Design and synthesis of n-phenyl phthalimides as potent protoporphyrinogen oxidase inhibitors

Article abstract of DOI:10.3390/molecules24234363

Protoporphyrinogen oxidase (PPO) has been identified as one of the most promising targets for herbicide discovery. A series of novel phthalimide derivatives were designed by molecular docking studies targeting the crystal structure of mitochondrial PPO from tobacco (mtPPO, PDB: 1SEZ) by using Flumioxazin as a lead, after which the derivatives were synthesized and characterized, and their herbicidal activities were subsequently evaluated. The herbicidal bioassay results showed that compounds such as 3a (2-(4-bromo-2,6-difluorophenyl)isoindoline-1,3-dione), 3d (methyl 2-(4-chloro-1,3-dioxoisoindolin-2-yl)-5-fluorobenzoate), 3g (4-chloro-2-(5-methylisoxazol-3-yl) isoindoline-1,3-dione), 3j (4-chloro-2-(thiophen-2-ylmethyl)isoindoline-1,3-dione) and 3r (2-(4-bromo-2,6-difluorophenyl)-4-fluoroisoindoline-1,3-dione) had good herbicidal activities; among them, 3a showed excellent herbicidal efficacy against A. retroflexus and B. campestris via the small cup method and via pre-emergence and post-emergence spray treatments. The efficacy was comparable to that of the commercial herbicides Flumioxazin, Atrazine, and Chlortoluron. Further, the enzyme activity assay results suggest that the mode of action of compound 3a involves the inhibition of the PPO enzyme, and 3a showed better inhibitory activity against PPO than did Flumioxazin. These results indicate that our molecular design strategy contributes to the development of novel promising PPO inhibitors.

Full text of DOI:10.3390/molecules24234363

molecules
Article
Design and Synthesis of N-phenyl Phthalimides as
Potent Protoporphyrinogen Oxidase Inhibitors
Wei Gao, Xiaotian Li, Da Ren, Susu Sun, Jingqian Huo, Yanen Wang, Lai Chen *
and Jinlin Zhang *
Plant Protection College, Hebei Agricultural University, Baoding 071000, China; hbaugaow@163.com (W.G.);
kddanongyao@163.com (X.L.); 18233470747@163.com (D.R.); susu8023d@163.com (S.S.);
huojingqian@163.com (J.H.); yanenwang@163.com (Y.W.)
* Correspondence: chenlai@hebau.edu.cn (L.C.); zhangjinlin@hebau.edu.cn (J.Z.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Received: 19 November 2019; Accepted: 27 November 2019; Published: 29 November 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Protoporphyrinogen oxidase (PPO) has been identified as one of the most promising
targets for herbicide discovery. A series of novel phthalimide derivatives were designed by
molecular docking studies targeting the crystal structure of mitochondrial PPO from tobacco (mtPPO,
PDB: 1SEZ) by using Flumioxazin as a lead, after which the derivatives were synthesized and
characterized, and their herbicidal activities were subsequently evaluated. The herbicidal bioassay
results showed that compounds such as 3a (2-(4-bromo-2,6-difluorophenyl)isoindoline-1,3-dione),
3d (methyl 2-(4-chloro-1,3-dioxoisoindolin-2-yl)-5-fluorobenzoate), 3g (4-chloro-2-(5-methylisoxazol-3-yl)
isoindoline-1,3-dione), 3j (4-chloro-2-(thiophen-2-ylmethyl)isoindoline-1,3-dione) and 3r (2-(4-bromo-
2,6-difluorophenyl)-4-fluoroisoindoline-1,3-dione) had good herbicidal activities; among them,
3a showed excellent herbicidal efficacy against A. retroflexus and B. campestris via the small cup method
and via pre-emergence and post-emergence spray treatments. The efficacy was comparable to that of
the commercial herbicides Flumioxazin, Atrazine, and Chlortoluron. Further, the enzyme activity
assay results suggest that the mode of action of compound 3a involves the inhibition of the PPO
enzyme, and 3a showed better inhibitory activity against PPO than did Flumioxazin. These results
indicate that our molecular design strategy contributes to the development of novel promising
PPO inhibitors.
Keywords:
protoporphyrinogen oxidase; molecular design; N-phenyl phthalimides;
herbicidal activity
1. Introduction
Protoporphyrinogen oxidase (PPO) is the last common enzyme in both chlorophyll (in plants)
and haem (in animals) biosynthesis [
protoporphyrin IX via molecular oxygen [
most significant targets for herbicide research [10
compounds inhibiting the enzyme PPO have been synthesized [12
1
–
4
], catalyzing the oxidation of protoporphyrinogen IX to
], and this enzyme has been identified as one of the
11]. During the last thirty years, a number of active
13], some of which have been
5–9
,
,
developed for use as low-toxicity, efficient, broad-spectrum commercial herbicides [14,15], such as
Flumioxazin [16,17], sulfentrazone [18] and saflufenacil [19].
Among the protox herbicides, N-phenyl phthalimides, which exhibit broad structural diversity [20],
have attracted considerable attention; their representative commercial products, Cinidon-ethyl,
Flumiclorac-pentyl and Flumioxazin, were identified as a result of a stepwise optimization procedure
from Chlorphthalim. Their common structural feature consists of two parts: An N-substituted
phenyl group and tetrahydrophthalimide, which can interact with key active centre residues of the
Molecules 2019, 24, 4363; doi:10.3390/molecules24234363
www.mdpi.com/journal/molecules

Molecules 2019, 24, 4363
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mtPPO enzyme, such as Arg98, Gly175, Leu372, Phe392, and FAD600, via H-bonding or
π
–π
stacking
interactions [21]. These findings have contributed to the development of PPO inhibitors.
To obtain novel PPO inhibitors, a series of N-phenyl phthalimides were designed by molecular
docking using the mtPPO as a target, and by using Flumioxazin as a lead (Figure 1) [22 23],
,
after which they were synthesized and characterized by NMR and High resolution mass spectrometry
(HR-MS). Their herbicidal activities were also evaluated against Brassica campestris (B. campestris),
Amaranthus retroflexus (A. retroflexus) and Digitaria sanguinalis (D. sanguinalis) to verify our molecular
design strategy.
Figure 1. Design of the title compounds.
2. Results and Discussion
2.1. Docking Analysis
The affinity values of molecular docking between the ligand-mtPPO complexes were determined,
which demonstrated that the affinity between 3a, 3c and 3s and mtPPO exhibited the highest values with
−
10.0, 10.1, 10.2 Kcal/mol (Table 1), respectively, suggesting that these compounds may have good
herbicidal activity. Their docking models were displayed by Pymol software (Figure 2), which indicated
−
that no hydrogen bonding was found between these compounds and mtPPO, which differs from the
results of Flumioxazin (three hydrogen bonds). However, the phthalimide rings of compounds 3a
3c and 3s shape stacking interactions with Phe392, which is conserved in plant PPO enzymes.
The phenyl rings of compounds 3a 3c and 3s were sandwiched by the residues Leu356 and Leu372.
,
π-π
,
These results suggest that these compounds, with a phenyl ring substituting phthalimide rings, could be
PPO inhibitors that exhibit good herbicidal activity.
Table 1. The affinity between compounds and mtPPO. (Affinity, Kcal/mol).
Compd.
Affinity
Compd.
Affinity
Compd.
Affinity
Compd.
Affinity
3a
3b
3c
3d
3e
3f
−10.0
−8.6
−10.1
−9.1
−9.8
−9.7
3h
3i
3j
3k
3l
3m
−8.7
−8.4
−7.7
−9.4
−8.9
−9.9
3o
3p
3q
3r
3s
3t
−9.5
−8.7
−9.3
−9.0
−10.2
−9.7
3v
3w
3x
−8.1
−7.7
−9.8
−9.5
−9.5
−7.1
3y
Flumioxazin
Chlortoluron

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Figure 2. Docking model shown in Pymol, molecule (blade yellowing) and residues of mtPPO
(wrinkling); Flumioxazin (A), 3a (B), 3c (C), and 3s (D).
2.2. Chemistry
The starting material
nucleophilic substitution reaction between phthalic anhydrides
1
and
2
could be commercially available. Compounds
3 were prepared by
1
and amines in glacial acetic acid,
2
with yields ranging from 28% to 87% (Table 2). The reaction solvent and temperature were the key
conditions for the reaction yield. The yield of the reaction stirred at 110 ◦C was much better than that
of 80 ◦C. Besides, the yield of the reaction in glacial acetic acid was higher than that of other solvents,
such as ethanol.

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Table 2. General Synthetic Route for Compounds 3a–3y.
Compd. R1
R2
Ar
Yield (%)
Compd. R1
R2
Ar
Yield (%)
Compd. R1
R2
Ar
Yield (%)
3a
H
H
73
3j
Cl
H
71
3s
F
H
79
3b
3c
H
Cl
H
77
81
3k
3l
Cl
Cl
H
H
63
86
3t
H
H
Cl
Cl
79
70
F
O
Cl
3u
3d
3e
Cl
Cl
H
H
28
80
3m
3n
Cl
Cl
H
H
84
52
3v
H
H
Cl
Cl
76
79
3w
3f
Cl
Cl
H
H
78
58
3o
3p
Cl
Cl
H
H
87
30
3x
3y
H
H
CH3
NO2
47
57
3g
3h
3i
Cl
Cl
H
H
60
67
3q
3r
F
F
H
H
57
78

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2.3. Herbicidal Activity
The herbicidal activity against B. campestris, A. retroflexus and D. sanguinalis at 200 mg/L of
compound were evaluated by the small cup method, and the results are shown in Table 3. The data
3
showed that 3a at 200 mg/L displayed 92% growth inhibition against B. campestris roots, which was
better than that of the positive controls Chlortoluron (85%), Atrazine (80%), and Flumioxazin (85%);
3a also showed 61% growth inhibition against stems of B. campestris, which was better than that of
Atrazine (51%) and similar to that of Chlortoluron (58%). During the test, the B. campestris plants
treated with 3a became chlorotic, and blade yellowing was also observed. In addition, with 87% growth
inhibition, 3a exhibited better efficacy against A. retroflexus stems than did Atrazine, which was similar
to that of Flumioxazin (88%); the leaves of A. retroflexus were wrinkled. Furthermore, 3a displayed
68% and 83% growth inhibition against the roots and stems of D. sanguinalis, respectively, which were
better than the inhibition due to Atrazine. Compounds 3d and 3o exhibited high activity against the
roots of A. retroflexus, with 81% and 83% growth inhibition, respectively, while Atrazine showed only
32% inhibition under the same conditions; 3o also showed 89% growth inhibition activity against B.
campestris roots, which was slightly better than that of the three positive controls. 3g showed excellent
efficacy against D. sanguinalis roots and stems, with 91% and 83% growth inhibition, respectively,
these percentages were better than those due to Atrazine.
Table 3. Herbicidal activity of compounds 3a–3y at 200 mg/L by the small cup method.
BC a
AR a
DS a
Compd.
Root
Stem
Root
Stem
Root
Stem
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
3t
3u
3v
3w
3x
3y
92 ± 1
74 ± 2
78 ± 5
82 ± 1
28 ± 2
46 ± 2
0
61 ± 2
37 ± 1
28 ± 4
65 ± 1
81 ± 4
51 ± 1
68 ± 0
74 ± 2
62 ± 2
55 ± 1
66 ± 1
53 ± 3
44 ± 3
55 ± 4
59 ± 2
83 ± 5
37 ± 3
22 ± 1
62 ± 2
35 ± 4
16 ± 1
0
87 ± 2
66 ± 4
22 ± 1
33 ± 2
55 ± 3
42 ± 0
41 ± 2
33 ± 1
27 ± 2
34 ± 1
36 ± 2
31 ± 2
45 ± 3
33 ± 2
51 ± 3
72 ± 4
21 ± 2
75 ± 3
61 ± 5
39 ± 4
53 ± 2
42 ± 3
61 ± 2
55 ± 3
60 ± 4
90 ± 5
66 ± 2
88 ± 1
68 ± 3
77 ± 2
61 ± 1
65 ± 2
28 ± 2
47 ± 3
91 ± 1
55 ± 1
67 ± 5
87 ± 2
57 ± 1
48 ± 4
57 ± 2
59 ± 4
63 ± 4
58 ± 1
38 ± 3
87 ± 2
53 ± 3
46 ± 2
51 ± 2
22 ± 1
79 ± 2
31 ± 2
44 ± 1
98 ± 0
58 ± 1
71 ± 2
83 ± 2
62 ± 3
0
10 ± 1
38 ± 3
0
83 ± 1
34 ± 1
17 ± 3
11 ± 0
0
0
0
35 ± 1
21 ± 2
73 ± 3
18 ± 0
0
0
0
0
77 ± 3
58 ± 4
75 ± 0
65 ± 1
68 ± 2
45 ± 3
49 ± 1
89 ± 1
51 ± 2
28 ± 1
65 ± 4
64 ± 2
54 ± 1
77 ± 1
60 ± 5
58 ± 2
59 ± 4
68 ± 2
85 ± 4
81 ± 1
85 ± 3
a
6 ± 2
51 ± 3
0
0
0
0
0
77 ± 0
9 ± 2
22 ± 1
54 ± 3
15 ± 1
0
45 ± 2
0
86 ± 2
28 ± 5
18 ± 1
32 ± 1
21 ± 2
11 ± 2
24 ± 1
37 ± 3
97 ± 1
60 ± 2
91 ± 0
53 ± 3
0
10 ± 2
28 ± 1
36 ± 3
34 ± 4
92 ± 3
32 ± 2
82 ± 2
0
0
9 ± 0
58 ± 3
52 ± 1
72 ± 5
Chlortoluron
Atrazine
Flumioxazin
BC for B. campestris; AR for A. retroflexus; DS for D. sanguinalis.
The results of the evaluation of the post-emergence herbicidal activities are shown in Table 4.
The data showed that compounds 3a and 3d exhibited 82% and 73% fresh weight growth inhibition
at 90 g ai/ha against A. retroflexus, respectively. Therefore, 3a and 3d were also chosen for further

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pre-emergence herbicidal activity tests. As shown in Table 5, 3a at 90 g ai/ha exhibited a 98% inhibitory
effect against A. retroflexus, and the effects were not significantly different from those of Flumioxazin.
Table 4. Post-emergence herbicidal activity of compounds 3a–3y at 90 g ai/ha.
Compd.
BC
AR
DS
Compd.
BC
AR
DS
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
25 ± 2
27 ± 2
48 ± 1
60 ± 3
35 ± 4
56 ± 1
38 ± 2
58 ± 1
28 ± 2
23 ± 2
35 ± 5
33 ± 1
22 ± 4
25 ± 2
82 ± 3
18 ± 2
13 ± 1
73 ± 2
47 ± 3
10 ± 2
0
37 ± 1
12 ± 1
19 ± 3
20 ± 1
33 ± 1
11 ± 1
23 ± 2
27 ± 2
37 ± 1
31 ± 1
0
3o
3p
3q
3r
3s
3t
3u
3v
3w
16 ± 2
19 ± 4
30 ± 2
37 ± 1
22 ± 1
29 ± 2
60 ± 2
33 ± 1
35 ± 1
17 ± 2
28 ± 1
nd a
27 ± 2
20 ± 1
19 ± 2
22 ± 3
44 ± 2
62 ± 3
5 ± 1
0
22 ± 4
7 ± 2
0
91 ± 1
91 ± 2
92 ± 4
23 ± 2
14 ± 0
36 ± 1
36 ± 5
18 ± 1
17 ± 1
28 ± 2
0
36 ± 4
7 ± 1
0
65 ± 2
47 ± 3
86 ± 6
7 ± 1
34 ± 2
20 ± 5
24 ± 3
37 ± 2
35 ± 1
30 ± 5
3x
3y
26 ± 2
34 ± 2
11 ± 1
a
Chlortoluron
Atrazine
Flumioxazin
nd a
85 ± 6
nd, not detect.
Table 5. Per-emergence herbicidal activity of compounds 3a, 3d and Flumioxazin at 90 g ai/ha.
Compd.
AR
DS
98 ± 2
36 ± 3
100
61 ± 2
3a
3d
0
Flumioxazin
100
In summary, 3a exhibited excellent herbicidal activity and should be further developed. Moreover,
its herbicidal activity conformed to the predictions of the molecular docking studies.
2.4. Crystal Structure Determination of Compound 3a
The X-ray diffraction structure of compound 3a, cultured from the mixture of ethanol and
chloroform, was shown in Figure 3, and the data were available at the Cambridge Crystallographic
Data Centre (CCDC 1923372).
Figure 3. Crystal structure for 3a by X-ray diffraction determination.
2.5. PPO Enzyme Assays
The PPO enzyme activity of D. sanguinalis, treated with 3a and Flumioxazin at 90 g ai/ha,
were measured, and the control was sprayed with blank solution without any compounds. The results,
shown in Figure 4, indicated that the PPO activity values of the 3a treatment and the control Flumioxazin
were affected; in addition, 3a showed a stronger effect on the PPO enzyme, with 33.01%, compared to
that of the positive control Flumioxazin, with 21.80%.

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Figure 4. PPO enzyme activity involved by 3a and Flumioxazin. (The control was sprayed with blank
solution without any compounds; the 3a and Flumioxazin were sprayed with 3a and Flumioxazin at
90 g ai/ha, respectively).
3. Materials and Methods
3.1. Molecular Docking
The structure of mtPPO was available at the National Center for Biotechnology Information
(NCBI) database. The modelled complexes of mtPPO and ligands (Flumioxazin, Chlorotoluron,
Atrazine and designed target compounds) were prepared by using AutoDockTools 1.5.6 (Molecular
Graphics Laboratory, La Jolla, CA 92037-1000, USA) and analysed via the AutoDock Vina program
(Molecular Graphics Laboratory, La Jolla, CA 92037-1000, USA) [24
were generated by the Pymol tool 2.2.0 [26,27].
,25]. The structures of the complexes
3.2. Equipment and Materials
The melting points of the new compounds were measured in a microfusion melting point apparatus
1
(X-4) (Taike, Beijing, China) and uncorrected. H-NMR and 13C-NMR spectra were recorded on Varian
400 spectrometer at 400 MHz and 101 MHz using tetramethylsilane as internal standard (solvent CDCl3
or DMSO-d6). HR-MS date were detected on an FTICR-MS Varian 7.0T FTICR-MS equipment (Agilent,
Lexington, MA, USA). Crystal structure was recorded on a Bruker SMART 1000CCD diffraction meter.
3.3. General Synthetic Procedure for Compounds 3
The starting materials
1
and
2 were commercially available (Energy Chemical, Shanghai, China).
Compound
2
(3.72 mmol) was added to a stirred solution of compound
1 (3.38 mmol) in glacial acetic
acid (10 mL). The reaction mixture was then stirred at 110 ◦C for 4 h. After completion of the reaction,
the solvent was evaporated, and the residue was purified on a silica gel column chromatography and
eluted with ethyl acetate/petroleum ether (bp 60–90 ◦C) (1:3, v/v) to give compounds
3. Among them,
compounds 3k
,
3l
,
3n
,
3x and 3y were published [28
–
32], while their herbicidal activities were
1
not studied. Their yields, physical properties, H-NMR, 13C-NMR, and HR-MS results are shown
as follows:
Data for 3a (2-(4-bromo-2,6-difluorophenyl)isoindoline-1,3-dione): white solid; yield, 73%; m.p.:
◦
1
175–176 C; H-NMR (400 MHz, CDCl3)
δ
7.98 (dd, J = 5.5, 3.1 Hz, 2H), 7.83 (dd, J = 5.5, 3.1 Hz,
165.41 (s), 160.09 (s), 157.47 (s), 134.73 (s),
2H), 7.29 (d, J = 6.7 Hz, 2H).13C-NMR (101 MHz, CDCl3)
δ
131.93 (s), 124.22 (s), 123.29 (s), 116.52 (s), 116.25 (s). HR-MS (ESI) [M + H]+ calcd for C14H6BrF2NO2:
337.9500, found: 337.9621.
Data for 3b (5-chloro-2-(5-methylisoxazol-3-yl)isoindoline-1,3-dione): white solid; yield, 77%;
m.p.: 164–165 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.95 (dd, J = 12.4, 4.8 Hz, 2H), 7.79 (dd, J = 8.0, 1.7 Hz,
1H), 6.48 (s, 1H), 2.51 (s, 3H). 13C-NMR (101 MHz, CDCl3)
δ
170.98 (s), 163.90 (s), 163.65 (s), 153.28

Molecules 2019, 24, 4363
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(s), 141.86 (s), 135.13 (s), 133.13 (s), 129.55 (s), 125.56 (s), 124.67 (s), 97.96 (s), 12.73 (s). HR-MS (ESI)
[M + H]+ calcd for C12H7N2O3: 263.0145, found: 263.0223.
Data for 3c (4-chloro-2-(3-fluoro-4-methylphenyl)isoindoline-1,3-dione): white solid; yield, 81%;
m.p.: 197–198 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.95–7.80 (m, 1H), 7.79–7.60 (m, 2H), 7.30 (d, J = 8.1 Hz,
1H), 7.14 (t, J = 8.3 Hz, 2H), 2.32 (d, J = 7.1 Hz, 3H). 13C-NMR (101 MHz, CDCl3)
δ
165.46 (s), 162.00 (s),
159.55 (s), 136.11 (s), 135.23 (s), 133.58 (s), 131.88 (s), 131.49 (s), 129.90 (s), 127.14 (s), 125.23 (s), 122.18 (s),
121.74 (s), 113.59 (s), 14.27 (s). HR-MS (ESI) [M + H]+ calcd for C15H9ClFNO2: 290.0306, found: 290.0380.
Data for 3d (methyl 2-(4-chloro-1,3-dioxoisoindolin-2-yl)-5-fluorobenzoate): white solid; yield,
28%; m.p.: 136–137 ◦C; 1H-NMR (400 MHz, CDCl3)
(d, J = 4.6 Hz, 2H), 3.80 (s, 3H). 13C-NMR (101 MHz, CDCl3)
δ
7.89 (d, J = 3.8 Hz, 2H), 7.74 (d, J = 3.8 Hz, 2H), 7.40
165.96 (s), 164.93 (s), 163.85 (s), 160.96 (s),
δ
136.04 (s), 135.19 (s), 134.03 (s), 132.32 (s), 131.86 (s), 129.68 (s), 127.72 (s), 127.45 (s), 122.25 (s), 120.48 (s),
118.88 (s), 52.62 (s). HR-MS (ESI) [M + H]+ calcd for C16H9ClFNO4: 334.0204, found: 334.0273.
Data for 3e (2-(4-bromo-2,6-difluorophenyl)-4-chloroisoindoline-1,3-dione): white solid; yield,
◦
1
80%; m.p.: 185–186 C; H-NMR (400 MHz, CDCl3)
δ
7.94–7.82 (m, 2H), 7.74 (d, J = 3.8 Hz, 2H),
163.97 (s), 162.87 (s), 159.98 (s), 157.41 (s),
7.45–7.35 (m, 2H), 3.80 (s, 3H).13C-NMR (101 MHz, CDCl3)
δ
136.47 (s), 135.60 (s), 133.91 (s), 132.39 (s), 127.72 (s), 123.57 (s), 122.67 (s), 116.55 (s), 116.28 (s), 108.22 (s).
HR-MS (ESI) [M + H]+ calcd for C14H5BrClF2NO2: 371.9160, found: 371.9232.
Data for 3f (4-chloro-2-(4-isopropylphenyl)isoindoline-1,3-dione): white solid; yield, 78%; m.p.:
154–155 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.89 (dd, J = 4.4, 3.9 Hz, 1H), 7.76–7.68 (m, 2H), 7.43–7.32 (m,
4H), 2.99 (s, 1H), 1.31 (d, J = 6.9 Hz, 6H). 13C-NMR (101 MHz, CDCl3)
δ
166.04 (s), 165.03 (s), 149.13 (s),
136.09 (s), 135.22 (s), 133.93 (s), 131.85 (s), 128.83 (s), 127.46 (s), 127.25 (s), 126.42 (s), 122.21 (s), 33.95 (s),
23.93 (s). HR-MS (ESI) [M + H]+ calcd for C17H14ClNO2: 300.0713, found: 300.0791.
Data for 3g (4-chloro-2-(5-methylisoxazol-3-yl)isoindoline-1,3-dione): White solid; yield, 58%;
m.p.: 160–161 ◦C; 1H-NMR (400 MHz, CDCl3)
δ 170.96 (s), 163.34 (s), 162.52 (s), 153.18 (s), 136.69 (s),
135.83 (s), 133.59 (s), 132.52 (s), 127.21 (s), 122.81 (s), 98.09 (s), 12.76 (s). HR-MS (ESI) [M + H]+ calcd for
C12H7ClN2O3: 263.0145, found: 263.0220.
δ
7.95–7.89 (m, 1H), 7.76 (d, J = 4.4 Hz, 2H), 6.49 (s,
1H), 2.52 (s, 3H). 13C-NMR (101 MHz, CDCl3)
Data for 3h (4-chloro-2-(1-methyl-1H-pyrazol-5-yl)isoindoline-1,3-dione): white solid; yield, 60%;
m.p.: 195–196 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.91 (dd, J = 4.6, 3.7 Hz, 1H), 7.81–7.74 (m, 2H), 7.60 (d,
J = 2.0 Hz, 1H), 6.33 (d, J = 2.0 Hz, 1H), 3.78 (s, 3H).13C-NMR (101 MHz, CDCl3)
δ
164.65 (s), 163.59 (s),
138.84 (s), 136.69 (s), 135.83 (s), 133.61 (s), 132.52 (s), 129.13 (s), 127.39 (s), 122.76 (s), 104.67 (s), 36.51 (s).
HR-MS (ESI) [M + H]+ calcd for C12H8ClN3O2: 262.0305, found: 262.0379.
Data for 3i (4-chloro-2-(3,4-dichlorobenzyl)isoindoline-1,3-dione): white solid; yield, 67%; m.p.:
143–144 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.78 (dd, J = 4.8, 3.4 Hz, 1H), 7.69–7.62 (m, 2H), 7.53 (d,
J = 2.0 Hz, 1H), 7.39 (d, J = 8.2 Hz, 1H), 7.30 (d, J = 2.0 Hz, 1H), 4.78 (s, 2H). 13C-NMR (101 MHz,
CDCl3) δ 166.32 (s), 165.47 (s), 135.99 (d, J = 4.7 Hz), 135.18 (s), 133.97 (s), 132.77 (s), 132.30 (s), 131.65 (s),
130.76 (d, J = 8.8 Hz), 128.30 (s), 127.63 (s), 122.05 (s), 40.70 (s). HR-MS (ESI) [M + H]+ calcd for
C15H8Cl3NO2: 339.9621, found: 339.9692.
Data for 3j (4-chloro-2-(thiophen-2-ylmethyl)isoindoline-1,3-dione): white solid; yield, 71%; m.p.:
135–136 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.78 (dd, J = 4.6, 3.6 Hz, 1H), 7.68–7.61 (m, 2H), 7.31–7.15
(m, 2H), 6.95 (dd, J = 5.1, 3.6 Hz, 1H), 5.03 (s, 2H).13C-NMR (101 MHz, CDCl3)
δ
166.09 (s), 165.20 (s),
137.56 (s), 135.81 (s), 135.03 (s), 134.09 (s), 131.54 (s), 128.08 (s), 127.74 (s), 126.95 (s), 126.08 (s), 121.95 (s),
35.82 (s). HR-MS (ESI) [M + H]+ calcd for C13H8ClNO2S: 277.9964, found: 278.0038.
Data for 3k (4-chloro-2-(3-fluoro-4-nitrophenyl)isoindoline-1,3-dione): brown solid, yield, 63%;
m.p.: 231–232 ◦C; 1H-NMR (400 MHz, DMSO)
δ
8.42 (dd, J = 9.7, 2.1 Hz, 1H), 8.31 (d, J = 8.7 Hz, 1H),
7.95 (ddd, J = 26.6, 16.0, 7.9 Hz, 4H).13C-NMR (101 MHz, DMSO)
δ
164.72 (s), 163.70 (s), 158.45 (s),
155.91 (s), 148.65 (s), 137.06 (s), 134.29 (s), 131.84 (s), 130.71 (s), 127.86 (s), 125.75 (s), 123.36 (s), 120.71 (s),
113.21 (s). HR-MS (ESI) [M + H]+ calcd for C14H6ClFN2O4: 321.0000, found: 321.0059.
Data for 3l (4-chloro-2-(4-phenoxyphenyl)isoindoline-1,3-dione): white solid; yield, 86%; m.p.:
172–173 ◦C; 1H-NMR (400 MHz, CDCl3)
δ 8.05–7.98 (m, 1H), 7.86 (d, J = 4.3 Hz, 2H), 7.52 (dd, J = 4.7,

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4.0 Hz, 4H), 7.35–7.14 (m, 5H).13C-NMR (101 MHz, CDCl3)
δ
165.97 (s), 164.98 (s), 157.39 (s), 156.44 (s),
136.16 (s), 135.29 (s), 133.86 (s), 131.92 (s), 129.94 (s), 128.07 (s), 127.40 (s), 125.97 (s), 123.97 (s), 122.25 (s),
119.60 (s), 118.81 (s). HR-MS (ESI) [M + H]+ calcd for C20H12ClNO3: 350.0506, found: 350.0580.
Data for 3m (3-(4-chloro-1,3-dioxoisoindolin-2-yl)benzamide): light yellow solid; yield, 84%; m.p.:
252–253 ◦C; 1H-NMR (400 MHz, DMSO)
13C-NMR (101 MHz, CDCl3)
δ
8.09 (s, 1H), 8.03–7.83 (m, 5H), 7.62 (s, 2H), 7.53 (s, 1H).
δ
172.19 (s), 170.82 (s), 169.85 (s), 141.41 (s), 141.12 (s), 140.34 (s), 139.16 (s),
136.99 (s), 135.44 (s), 135.05 (s), 134.05 (s), 132.53 (s), 132.23 (s), 132.10 (s), 127.60 (s). HR-MS (ESI)
[M + H]+ calcd for C15H9ClN2O3: 301.0302, found: 301.0380.
Data for 3n (2-(4-(tert-butyl)phenyl)-4-chloroisoindoline-1,3-dione): white solid; yield, 52%; m.p.:
178–179 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.93–7.82 (m, 1H), 7.70 (d, J = 3.8 Hz, 2H), 7.52 (d, J = 8.5 Hz,
2H), 7.35 (d, J = 8.4 Hz, 2H), 1.36 (s, 9H).13C-NMR (101 MHz, CDCl3)
δ
166.02 (s), 165.01 (s), 151.35 (s),
136.08 (s), 135.21 (s), 133.94 (s), 131.84 (s), 128.59 (s), 127.47 (s), 126.18 (s), 126.04 (s), 122.21 (s), 34.76 (s),
31.32 (s). HR-MS (ESI) [M + H]+ calcd for C18H16ClNO2: 314.0870, found: 314.0947.
Data for 3o (4-chloro-2-(3-fluoro-2-methoxyphenyl)isoindoline-1,3-dione): white solid; yield, 87%;
m.p.: 153–154 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
8.10–7.99 (m, 1H), 7.97–7.81 (m, 2H), 7.46–7.37 (m, 1H),
7.30 (td, J = 8.1, 5.2 Hz, 1H), 7.26–7.15 (m, 1H), 4.14 (d, J = 2.4 Hz, 3H).13C-NMR (101 MHz, CDCl3)
δ
165.63 (s), 164.59 (s), 156.89 (s), 154.43 (s), 144.39 (s), 136.11 (s), 135.25 (s), 134.10 (s), 131.93 (s), 127.76 (s),
125.01 (s), 123.03 (s), 122.32 (s), 118.40 (s), 61.42 (s). HR-MS (ESI) [M + H]+ calcd for C15H9ClFNO3:
306.0255, found: 306.0331.
Data for 3p (4-chloro-2-(5-methylthiazol-2-yl)isoindoline-1,3-dione): yellow solid; yield, 30%;
m.p.: 143–144 ◦C; 1H-NMR (400 MHz, CDCl3)
δ 163.32 (s), 162.62 (s), 150.64 (s), 150.45 (s), 136.77 (s), 135.92 (s),
133.30 (s), 132.62 (s), 126.87 (s), 122.81 (s), 113.01 (s), 17.48 (s). HR-MS (ESI) [M + H]+ calcd for
C12H7ClN2O2S: 278.9917, found: 278.9988.
δ 7.83 (dd, J = 63.0, 3.8 Hz, 3H), 6.93 (s, 1H), 2.52 (s,
3H).13C-NMR (101 MHz, CDCl3)
Data for 3q (methyl 5-fluoro-2-(4-fluoro-1,3-dioxoisoindolin-2-yl)benzoate): white solid; yield,
57%; m.p.: 179–180 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
8.09–7.65 (m, 3H), 7.56–7.29 (m, 3H), 3.79 (d,
J = 3.0 Hz, 3H). 13C-NMR (101 MHz, CDCl3)
δ 166.33 (s), 163.96 (s), 159.24 (s), 156.59 (s), 136.94 (s),
134.23 (s), 132.38 (s), 129.82 (s), 127.45 (s), 122.87 (s), 122.68 (s), 120.38 (s), 120.10 (s), 119.00 (s), 118.75 (s),
52.72 (s). HR-MS (ESI) [M + H]+ calcd for C16H9F2NO4: 318.0500, found: 318.0574.
Data for 3r (2-(4-bromo-2,6-difluorophenyl)-4-fluoroisoindoline-1,3-dione): shite solid; yield, 78%;
m.p.: 140–142 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.90–7.79 (m, 2H), 7.52 (t, J = 8.0 Hz, 1H), 7.32 (d,
J = 6.9 Hz, 2H). 13C-NMR (101 MHz, CDCl3)
δ 164.25 (s), 161.90 (s), 160.05 (s), 159.36 (s), 156.70 (s),
137.41 (s), 133.92 (s), 123.60 (s), 123.20 (s), 123.01 (s), 120.43 (s), 117.81 (s), 116.30 (s), 108.15 (s). HR-MS
(ESI) [M + H]+ calcd for C14H5BrF3NO2: 355.9456, found: 355.9522.
Data for 3s (4-fluoro-2-(4-isopropylphenyl)isoindoline-1,3-dione): shite solid; yield, 79%; m.p.:
116–117 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.68 (d, J = 2.4 Hz, 2H), 7.34 (ddd, J = 9.0, 6.1, 3.1 Hz, 1H),
7.25 (q, J = 8.6 Hz, 4H), 2.87 (dt, J = 13.8, 6.9 Hz, 1H), 1.19 (d, J = 6.9 Hz,6H).13C-NMR (101 MHz,
CDCl3)
δ 166.33 (s), 164.08 (s), 159.21 (s), 156.56 (s), 149.16 (s), 136.84 (s), 134.03 (s), 128.77 (s), 127.28 (s),
126.44 (s), 122.59 (s), 119.94 (s), 33.94 (s), 23.91 (s). HR-MS (ESI) [M + H]+ calcd for C17H14FNO2:
284.1009, found: 284.1086.
Data for 3t (5-chloro-2-(3-fluoro-4-methylphenyl)isoindoline-1,3-dione): shite solid; yield, 79%;
m.p.: 178–179 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.96–7.85 (m, 2H), 7.76 (dd, J = 8.0, 1.4 Hz, 1H), 7.31 (t,
J = 8.1 Hz, 1H), 7.14 (d, J = 8.8 Hz, 2H), 2.33 (s, 3H). 13C-NMR (101 MHz, CDCl3)
δ
166.07 (s), 165.76 (s),
162.14 (s), 141.29 (s), 134.61 (s), 133.29 (s), 131.68 (s), 129.67 (s), 125.10 (s), 124.26 (s), 121.79 (s), 113.65 (s),
113.40 (s), 99.99 (s), 14.41 (s). HR-MS (ESI) [M + H]+ calcd for C15H9ClFNO2: 290.0306, found: 290.0376.
Data for 3u (5-chloro-2-(4-isopropylphenyl)isoindoline-1,3-dione): white solid; yield, 70%; m.p.:
137–138 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.90 (dd, J = 13.7, 4.7 Hz, 2H), 7.74 (dd, J = 8.0, 1.8 Hz, 1H),
7.34 (d, J = 8.6 Hz, 4H), 2.97 (dt, J = 13.8, 6.9 Hz, 1H), 1.28 (d, J = 6.9 Hz, 6H).13C-NMR (101 MHz,
CDCl3) 166.50 (s), 166.19 (s), 149.16 (s), 141.06 (s), 134.41 (s), 133.50 (s), 129.88 (s), 128.91 (s), 127.29 (s),
δ

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126.34 (s), 124.97 (s), 124.14 (s), 33.94 (s), 23.91 (s). HR-MS (ESI) [M + H]+ calcd for C17H14ClNO2:
300.0713, found: 300.0791.
Data for 3v (5-chloro-2-(1-methyl-1H-pyrazol-5-yl)isoindoline-1,3-dione): white solid; yield, 76%;
m.p.: 175–176 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.95 (dd, J = 12.1, 4.7 Hz, 2H), 7.83 (dd, J = 8.0, 1.6
Hz, 1H), 7.61 (d, J = 1.9 Hz, 1H), 6.33 (d, J = 1.9 Hz, 1H), 3.78 (s, 3H).13C-NMR (101 MHz, CDCl3)
δ
165.09 (s), 164.81 (s), 141.88 (s), 138.85 (s), 135.10 (s), 133.20 (s), 129.59 (s), 129.20 (s), 125.55 (s), 124.69 (s),
104.63 (s), 36.47 (s). HR-MS (ESI) [M + H]+ calcd for C12H8ClN3O2: 262.0305, found: 262.0384.
Data for 3w (5-chloro-2-(thiophen-2-ylmethyl)isoindoline-1,3-dione): white solid; yield, 79%; m.p.:
93–94 ◦C; 1H-NMR (400 MHz, CDCl3)
(m, 2H), 7.04 (dd, J = 6.0, 2.4 Hz, 1H), 5.11 (s, 2H).13C-NMR (101 MHz, CDCl3)
δ
7.89 (dd, J = 11.9, 4.9 Hz, 2H), 7.78 (d, J = 8.0 Hz, 1H), 7.39–7.22
166.60 (s), 166.27 (s),
δ
140.81 (s), 137.67 (s), 134.12 (s), 133.72 (s), 130.11 (s), 127.90 (s), 126.94 (s), 126.05 (s), 124.70 (s), 123.93 (s),
35.91 (s). HR-MS (ESI) [M + H]+ calcd for C13H8ClNO2S: 277.9964, found: 278.0032.
Data for 3x (2-(4-isopropylphenyl)-5-methylisoindoline-1,3-dione): white solid; yield, 47%; m.p.:
161–162 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
7.82 (d, J = 7.6 Hz, 1H), 7.75 (s, 1H), 7.57 (d, J = 7.5 Hz,
1H), 7.35 (s, 4H), 2.96 (dt, J = 13.7, 6.9 Hz, 1H), 2.54 (s, 3H), 1.28 (d, J = 6.9 Hz, 6H).13C-NMR (101
MHz, CDCl3)
δ 167.66 (s), 167.54 (s), 148.77 (s), 145.67 (s), 134.89 (s), 132.22 (s), 129.32 (s), 129.25 (s),
127.19 (s), 126.44 (s), 124.21 (s), 123.62 (s), 33.94 (s), 23.94 (s), 22.07 (s). HR-MS (ESI) [M + H]+ calcd for
C18H17NO2: 280.1259, found: 280.1338.
Data for 3y (2-(4-isopropylphenyl)-5-nitroisoindoline-1,3-dione): white solid; yield, 57%; m.p.:
179–180 ◦C; 1H-NMR (400 MHz, CDCl3)
δ
8.77 (d, J = 1.7 Hz, 1H), 8.67 (dd, J = 8.1, 1.9 Hz, 1H), 8.15 (d,
J = 8.1 Hz, 1H), 7.44–7.30 (m, 4H), 3.09–2.84 (m, 1H), 1.29 (d, J = 6.9 Hz, 6H).13C-NMR (101 MHz,
CDCl3)
δ 165.11 (s), 151.97 (s), 149.66 (s), 136.21 (s), 133.21 (s), 129.56 (s), 128.51 (s), 127.44 (s), 126.23 (s),
124.97 (s), 119.14 (s), 33.97 (s), 23.90 (s). HR-MS (ESI) [M + H]+ calcd for C17H14N2O4: 311.0954,
found: 311.1032.
3.4. Herbicidal Activity
The herbicidal activities of the test compounds against B. campestris, A. retroflexus and D. sanguinalis
were evaluated by the small cup method and foliar spray method at 200 mg/L, according the following
procedure [33,34]. With respect to the small cup method, generally, the test compound (20 mg) was
dissolved in N,N-dimethylformamide (DMF) (1 mL) and then diluted in water containing 0.1% Tween
80 to a final concentration of 200 mg/L. The controls were treated with the same solution but without
any test compound. A piece of filter paper in a 50 mL beaker was treated with the test compound
solution (1 mL), and then 10 seeds that were soaked in water for 24 h were added. All treatments were
repeated three times.
The pre- and post-emergence herbicidal activities of the title compounds were evaluated at an
application rate of 90 g ai/ha in a greenhouse according to a reported method [35,36]. Nine seeds of these
plants (A. retroflexus seeds, B. campestris seeds or D. sanguinalis seeds) were sown at a depth of 5 mm in a
mixture of vermiculite/nutrient-enriched soil (1:1, m/m) with some water at 4 cm below the surface and
then cultivated at 20–25 ◦C. To test the pre-emergence herbicidal activities, the abovementioned soil was
sprayed with the title compound solution before germination, and the results were determined after
two weeks. To test the post-emergence herbicidal activities, plants at the 2–4 leaf stage after germination
were treated with the test compound solution by a walking spray tower and then cultivated for one
week. The percentage of herbicidal activity was calculated by comparing the fresh weight of the
growth-inhibited plants with that of the healthy control plants, where completely inhibited growth
was set as 100 and the healthy control was set as 0.
3.5. PPO Enzyme Assays
To further explore the mode of action of these target compounds, 3a was selected as a
representative to confirm whether it can act on PPO. Briefly, the procedures were followed as
described here. D. sanguinalis plants were treated with 90 g ai/ha of compound 3a and Flumioxazin by

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the post-emergence method. After 3 days, D. sanguinalis leaves (0.2 g) were collected and dissolved in
extraction medium (1.5 mL) in an ice bath, and the mixture was then centrifuged at 4 ◦C × 12,000 rpm
for 15 min. A polyphenol oxidase kit (G0113W, Suzhou Grace Bio-technology Co., Ltd., Suzhou, China)
was obtained from commercial sources to determine the PPO activity. The change in absorbance
(every 5 min) was measured on a POLARstar Optima/Galaxy instrument (BMG) (Shanghai Microplate
Co., Ltd., Shanghai, China) at 420 nm [37
absorbance of 0.01 per minute.
–39]. One unit of PPO activity was defined as a change in
4. Conclusions
In summary, a series of phthalimide derivatives were designed by molecular docking and by
using Flumioxazin as a lead, then synthesized and characterized by NMR, HR-MS, and the typical
crystal structure was determined by X-ray diffraction. The herbicidal activities of these compounds
were assessed against B. campestris, A. retroflexus and D. sanguinalis, by the small cup, pre-emergence,
and post-emergence methods, respectively. Most of the synthesized compounds exhibited good to
excellent herbicidal activities, and especially 3a displayed the same efficacy against A. retroflexus and
B. campestris to commercial standards of Flumioxazin. Further PPO activity assays confirmed that the
mode of action of 3a is similar to PPO inhibitors. These results suggest that our molecular design
strategy is effective.
Author Contributions: The experiments were conceived and planned by W.G., L.C. and J.Z., the manuscript was
written by W.G., L.C. and J.H. Experiments were performed by W.G., X.L., D.R., S.S. and Y.W. The final version of
this manuscript has been read and approved by all authors.
Funding: This research was in financial supported by the National Natural Science Foundation of China
(Nos. 31871981); the starting Scientific Research Foundation for the introduced talents of Hebei Agricultural
University (Nos. 201842); Youth Natural Science Foundation of Hebei Province (Nos. B2019204030); Modern
Agriculture Industry Technology System Innovation Team of Phase II of Hebei Province (Nos. HB2018020205).
Conflicts of Interest: Authors hereby declare that there are no financial/commercial conflict of interest.
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