
2-Cl
3-Cl
4-Cl
2-Br
3-Br
4-Br
2-CH3
3-CH3
4-CH3
4-CF3
4-t-Bu
4-NO2
2,3-Cl2
2,4-Cl2
2,5-Cl2
2,6-Cl2
2,6-F2
—
22.07
20.70
NT
13.38
NT
NT
NT
NT
Supplementary data
5.36
13.65
8.51
8.33
8.34
NT
13.62
NT
NT
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2016.03.
085.
NT
7.50
11.65
NT
References and notes
NT
8.84
12.71
3.44
NT
NT
15.10
NT
NT
NT
9.39
0.01
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NT
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9.25
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5r
5s
5t
Flutolanil
Pyraclostrobin
4.61
0.42
—
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Table 2
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EC50 (mg/L)
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Compd.
EC50 (mg/L)
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Pyraclostrobin
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Table 2, compound 5n still displayed good in vivo fungicidal activ-
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was, however, weaker than that of the commercial fungicide
pyraclostrobin.
In summary, a series of N-substituted benzoyl-1,2,3,4-tetrahy-
droquinolyl-1-carboxamide were designed, synthesized and their
structures were confirmed by 1H NMR, IR spectrum and elemental
analysis. This series of compounds exhibited excellent in vitro
fungicidal activities. It is observed that the position of the
substituents played an important role in fungicidal activities. Of
particular interesting, compound 5n, exhibited better in vitro
fungicidal activities than flutolanil against both two tested fungi
V. mali and S. sclerotiorum, with EC50 values of 3.44 and 2.63
mg/L, respectively. And it also displayed good in vivo fungicidal
activity against S. sclerotiorum with the EC50 value of 29.52 mg/L.
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