Design, Synthesis, and Synergistic Activity of Eight-Membered Oxabridge Neonicotinoid Analogues

Article abstract of DOI:10.1021/acs.jafc.0c04786

Insecticide synergists are sought-after due to their potential in improving the pesticide control efficacy with a reduced dose of an active ingredient. We previously reported that a cis-configuration neonicotinoid (IPPA08) exhibited specific synergistic activity toward neonicotinoid insecticides. In this study, we synthesized a series of structural analogues of IPPA08 by converting the pyridyl moiety of IPPA08 into phenyl groups, via facile double-Mannich condensation reactions between nitromethylene compounds and glutaraldehyde. All of the oxabridged neonicotinoid compounds were found to increase the toxicity of imidacloprid against Aphis craccivora. Notably, compound 25 at 0.75 mg/L lowered the LC50 value of imidacloprid against A. craccivora by 6.54-fold, while a 3.50-fold reduction of the LC50 value was observed for IPPA08. The results of bee toxicity test showed that compound 25 display selectivity in its effects on imidacloprid toxicity against the honey bee (Apis mellifera L.). In summary, replacing the pyridyl ring with a phenyl ring was a viable approach to obtain a novel synergist with oxabridged moiety for neonicotinoid insecticides.

Full text of DOI:10.1021/acs.jafc.0c04786

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Article
Design, Synthesis, and Synergistic Activity of Eight-Membered
Oxabridge Neonicotinoid Analogues
Xiao Zhang, Yiping Wang, Zhiping Xu, Xusheng Shao, Zewen Liu, Xiaoyong Xu, Peter Maienfisch,
and Zhong Li*
Cite This: J. Agric. Food Chem. 2021, 69, 3005−3014
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ABSTRACT: Insecticide synergists are sought-after due to their potential in improving the pesticide control efficacy with a reduced
dose of an active ingredient. We previously reported that a cis-configuration neonicotinoid (IPPA08) exhibited specific synergistic
activity toward neonicotinoid insecticides. In this study, we synthesized a series of structural analogues of IPPA08 by converting the
pyridyl moiety of IPPA08 into phenyl groups, via facile double-Mannich condensation reactions between nitromethylene
compounds and glutaraldehyde. All of the oxabridged neonicotinoid compounds were found to increase the toxicity of imidacloprid
against Aphis craccivora. Notably, compound 25 at 0.75 mg/L lowered the LC50 value of imidacloprid against A. craccivora by 6.54-
fold, while a 3.50-fold reduction of the LC50 value was observed for IPPA08. The results of bee toxicity test showed that compound
25 display selectivity in its effects on imidacloprid toxicity against the honey bee (Apis mellifera L.). In summary, replacing the
pyridyl ring with a phenyl ring was a viable approach to obtain a novel synergist with oxabridged moiety for neonicotinoid
insecticides.
KEYWORDS: neonicotinoid, insecticide synergist, imidacloprid, selectivity
recently approved in China.10−14 Interestingly, an eight-
membered oxabridged homologue of cycloxaprid, i.e.,
INTRODUCTION
■
Since the introduction of imidacloprid in the 1990s,
IPPA08 (Figure 1), was almost inactive to insects and yet
neonicotinoid insecticides have played crucial roles in pest
exhibited synergistic effects on traditional neonicotinoids,
control due to their safety profile, insecticidal spectrum, and
systemic properties.1−3 Currently, neonicotinoids represent the
including imidacloprid, acetamiprid, thiacloprid, and clothia-
nidin. We also find that IPPA08 presented little effects on the
largest insecticide group, accounting for over 25% of the global
insecticide market.4 However, the abusive use of neonicoti-
toxicities of dinotefuran and cycloxaprid. Also, IPPA08 did not
display selectivity in its effects on imidacloprid toxicity against
the honey bee (A. mellifera L.).15,16 Therefore, we sought to
modify the oxabridged structure, by changing the peripheral
pyridyl ring into a phenyl ring, to further improve its
synergistic activity as well as with selectivity and safety toward
honey bees.
Herein, a focused library of eight-membered oxabridged
compounds were synthesized from nitromethylene analogues
and glutaraldehyde, and their structure−activity relationships
(SARs) were thoroughly tested.
noids in the past three decades has brought about tough
challenges, such as resistance and toxicity to honey bees.5,6
Also, neonicotinoids are considered to have side effects on
insect pollinators at sublethal concentrations through the
potential uptake from crops and wild plants.7,8 Recently, these
concerns about sublethal effects of neonicotinoids on bees
have led to a reevaluation of the overall biological safety of
neonicotinoids in Europe and elsewhere.9 Insecticide syner-
gists are a potential solution for these issues. They can greatly
enhance the activity of the existing pesticides and therefore
lower the necessary dose of the active ingredient.
Cycloxaprid is a new class of neonicotinoid with a unique
oxabridged substructure (Figure 1). It exhibited excellent
activity, high selectivity, and good safety profiles, and was
MATERIALS AND METHODS
■
Chemicals. Cycloxaprid, paichongding, and IPPA08 were
synthesized and purified according to a literature procedure.17
Other insecticides were purchased from Sigma-Aldrich (St. Louis,
MO).
Received: July 27, 2020
Revised: December 23, 2020
Accepted: February 5, 2021
Published: March 2, 2021
Figure 1. Oxabridged neonicotinoid compounds cycloxaprid (CYC)
and IPPA08.
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a
Scheme 1. Synthetic Routes of Target Compounds
a
Reagents and conditions: (a) ethane-1,2-diamine, CH3CN, 0−5 °C; (b) 1,1-bis(methylthio)-2-nitroethene, CH3CH2OH, refluxing; (c)
glutaraldehyde, concentrated HCl, rt.
Table 1. Compounds 1−33
Experiment Design. First, we tested the standard curve of
imidacloprid and calculated the LC30 (0.997 mg/L) and LC50 (1.72
mg/L) values. To screen out candidates with significant synergistic
effect, the mortality of a mixture of imidacloprid (1 mg/L) with the
new compounds at low concentrations (0.5, 1, 2, 4 mg/L) against
Aphis craccivora was analyzed. To obtain the initial synergistic
concentration a and the maximum synergistic concentration b (the
minimum concentration when the mortality reaches the maximum), a
further refined concentration gradient was performed. Finally, we
assessed the LC50 of imidacloprid when the candidate compound is
combined with imidacloprid at the initial synergistic concentration a
and maximum synergistic concentration b, and compared with the
blank to calculate the synergism ratio.
Insects and Bioassay. Cowpea Aphid (A. craccivora) is a
susceptible strain from our laboratory, and the bioassay was carried
out using the leaf dip bioassay.18 Aqueous solution of each compound
was prepared, to which 0.1% Triton X-100 (0.1 mg/L) as a surfactant
and dimethyl sulfoxide (DMSO) as a solvent were added. The
mixture was then diluted with 0.1% Triton X-100 (0.1 mg/L) to
obtain a series concentrations of 4, 2, 1, and 0.5 mg/L and so on.
Several adults of aphid were selected, avoiding light, and given a
starvation treatment about an hour in the dark. Then, they were let to
eat broad bean seedling until their mouthparts pierce the bean sprout
(2−3 h). Then, the leaves of the horsebean plant, infested with 20−30
aphids, were dipped for 5 s in each concentration three times. This
procedure was repeated three times in each group. After treatment,
the burgeoning shoots were placed in a conditioned room (25 1 °C,
50% RH). Water containing Triton X-100 (0.1 mg/L) was used as
control. The mortality was recorded in 48 h. The data obtained were
analyzed using Polo software (LeOra Software, Inc., Cary, NC) to
determine the LC50 values.
Bee Toxicity Assay. Honey bees (Apis mellifera L.) were provided
by Chinese Academy of Agricultural Sciences (Beijing, China), and a
bioassay was performed following the Organization for Economic
Cooperation and Development (OECD) method for contact toxicity
and oral toxicity tests.19
Contact Assay. Honey bees (A. mellifera L.) were put into a dryer
and anesthetized by 5 mL of diethyl ether for 3 min before the test.
Then, solutions of different concentrations were added dropwise on
the pronotum of the bees by a 1.00 μL microdropper. The bees were
enclosed in the cage in time before the bees fully recovered and were
fed with 33% honey water. The cage was put on the laboratory table
and covered by a black cloth, and acetone was used as control.
Uptake Assay. Degrease cotton was dipped in diluted solutions of
the chemicals, to which was added Tween 80 and diluted by 33%
honey water until saturation. Then, the degrease cotton was spread on
a gauze net in the cage and a 50 mL beaker was placed on the
degrease cotton so that the honey bees (A. mellifera L.) could suck the
liquid. The cage was placed on a laboratory table and covered by a
black cloth. The amount of acetone with Tween 80 was the same as
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Article
the maximum concentration of diluted solution. It was used as
control.
2H), 7.23−7.19 (m, 2H), 5.12 (s, 1H), 5.03 (s, 1H), 4.97 (d, J
= 15.8 Hz, 1H), 4.89 (d, J = 15.8 Hz, 1H), 3.82−3.54 (m, 4H),
1.80−1.61 (m, 4H), 1.52 (d, J = 13.8 Hz, 1H), 1.38−1.35 (m,
1H) ppm. 19F NMR (376 MHz, DMSO-d6) δ-118.36 (s, 1F)
ppm. 13C NMR (100 MHz, DMSO-d6) δ 161.31, 155.99 (d,
JC‑F = 255.8 Hz, 1C), 129.88, 129.44, 124.59 (d, JC‑F = 19.2
Hz, 1C), 123.87, 115.21 (d, JC‑F = 3.9 Hz, 1C), 105.33, 81.24,
68.43, 49.44, 47.95, 45.83, 28.16, 26.78, 14.21 ppm. HRMS
(ESI) calcd for C16H19N3O3F [M + H]+, 320.1410, found,
320.1411.
The mortality rates and poisoning symptoms were recorded 24 and
48 h after treatment. The test data were processed by SPSS12.0 to
obtain the LD50 (bee contact toxicity) for 24 and 48 h, and LD50 (bee
oral toxicity) for 24 h and 48 h and 95% confidence intervals.
Instruments. All melting points were obtained with a Buchi
Melting Point B540 and are uncorrected. Nuclear magnetic resonance
spectra were recorded on a Bruker AM-400 spectrometer with CDCl3
or DMSO-d6 as the solvent (1H at 400 MHz and 13C at 100 MHz)
and TMS as the internal standard. Chemical shifts are reported in
parts per million (δ). Coupling constants (J) are reported in hertz
(Hz). High-resolution electron mass spectra (electrospray ionization
time-of-flight (ESI-TOF)) were performed on a Micromass liquid
chromatography-TOF (LC-TOF) spectrometer. High-resolution
mass spectra (HRMS) EI were recorded under electron impact (70
eV) condition using a Micromass GCT CA 055 instrument. Analytical
thin-layer chromatography (TLC) was carried out on precoated plates
(silica gel 60 F254), and spots were visualized with ultraviolet (UV)
light. All other solvents and reagents were obtained from commercial
sources without further purification.
1-(3-Fluorobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (4). Yield, 86%; mp, 129.3−
1
130.7 °C; H NMR (400 MHz, DMSO-d6) δ 7.53−7.03 (m,
4H), 5.07−5.01 (m, 3H), 4.75 (d, J = 15.6 Hz, 1H), 3.68 (d, J
= 18.2 Hz, 4H), 1.90−1.29 (m, 6H) ppm. 19F NMR (376
MHz, DMSO-d6) δ-113.32 (s, 1F) ppm. 13C NMR (100 MHz,
DMSO-d6) δ 163.42, 156.08 (d, JC‑F = 255.1 Hz, 1C), 140.09,
130.32 (d, JC‑F = 9.8 Hz, 1C), 123.69 (d, JC‑F = 4.6 Hz, 1C),
114.44, 114.12, 105.30, 81.23, 68.45, 53.40, 49.37, 45.79,
28.25, 26.70, 14.32 ppm. HRMS (ESI) calcd for C16H19N3O3F
[M + H]+, 320.1410, found, 320.1411.
RESULTS
■
1-(4-Fluorobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (5). Yield, 76%; mp, 143.3−
Synthesis. The synthetic route of the target compounds is
summarized in Scheme 1. The structures of the title
1
compounds were well characterized by H NMR, 13C NMR,
1
144.1 °C; H NMR (400 MHz, DMSO-d6) δ 7.42−7.36 (m,
2H), 7.23−7.16 (m, 2H), 5.11 (s, 1H), 5.06 (d, J = 2.4 Hz,
1H), 4.96 (d, J = 15.0 Hz, 1H), 4.72 (d, J = 15.0 Hz, 1H),
3.73−3.57 (m, 4H), 1.85−1.65 (m, 4H), 1.53 (d, J = 13.8 Hz,
1H), 1.43−1.29 (m, 1H) ppm. 19F NMR (376 MHz, DMSO-
d6) δ-115.23 (s, 1F) ppm. 13C NMR (100 MHz, DMSO-d6) δ
161.52, 155.97 (d, JC‑F = 249.2 Hz, 1C), 133.16, 129.89 (d, JC‑F
= 10.5 Hz, 1C), 115.18 (d, JC‑F = 5.0 Hz, 1C), 105.34, 81.23,
68.48, 52.96, 49.14, 45.77, 28.23, 26.69, 14.33 ppm. HRMS
(ESI) calcd for C16H19N3O3F [M + H]+, 320.1410, found,
320.1415.
and HRMS (ESI).
General Procedure for the Preparation of Com-
pounds. A mixture of starting material A (nitromethylene
analogues) (2 mmol) and concentrated hydrochloric acid (0.1
mL) in MeCN (6 mL) was stirred at room temperature. After
5 min of stirring, a 25% glutaraldehyde aqueous solution (1
mL) was added slowly. Then, the reaction mixture was stirred
at 0 °C to room temperature and monitored by TLC. After
completion, the pH value of the reaction mixture was adjusted
to 7−8 by saturated aqueous NaHCO3 and the solvent was
removed under reduced pressure. The resulting mixture was
extracted with dichloromethane (3 × 20 mL), and the organic
layer was washed with brine, dried over anhydrous sodium
sulfate, filtered, and concentrated. The residue was purified by
silica gel column chromatography (DCM/MeOH as eluent) to
afford the pure products (1−33, Table 1). The data of 1−33
are shown as follows.
1-(2-Chlorobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (6). Yield, 86%; mp, 130.4−
1
131.7 °C; H NMR (400 MHz, DMSO-d6) δ 7.50−7.44 (m,
2H), 7.38−7.33 (m, 2H), 5.15 (s, 1H), 5.06 (d, J = 2.2 Hz,
1H), 4.94 (d, J = 16.4 Hz, 1H), 4.85 (d, J = 16.4 Hz, 1H),
3.82−3.61 (m, 4H), 1.88−1.66 (m, 4H), 1.64−1.40 (m, 2H)
ppm. 13C NMR (100 MHz, DMSO-d6) δ 156.46, 134.61,
131.68, 129.17, 129.00, 128.79, 127.26, 105.03, 81.30, 68.46,
52.55, 49.48, 45.91, 28.35, 26.76, 14.37 ppm. HRMS (ESI)
calcd for C16H19N3O335Cl [M + H]+, 336.1115, found,
336.1113; calcd for C16H19N3O337Cl [M + H]+, 338.1085,
found, 338.1089.
1-Methyl-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (1). Yield, 80%; mp, 109.8−
1
110.5 °C; H NMR (400 MHz, DMSO-d6) δ 5.11−4.99 (m,
2H), 3.86−3.56 (m, 4H), 3.14 (s, 3H), 1.84 (d, J = 13.4 Hz,
1H), 1.76−1.65 (m, 3H), 1.58−1.49 (m, 1H), 1.47−1.36 (m,
1H) ppm. 13C NMR (100 MHz, DMSO-d6) δ 156.45, 104.44,
81.46, 68.64, 51.73, 45.97, 38.62, 28.56, 26.49, 14.45 ppm.
HRMS (ESI) calcd for C10H16N3O3 [M + H]+, 226.1192,
found, 226.1193.
1-(3-Chlorobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (7). Yield, 91%; mp, 132.7−
1
133.6 °C; H NMR (400 MHz, DMSO-d6) δ 7.39−7.34 (m,
4H), 5.13 (s, 1H), 5.07 (s, 1H), 5.00 (d, J = 15.4 Hz, 1H), 4.71
(d, J = 15.4 Hz, 1H), 3.76−3.58 (m, 4H), 1.86−1.66 (m, 4H),
1.54 (d, J = 13.8 Hz, 1H), 1.48−1.32 (m, 1H) ppm.13C NMR
(100 MHz, DMSO-d6) δ 156.11, 139.69, 133.08, 130.24,
127.57, 127.30, 126.38, 105.30, 81.21, 68.44, 53.42, 49.37,
45.76, 28.22, 26.72, 14.31 ppm. HRMS (ESI) calcd for
C16H18N3O335ClNa [M + Na]+, 358.0934, found, 358.0933;
calcd for C16H18N3O337ClNa [M + Na]+, 360.0905, found,
360.0903.
1-Benzyl-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (2). Yield, 81%; mp, 154.5−
1
155.6 °C; H NMR (400 MHz, DMSO-d6) δ 7.43−7.25 (m,
5H), 5.16−5.04 (m, 2H), 4.98 (d, J = 15.2 Hz, 1H), 4.78 (d, J
= 15.2 Hz, 1H), 3.75−3.56 (m, 4H), 1.85−1.66 (m, 4H),
1.54−1.50 (m, 1H), 1.44−1.30 (m, 1H) ppm. 13C NMR (100
MHz, DMSO-d6) δ 156.02, 137.01, 128.45, 127.75, 127.39,
105.35, 81.25, 68.52, 53.60, 49.15, 45.80, 28.23, 26.70, 14.33
ppm. HRMS (ESI) calcd for C16H20N3O3 [M + H]+, 302.1505,
found, 302.1506.
1-(4-Chlorobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (8). Yield, 83%; mp, 139.0−
1
1-(2-Fluorobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (3). Yield, 85%; mp, 144.1−
141.4 °C; H NMR (400 MHz, DMSO-d6) δ 7.42−7.36 (m,
2H), 7.23−7.16 (m, 2H), 5.11 (s, 1H), 5.06 (d, J = 2.4 Hz,
1H), 4.96 (d, J = 15.0 Hz, 1H), 4.72 (d, J = 15.0 Hz, 1H),
1
144.8 °C; H NMR (400 MHz, DMSO-d6) δ 7.38−7.35 (m,
3007
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3.73−3.57 (m, 4H), 1.85−1.65 (m, 4H), 1.53 (d, J = 13.8 Hz,
1H), 1.43−1.29 (m, 1H) ppm. 13C NMR (100 MHz, DMSO-
d6) δ 156.03, 136.11, 131.94, 129.65, 128.36, 105.31, 81.24,
68.47, 53.17, 49.25, 45.79, 28.27, 26.67, 14.35 ppm. HRMS
(ESI) calcd for C16H19N3O335Cl [M + H]+, 336.1115, found,
336.1114; calcd for C16H19N3O337Cl [M + H]+, 338.1085,
found, 338.1087.
14.28 ppm. HRMS (ESI) calcd for C17H21N3O3Na [M + Na]+,
338.1481, found, 338.1482.
1-(4-Methylbenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (14). Yield, 89%; mp, 122.9−
124.1 °C; 1H NMR (400 MHz, CDCl3) δ 7.21−7.17 (m, 4H),
5.37−5.29 (m, 1H), 4.96 (d, J = 14.8 Hz, 1H), 4.83 (d, J =
14.8 Hz, 1H), 3.75−3.64 (m, 1H), 3.63−3.45 (m, 4H), 2.34
(s, 3H), 2.16 (d, J = 13.6 Hz, 1H), 1.94−1.83 (m, 2H), 1.68−
1.65 (m, 3H) ppm. 13C NMR (101 MHz, CDCl3) δ 156.53,
137.77, 133.27, 129.53, 128.34, 106.72, 83.02, 69.74, 54.48,
48.37, 46.72, 29.56, 26.48, 21.15, 15.01 ppm. HRMS (ESI)
calcd for C17H21N3O3Na [M + Na]+, 338.1481, found,
338.1483.
1-(2-Bromobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (9). Yield, 84%; mp, 142.3−
1
143.1 °C; H NMR (400 MHz, DMSO-d6) δ 7.68−7.60 (m,
1H), 7.49−7.39 (m, 2H), 7.30−7.21 (m, 1H), 5.15 (s, 1H),
5.07 (d, J = 2.4 Hz, 1H), 4.89 (d, J = 16.6 Hz, 1H), 4.79 (d, J =
16.6 Hz, 1H), 3.80−3.60 (m, 4H), 1.87−1.67 (m, 4H), 1.61−
1.47 (m, 2H) ppm. 13C NMR (100 MHz, DMSO-d6) δ 156.53,
136.16, 132.39, 129.01, 128.97, 127.79, 121.86, 104.99, 81.31,
68.46, 55.16, 49.50, 45.93, 28.40, 26.74, 14.41 ppm. HRMS
(ESI) calcd for C16H19N3O379Br [M + H]+, 380.0610, found,
380.0609; calcd for C16H19N3O381Br [M + H]+, 382.0589,
found, 382.0588.
1-(2-Methoxybenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-
5,9-epoxyimidazo[1,2-a]azocine (15). Yield, 77%; mp,
1
127.5−128.7 °C; H NMR (400 MHz, DMSO-d6) δ 7.32−
7.24 (m, 1H), 7.19 (d, J = 7.4 Hz, 1H), 7.00 (d, J = 8.0 Hz,
1H), 6.91 (t, J = 7.4 Hz, 1H), 5.08 (s, 1H), 5.02 (s, 1H), 4.81
(s, 2H), 3.78 (d, J = 8.4 Hz, 3H), 3.76−3.61 (m, 2H), 3.59−
3.52 (m, 2H), 1.79−1.60 (m, 4H), 1.49 (d, J = 13.8 Hz, 1H),
1.40−1.22 (m, 1H) ppm. 13C NMR (100 MHz, DMSO-d6) δ
156.93, 156.15, 128.90, 128.83, 124.44, 120.21, 110.61, 105.33,
81.35, 68.53, 55.18, 49.51, 49.18, 45.96, 28.07, 26.90, 14.12
ppm. HRMS (ESI) calcd for C17H21N3O4Na [M + Na]+,
354.1430, found, 354.1431.
1-(3-Bromobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (10). Yield, 72%; mp, 140.9−
1
142.1 °C; H NMR (400 MHz, DMSO-d6) δ 7.56 (s, 1H),
7.52−7.49 (m, 1H), 7.38−7.30 (m, 2H), 5.13 (s, 1H), 5.06 (d,
J = 2.2 Hz, 1H), 5.00 (d, J = 15.4 Hz, 1H), 4.69 (d, J = 15.4
Hz, 1H), 3.74−3.59 (m, 4H), 1.87−1.66 (m, 4H), 1.54 (d, J =
13.8 Hz, 1H), 1.47−1.30 (m, 1H) ppm.13C NMR (100 MHz,
DMSO-d6) δ 156.11, 139.94, 130.54, 130.46, 130.20, 126.78,
121.72, 105.30, 81.20, 68.44, 53.37, 49.35, 45.76, 28.21, 26.74,
14.31 ppm. HRMS (ESI) calcd for C16H19N3O379Br [M + H]+,
380.0610, found, 380.0609; calcd for C16H19N3O381Br [M +
H]+, 382.0589, found, 382.0588.
1-(3-Methoxybenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-
5,9-epoxyimidazo[1,2-a]azocine (16). Yield, 81%; mp,
1
135.3−135.9 °C; H NMR (400 MHz, DMSO-d6) δ 7.31−
7.25 (m, 1H), 6.92−6.84 (m, 3H), 5.12 (s, 1H), 5.07 (d, J =
2.2 Hz, 1H), 4.93 (d, J = 15.0 Hz, 1H), 4.77 (d, J = 15.0 Hz,
1H), 3.74 (s, 3H), 3.73−3.58 (m, 4H), 1.84−1.65 (m, 4H),
1.53 (d, J = 14.0 Hz, 1H), 1.42−1.32 (m, 1H) ppm. 13C NMR
(100 MHz, DMSO-d6) δ 159.34, 156.02, 138.59, 129.55,
119.89, 113.31, 112.86, 105.42, 81.26, 68.52, 54.94, 53.45,
49.12, 45.83, 28.21, 26.73, 14.32 ppm. HRMS (ESI) calcd for
C17H21N3O4Na [M + Na]+, 354.1430, found, 354.1431.
1-(4-Methoxybenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-
5,9-epoxyimidazo[1,2-a]azocine (17). Yield, 86%; mp,
1-(4-Bromobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (11). Yield, 77%; mp, 149.1−
1
150.0 °C; H NMR (400 MHz, DMSO-d6) δ 7.63−7.47 (m,
2H), 7.31 (d, J = 8.4 Hz, 2H), 5.11 (s, 1H), 5.06 (d, J = 2.6
Hz, 1H), 4.94 (d, J = 15.2 Hz, 1H), 4.71 (d, J = 15.2 Hz, 1H),
3.78−3.58 (m, 4H), 1.87−1.64 (m, 4H), 1.54 (d, J = 13.8 Hz,
1H), 1.47−1.28 (m, 1H) ppm.13C NMR (100 MHz, DMSO-
d6) δ 156.04, 136.55, 131.27, 129.99, 120.46, 105.30, 81.25,
68.47, 53.25, 49.26, 45.79, 28.28, 26.67, 14.36 ppm. HRMS
(ESI) calcd for C16H18N3O379BrNa [M + Na]+, 402.0429,
found, 402.0430; calcd for C16H18N3O381BrNa [M + Na]+,
404.0409, found, 404.0408.
1
130.7−132.5 °C; H NMR (400 MHz, DMSO-d6) δ 7.29−
7.24 (m, 2H), 6.94−6.90 (m, 2H), 5.15−5.03 (m, 2H), 4.90
(d, J = 14.6 Hz, 1H), 4.71 (d, J = 14.6 Hz, 1H), 3.78−3.71 (m,
3H), 3.71−3.54 (m, 4H), 1.85−1.44 (m, 7H) ppm. 13C NMR
(100 MHz, DMSO-d6) δ 158.68, 155.87, 129.31, 128.73,
113.84, 105.38, 81.26, 68.55, 55.02, 52.89, 48.83, 45.76, 28.21,
26.71, 14.32 ppm. HRMS (ESI) calcd for C17H21N3O4Na [M
+ Na]+, 354.1430, found, 354.1431.
1-(2-Methylbenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (12). Yield, 80%; mp, 141.3−
1
142.1 °C; H NMR (400 MHz, DMSO-d6) δ 7.32−7.15 (m,
2-((10-Nitro-2,3,6,7,8,9-hexahydro-5,9-epoxyimidazo[1,2-
a]azocin-1(5H)-yl)methyl)benzonitrile (18). Yield, 83%; mp,
135.2−136.4 °C; 1H NMR (400 MHz, DMSO-d6) δ 7.85 (dd,
J = 8.0, 1.0 Hz, 1H), 7.78−7.73 (m, 1H), 7.60 (d, J = 8.0 Hz,
1H), 7.49 (t, J = 7.6 Hz, 1H), 5.21−5.03 (m, 3H), 4.96 (d, J =
16.6 Hz, 1H), 3.84−3.67 (m, 4H), 1.85−1.69 (m, 4H), 1.55−
1.50 (m, 2H) ppm. 13C NMR (100 MHz, DMSO-d6) δ 156.26,
141.26, 133.27, 132.98, 127.93, 127.84, 117.25, 109.84, 105.15,
81.27, 68.39, 53.16, 49.94, 45.85, 28.36, 26.74, 14.32 ppm.
HRMS (ESI) calcd for C17H19N4O3 [M + H]+, 327.1457,
found, 327.1458.
4H), 5.14 (s, 1H), 5.08 (d, J = 2.6 Hz, 1H), 4.90 (d, J = 16.0
Hz, 1H), 4.82 (d, J = 16.0 Hz, 1H), 3.77−3.52 (m, 4H), 2.25
(s, 3H), 1.91−1.67 (m, 4H), 1.53−1.48 (m, 2H) ppm.13C
NMR (100 MHz, DMSO-d6) δ 156.38, 135.39, 134.99, 130.08,
127.04, 126.97, 125.92, 105.03, 81.40, 68.62, 51.95, 48.98,
45.85, 28.43, 26.67, 18.55, 14.48 ppm. HRMS (ESI) calcd for
C17H21N3O3Na [M + Na]+, 338.1481, found, 338.1482.
1-(3-Methylbenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (13). Yield, 75%; mp, 111.2−
1
111.9 °C; H NMR (400 MHz, DMSO-d6) δ 7.24 (t, J = 7.4
Hz, 1H), 7.19−7.06 (m, 3H), 5.09 (d, J = 15.2 Hz, 2H), 4.97
(d, J = 15.0 Hz, 1H), 4.70 (d, J = 15.0 Hz, 1H), 3.74−3.53 (m,
4H), 2.30 (s, 3H), 1.87−1.63 (m, 4H), 1.53 (d, J = 13.8 Hz,
1H), 1.46−1.29 (m, 1H) ppm.13C NMR (100 MHz, DMSO-
d6) δ 156.04, 137.55, 136.91, 128.43, 128.35, 128.04, 124.86,
105.34, 81.23, 68.52, 53.55, 49.04, 45.76, 28.17, 26.78, 20.99,
3-((10-Nitro-2,3,6,7,8,9-hexahydro-5,9-epoxyimidazo[1,2-
a]azocin-1(5H)-yl)methyl)benzonitrile (19). Yield, 89%; mp,
1
144.8−145.7 °C; H NMR (400 MHz, DMSO-d6) δ 7.85−
7.74 (m, 2H), 7.69 (d, J = 7.8 Hz, 1H), 7.58 (t, J = 7.8 Hz,
1H), 5.13 (s, 1H), 5.09−4.99 (m, 2H), 4.73 (d, J = 15.6 Hz,
1H), 3.79−3.61 (m, 4H), 1.84−1.66 (m, 4H), 1.55 (d, J = 13.8
3008
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Article
Hz, 1H), 1.46−1.39 (m, 1H) ppm. 13C NMR (100 MHz,
DMSO-d6) δ 156.19, 138.97, 132.50, 131.17, 131.06, 129.55,
118.77, 111.29, 105.23, 81.22, 68.41, 53.55, 49.56, 45.79,
28.28, 26.70, 14.33 ppm. HRMS (ESI) calcd for
C17H18N4O3Na [M + Na]+, 349.1277, found, 349.1276.
4-((10-Nitro-2,3,6,7,8,9-hexahydro-5,9-epoxyimidazo[1,2-
a]azocin-1(5H)-yl)methyl)benzonitrile (20). Yield, 72%; mp,
(d, J = 15.0 Hz, 1H), 4.77 (d, J = 15.0 Hz, 1H), 3.74−3.55 (m,
4H), 1.83−1.63 (m, 4H), 1.51−1.48 (m, 2H), 1.28 (s, 9H)
ppm. 13C NMR (100 MHz, DMSO-d6) δ 156.03, 149.76,
133.97, 127.44, 125.20, 99.49, 81.24, 68.52, 53.20, 49.14,
45.79, 34.20, 31.11, 28.22, 26.71, 14.31 ppm. HRMS (ESI)
calcd for C20H28N3O3 [M + H]+, 358.2131, found, 358.2132.
1-(4-Isopropylbenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-
5,9-epoxyimidazo[1,2-a]azocine (26). Yield, 92%; mp,
1
140.9−142.2 °C; H NMR (400 MHz, DMSO-d6) δ 7.88−
1
7.81 (m, 2H), 7.54 (d, J = 8.4 Hz, 2H), 5.14 (s, 1H), 5.06−
5.03 (m, 2H), 4.81 (d, J = 16.0 Hz, 1H), 3.79−3.60 (m, 4H),
1.84−1.66 (m, 4H), 1.57−1.52 (m, 1H), 1.48−1.37 (m, 1H)
ppm. 13C NMR (101 MHz, DMSO-d6) δ 156.21, 143.24,
132.26, 128.39, 118.80, 109.94, 105.23, 81.25, 68.41, 53.97,
49.62, 45.84, 28.33, 26.65, 14.37 ppm. HRMS (ESI) calcd for
C17H19N4O3 [M + H]+, 327.1457, found, 327.1456.
140.2−141.3 °C; H NMR (400 MHz, DMSO-d6) δ 7.34−
7.14 (m, 4H), 5.09−5.06 (m, 2H), 4.92 (d, J = 15.0 Hz, 1H),
4.75 (d, J = 15.0 Hz, 1H), 3.76−3.54 (m, 4H), 2.94−2.80 (m,
1H), 1.83−1.44 (m, 6H), 1.22−1.15 (m, 6H) ppm. 13C NMR
(100 MHz, DMSO-d6) δ 156.01, 147.53, 134.33, 127.75,
126.35, 105.33, 81.24, 68.51, 53.29, 49.11, 45.78, 33.08, 28.21,
26.71, 23.85, 14.31 ppm. HRMS (ESI) calcd for C19H26N3O3
[M + H]+, 344.1974, found, 344.1976.
10-Nitro-1-(4-nitrobenzyl)-1,2,3,5,6,7,8,9-octahydro-5,9-
epoxyimidazo[1,2-a]azocine (21). Yield, 70%; mp, 146.1−
1-(2,6-Dichlorobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-
5,9-epoxyimidazo[1,2-a]azocine (27). Yield, 75%; mp,
1
146.8 °C; H NMR (400 MHz, DMSO-d6) δ 8.24 (d, J = 8.8
1
134.4−134.9 °C; H NMR (400 MHz, DMSO-d6) δ 7.71−
Hz, 2H), 7.63 (d, J = 8.8 Hz, 2H), 5.17−5.05 (m, 4H), 4.85
(d, J = 16.2 Hz, 1H), 3.79−3.62 (m, 4H), 1.79−1.72 (m, 3H),
1.55 (s, 1H), 1.51−1.41 (m, 2H) ppm. 13C NMR (100 MHz,
DMSO-d6) δ 156.30, 146.69, 145.48, 128.58, 123.43, 105.23,
81.25, 68.41, 53.94, 49.65, 45.87, 28.36, 26.65, 14.39 ppm.
HRMS (ESI) calcd for C16H18N4O5Na [M + Na]+, 369.1175,
found, 369.1176.
7.56 (m, 2H), 7.53−7.42 (m, 1H), 5.17−5.01 (m, 3H), 4.95
(d, J = 14.6 Hz, 1H), 3.57−3.33 (m, 4H), 1.90−1.46 (m, 6H)
ppm. 13C NMR (100 MHz, DMSO-d6) δ 156.01, 135.95,
131.37, 130.51, 129.09, 105.04, 81.55, 68.80, 48.95, 45.95,
45.35, 28.22, 26.90, 14.30 ppm. HRMS (ESI) calcd for
C16H18N3O335Cl2 [M + H]+, 370.0725, found, 370.0724; calcd
for C16H18N3O335Cl37Cl [M + H]+, 372.0696, found,
372.0699; calcd for C16H18N3O337Cl2 [M + H]+, 374.0666,
found, 374.0671.
10-Nitro-1-(4-(trifluoromethyl)benzyl)-1,2,3,5,6,7,8,9-oc-
tahydro-5,9-epoxyimidazo[1,2-a]azocine (22). Yield, 81%;
1
mp, 128.8−129.5 °C; H NMR (400 MHz, DMSO-d6) δ
1-(2,6-Difluorobenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahydro-
5,9-epoxyimidazo[1,2-a]azocine (28). Yield, 72%; mp,
7.78−7.53 (m, 4H), 5.18−4.96 (m, 3H), 4.82 (d, J = 15.8 Hz,
1H), 3.79−3.61 (m, 4H), 1.85−1.65 (m, 4H), 1.60−1.35 (m,
2H) ppm. 19F NMR (376 MHz, DMSO-d6) δ-60.78 (s, 3F)
ppm. 13C NMR (100 MHz, DMSO-d6) δ 157.01, 143.25 (d, J
= 249.5 Hz, 1C), 132.20, 128.34 (t, J = 7.4 Hz, 1C), 118.77,
109.96 (d, J = 3.8 Hz, 1C), 105.13, 81.22, 68.45, 53.94, 49.63,
45.85, 28.31, 26.66, 14.29 ppm. HRMS (ESI) calcd for
C17H19N3O3F3 [M + H]+, 370.1379, found, 370.1376.
1
143.3−144.8 °C; H NMR (400 MHz, DMSO-d6) δ 7.51−
7.41 (m, 1H), 7.21−7.11 (m, 2H), 5.15−5.05 (m, 3H), 5.05−
4.99 (m, 1H), 3.73−3.59 (m, 2H), 3.58−3.45 (m, 2H), 1.83−
1.66 (m, 4H), 1.58−1.48 (m, 1H), 1.42−1.27 (m, 1H) ppm.
19F NMR (376 MHz, DMSO-d6) δ -114.24 (s, 1F), -114.27 (s,
1F) ppm. HRMS (ESI) calcd for C16H18N3O3F2 [M + H]+,
338.1316, found, 338.1315.
10-Nitro-1-((6-(trifluoromethyl)pyridin-3-yl)methyl)-
1,2,3,5,6,7,8,9-octahydro-5,9-epoxyimidazo[1,2-a]azocine
1-(2,6-Dimethylbenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahy-
dro-5,9-epoxyimidazo[1,2-a]azocine (29). Yield, 84%; mp,
1
(23). Yield, 73%; mp, 147.1−148.4 °C; H NMR (400 MHz,
1
DMSO-d6) δ 8.75 (d, J = 1.2 Hz, 1H), 8.06 (dd, J = 8.0, 1.2
Hz, 1H), 7.92 (d, J = 8.0 Hz, 1H), 5.15 (s, 1H), 5.08 (d, J =
16.2 Hz, 2H), 4.83 (d, J = 16.2 Hz, 1H), 3.82−3.65 (m, 4H),
1.85−1.68 (m, 4H), 1.55 (d, J = 13.8 Hz, 1H), 1.49−1.35 (m,
1H) ppm. 19F NMR (376 MHz, DMSO-d6) δ-66.36 (s, 3F)
ppm. 13C NMR (100 MHz, DMSO-d6) δ 162.91, 156.08,
135.93, 131.36, 130.52, 129.09 (q, J = 22.8 Hz, 1C), 124.83,
105.01, 81.59, 68.79, 48.92, 45.94, 45.31, 28.23, 26.88, 14.35
ppm. HRMS (ESI) calcd for C16H18N4O3F3 [M + H]+,
371.1331, found, 371.1332.
130.7−131.7 °C; H NMR (400 MHz, DMSO-d6) δ 7.18−
7.12 (m, 3H), 5.13 (d, J = 2.8 Hz, 1H), 5.07 (s, 1H), 4.84 (d, J
= 14.4 Hz, 1H), 4.68 (d, J = 14.4 Hz, 1H), 3.59−3.47 (m, 1H),
3.47−3.28 (m, 2H), 3.25−3.13 (m, 1H), 2.31 (s, 6H), 1.91 (d,
J = 13.2 Hz, 1H), 1.81−1.67 (m, 3H), 1.63−1.40 (m, 3H)
ppm. 13C NMR (100 MHz, DMSO-d6) δ 155.96, 137.80,
131.66, 128.48, 128.15, 104.97, 81.82, 68.98, 47.67, 46.07,
45.37, 28.54, 26.53, 19.49, 14.54 ppm. HRMS (ESI) calcd for
C18H23N3O3Na [M + Na]+, 352.1637, found, 352.1636.
1-(2,5-Dimethylbenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahy-
dro-5,9-epoxyimidazo[1,2-a]azocine (30). Yield, 80%; mp,
129.4−130.6 °C; 1H NMR (400 MHz, DMSO-d6) δ 7.07 (d, J
= 8.6 Hz, 2H), 7.00 (d, J = 7.6 Hz, 1H), 5.13 (s, 1H), 5.08 (d,
J = 2.4 Hz, 1H), 4.90 (d, J = 15.8 Hz, 1H), 4.72 (d, J = 15.8
Hz, 1H), 3.73−3.51 (m, 4H), 2.26 (s, 3H), 2.19 (s, 3H),
1.89−1.66 (m, 4H), 1.55−1.49 (m, 2H) ppm. 13C NMR (100
MHz, DMSO-d6) δ 156.40, 134.76, 134.67, 132.30, 130.05,
127.92, 127.62, 105.03, 81.37, 68.62, 51.91, 48.81, 45.81,
28.34, 26.79, 20.72, 18.13, 14.43 ppm. HRMS (ESI) calcd for
C18H24N3O3 [M + H]+, 330.1818, found, 330.1819.
1-([1,1′-Biphenyl]-4-ylmethyl)-10-nitro-1,2,3,5,6,7,8,9-oc-
tahydro-5,9-epoxyimidazo[1,2-a]azocine (24). Yield, 84%;
1
mp, 129.6−130.8 °C; H NMR (400 MHz, DMSO-d6) δ
7.79−7.56 (m, 4H), 7.55−7.33 (m, 5H), 5.19−4.96 (m, 3H),
4.83 (d, J = 15.2 Hz, 1H), 3.79−3.58 (m, 4H), 1.89−1.65 (m,
4H), 1.63−1.34 (m, 2H) ppm. 13C NMR (100 MHz, DMSO-
d6) δ 156.06, 139.76, 139.25, 136.27, 128.92, 128.38, 127.41,
126.72, 126.59, 105.35, 81.27, 68.53, 53.39, 49.22, 45.82,
28.28, 26.70, 14.37 ppm. HRMS (ESI) calcd for C22H24N3O3
[M + H]+, 378.1818, found, 378.1816.
1-(4-(tert-Butyl)benzyl)-10-nitro-1,2,3,5,6,7,8,9-octahy-
dro-5,9-epoxyimidazo[1,2-a]azocine (25). Yield, 85%; mp,
137.9−138.5 °C; H NMR (400 MHz, DMSO-d6) δ 7.44−
7.33 (m, 2H), 7.25−7.21 (m, 2H), 5.13−5.04 (m, 2H), 4.92
1-(3,4-Dimethylbenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahy-
dro-5,9-epoxyimidazo[1,2-a]azocine (31). Yield, 88%; mp,
137.1−137.8 °C; H NMR (400 MHz, DMSO-d6) δ 7.15−
6.99 (m, 3H), 5.16−5.04 (m, 2H), 4.92 (d, J = 14.8 Hz, 1H),
1
1
3009
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Article
Table 2. Mortality of a Mixture of Imidacloprid (1 mg/L) with the New Compounds 1−33 at Low Concentrations (0.5−4 mg/
a
L) against A. craccivora
mortality (co-applied with imidacloprid) (%)
compound
R
0.5 mg/L
nt
1 mg/L
2 mg/L
4 mg/L
1
H
35.7 3.9
41.2 2.3
39.2 2.0
44.5 3.1
53.9 1.2
34.0 5.5
38.9 2.4
55.3 1.8
36.7 2.3
44.4 1.9
45.3 1.5
39.0 1.9
54.1 2.7
82.9 2.5
39.2 1.5
55.0 2.0
84.8 2.5
39.0 1.5
38.2 1.2
40.4 2.6
55.7 1.2
58.5 4.0
49.6 2.4
71.9 1.9
94.2 0.7
82.0 4.4
51.6 1.1
54.7 1.5
60.8 2.8
64.6 1.7
64.7 1.1
60.0 1.9
48.8 2.4
86.6 3.1
66.6 2.4
72.1 2.8
69.5 1.7
76.5 3.2
81.6 1.9
69.8 1.3
74.3 2.1
72.5 1.8
74.8 2.0
76.9 2.5
73.6 2.2
75.6 1.7
75.4 1.2
88.5 3.1
80.7 3.2
87.6 2.8
90.4 1.8
76.1 1.6
82.9 2.1
85.6 2.9
79.7 1.5
81.7 2.7
86.0 1.8
88.2 1.3
97.0 2.2
90.1 1.9
69.8 2.4
76.0 1.8
81.7 1.1
81.9 3.8
82.0 1.4
81.4 1.6
74.5 2.9
90.1 2.5
71.8 3.7
79.8 4.2
76.4 1.7
77.9 1.7
85.3 1.6
74.6 1.8
80.1 1.3
84.4 2.1
80.9 1.7
81.7 1.8
75.4 3.9
76.4 2.4
82.4 2.3
91.4 1.9
81.3 1.5
88.4 2.3
93.4 1.5
81.2 2.5
85.7 3.0
87.3 1.0
82.5 2.2
83.0 1.9
88.7 1.8
92.8 1.3
98.1 2.7
92.5 1.6
77.7 6.0
80.9 2.0
87.6 3.9
86.2 1.1
87.3 1.5
86.3 1.8
84.4 1.6
93.0 0.9
2
3
4
5
6
7
8
9
C6H5
2-F-C6H5
3-F-C6H5
4-F-C6H5
2-Cl-C6H5
3-Cl-C6H5
4-Cl-C6H5
2-Br-C6H5
3-Br-C6H5
4-Br-C6H5
2-Me-C6H5
3-Me-C6H5
nt
nt
nt
nt
nt
nt
nt
nt
nt
nt
nt
nt
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
IPPA08
4-Me-C6H5
59.8 2.4
nt
nt
63.0 1.1
nt
nt
nt
nt
nt
nt
34.6 1.2
77.0 2.0
56.5 2.0
nt
nt
nt
nt
nt
2-OMe-C6H5
3-OMe-C6H5
4-OMe-C6H5
2-CN-C6H5
3-CN-C6H5
4-CN-C6H5
4-NO2-C6H5
4-CF3-C6H5
6-CF3-pyridyl
4-Ph-C6H5
4-t-Bu -C6H5
4-i-Pr-C6H5
2,6-(Cl)2-C6H5
2,6-(F)2-C6H5
2,6-(Me)2-C6H5
2,5-(Me)2-C6H5
3,4-(Me)2-C6H5
3,4-(OMe)2-C6H5
2-chlorothiazol-5-yl
nt
nt
70.6 2.1
a
Effects of compounds 1−33 and IPPA08 on imidacloprid (1 mg/L) toxicity against A. craccivora. Data are mean of at least six independent
experiments standard error of the mean (SEM). Imidacloprid alone at a concentration of 1 mg/L gives 29% mortality. nt = not test.
4.66 (d, J = 14.8 Hz, 1H), 3.69−3.52 (m, 4H), 2.20 (s, 6H),
1.87−1.62 (m, 4H), 1.53 (d, J = 13.8 Hz, 1H), 1.45−1.28 (m,
1H) ppm. 13C NMR (100 MHz, DMSO-d6) δ 155.97, 136.17,
135.31, 134.24, 129.53, 129.09, 125.33, 105.35, 81.23, 68.54,
53.22, 48.84, 45.74, 28.17, 26.77, 19.39, 19.01, 14.29 ppm.
HRMS (ESI) calcd for C18H23N3O3Na [M + Na]+, 352.1637,
found, 352.1638.
1H), 5.31 (s, 1H), 5.24 (d, J = 15.4 Hz, 1H), 4.97 (s, 1H), 4.78
(d, J = 15.4 Hz, 1H), 3.85−3.74 (m, 1H), 3.73−3.58 (m, 3H),
2.12 (d, J = 13.4 Hz, 1H), 1.99−1.78 (m, 2H), 1.77−1.51 (m,
4H) ppm. 13C NMR (100 MHz, CDCl3) δ 155.68, 154.16,
140.49, 135.56, 106.97, 82.71, 69.36, 49.40, 48.31, 46.17,
29.43, 26.47, 14.93 ppm. HRMS (ESI) calcd for
C13H15N4O3S35ClNa [M + Na]+, 365.0451, found, 365.0452;
calcd for C13H15N4O3S37ClNa [M + Na]+, 367.0422, found,
367.0422.
1-(3,4-Dimethoxybenzyl)-10-nitro-1,2,3,5,6,7,8,9-octahy-
dro-5,9-epoxyimidazo[1,2-a]azocine (32). Yield, 70%; mp,
1
121.9−123.0 °C; H NMR (400 MHz, DMSO-d6) δ 6.91−
BIOLOGICAL RESULTS
6.87 (m, 3H), 5.14−5.04 (m, 2H), 4.86 (d, J = 14.8 Hz, 1H),
4.71 (d, J = 14.8 Hz, 1H), 3.73 (d, J = 6.4 Hz, 6H), 3.70−3.53
(m, 4H), 1.84−1.64 (m, 4H), 1.51 (d, J = 13.8 Hz, 1H), 1.38−
1.27 (m, 1H) ppm. 13C NMR (100 MHz, DMSO-d6) δ 155.95,
148.66, 148.24, 129.15, 120.33, 111.67, 105.54, 81.29, 68.55,
55.46, 55.30, 53.11, 48.85, 45.87, 28.17, 26.77, 14.31 ppm.
HRMS calcd for C18H24N3O5 [M + H]+, 362.1716, found,
362.1717.
■
Synergistic Effect on Imidacloprid. The mortality
caused by the oxabridged compounds and their effects on
imidacloprid toxicity against A. craccivora were assessed. None
of the oxabridged homologues showed any mortality at a low
concentration (10 mg/L). When the oxabridged compounds
were applied at concentrations of 4 and 2 mg/L together with
1 mg/L imidacloprid, the toxicity against A. craccivora strongly
increased (Table 2). Moreover, even at a low concentration of
0.5 mg/L, four compounds (14, 17, 25, and 26) possessing an
electron-donating group (EDG) at position 4 of the phenyl
1-((2-Chlorothiazol-5-yl)methyl)-10-nitro-1,2,3,5,6,7,8,9-
octahydro-5,9-epoxyimidazo[1,2-a]azocine (33). Yield, 84%;
mp, 134.9−135.7 °C; 1H NMR (400 MHz, CDCl3) δ 7.48 (s,
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Figure 2. (A) Effects of IPPA08 on imidacloprid (1 mg/L) toxicity against A. craccivora. (B) Effects of compound 25 on imidacloprid (1 mg/L)
toxicity against A. craccivora. Data are means of at least six independent experiments SEM. Significant differences from blank control: *p < 0.05,
**p < 0.01, ***p < 0.001.
a
Table 3. Synergistic Effects of IPPA08 and Compound 25 on Imidacloprid Toxicity Against A. craccivora
a
b
Data are the means of at least six independent experiments. IPPA08 and compound 25 at concentrations in mg/L showed no insecticidal
c
activities against A. craccivora. CI = confidence interval (CI can be used to estimate population parameters; the narrower the scope of CI, the
better the reliability of population parameters estimated by sample indicators). SR = synergism ratio. Significant differences from blank control: *p
< 0.05, **p < 0.01, ***p < 0.001.
d
ring increased the mortality of A. craccivora by imidacloprid
and exhibited obvious synergy. In particular, compound 25
showed higher synergistic activity than IPPA08. In addition,
compared to compound 24, compound 25 was more effective,
which indicated that electronic factor could be the major factor
instead of steric factor.
Influence of IPPA08 and Compound 25 on the
Toxicities of Other Neonicotinoids. The effects of
IPPA08 and compound 25 on the toxicities of other
neonicotinoids were also tested, including nitenpyram,
dinotefuran, acetamiprid, thiamethoxam, paichongding, and
cycloxaprid. For A. craccivora, IPPA08 and compound 25 at
low concentrations (causing no toxicity by itself) increased the
moralities caused by fixed concentrations (1 mg/L) of
nitenpyram, acetamiprid, and thiamethoxam, and compound
25 showed higher effectiveness as a synergist (Figure 3A,C,D).
However, IPPA08 and compound 25 showed little impact on
the toxicities of dinotefuran and paichongding against A.
craccivora (Figure 3B,E). Interestingly, compound 25 showed
weak effects on cycloxaprid toxicity against A. craccivora at 4
mg/L (Figure 3F). IPPA08 and compound 25 showed good
stability under these conditions, excluding the possibility that
the synergistic effects were caused by metabolites (Table 4).
Safety of Compound 25 to Honey Bee. Bee mortalities
by the independent use of imidacloprid or by the combined
use of imidacloprid and compound 25 (1:1) were tested
(Table 5). The mixture of imidacloprid and compound 25 did
not exhibit a higher toxicity toward the bee than the
imidacloprid alone. However, for A. craccivora, the mortality
by imidacloprid (30% at 1 mg/L) could be increased to 90%
with 1 mg/L compound 25 (Figure 2B). These results
Further study showed that the mortality of A. craccivora by
imidacloprid (29% at 1 mg/L) could be increased to 90% with
1.0 mg/L IPPA08, at which IPPA08 alone showed no
insecticidal activity (Figure 2A). Similarly, 30% mortality of
A. craccivora by 1 mg/L imidacloprid was increased to 97%
when combined with compound 25 at 0.75 mg/L, a
concentration at which compound 25 itself caused no
mortality (Figure 2B). Compared to IPPA08, compound 25
presented higher effects to synergize imidacloprid at the same
co-applied concentrations (0.25, 0.75 mg/L). For a co-applied
concentration at 0.25 mg/L, IPPA08 and compound 25
increased the toxicity of imidacloprid against A. craccivora by
2.84- and 3.51-fold, respectively. For a co-applied concen-
tration at 0.75 mg/L, IPPA08 and compound 25 increased the
toxicity of imidacloprid against A. craccivora by 3.50- and 6.54-
fold, respectively. Even though IPPA08 and compound 25
showed optimal co-applied concentrations of 1 and 0.75 mg/L,
respectively, compound 25 (6.54-fold) showed a higher
synergism ratio than IPPA08 (4.50-fold) (Table 3).
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Figure 3. Effects of IPPA08 and compound 25 on the toxicity of neonicotinoid insecticides, including nitenpyram, dinotefuran, acetamiprid,
thiamethoxam, paichongding, and cycloxaprid. The concentration for each neonicotinoid insecticide was 1 mg/L, and the concentrations of
IPPA08 and compound 25 were (a−e) 0, 0.5, 1, 2, and 4 mg/L, respectively. Data are mean of at least six independent experiments SEM.
indicated that compound 25 displayed selectivity in its effects
on imidacloprid toxicity against the honey bee (A. mellifera L.).
Table 4. Stability of IPPA08 and Compound 25 in PBS at
Different pHs
a
compound
IPPA08
pH
detection time
degradation rate (%)
DISCUSSION
■
4.0
7.0
9.0
4.0
7.0
9.0
3 days
7 days
5 days
3 days
7 days
5 days
1.65
Synergists can improve the pesticidal performance of
insecticides, such as increasing insecticidal activity, reducing
application frequency, and decreasing the rate of resistance
development.20 In our previous study, we found that IPPA08, a
novel neonicotinoid with a unique oxabridged substructure,
has proven strong synergistic effects on neonicotinoid
insecticides, which is currently the most crucial class of
insecticides.15,16 To discover new synergists with higher
0.809
0.501
1.49
0.527
0.333
25 (t-Bu)
a
Stability was detected by UPLC.
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a b
,
Table 5. Bee Toxicity (A. mellifera L.) of Imidacloprid and a Mixture of Imidacloprid with Compound 25
compound
bee toxicity
times
toxic regression equation
LD50 (μg/worker)
95% CI
toxicological grade
25 + IMI (1:1)
contact toxicity
24 h
48 h
24 h
48 h
24 h
48 h
24 h
48 h
24 h
48 h
24 h
48 h
F
F
F
y = 1.018x + 2.341
y = 1.797x + 0.690
y = 1.057x + 2.970
F
y = 0.999x + 1.015
F
y = 1.383x + 0.234
F
0.118
1.93
1.06
F
0.096
F
0.677
F
>12
F
0.069−0.228
1.54−2.57
0.624−2.60
F
0.053−0.363
F
II
II
II
oral toxicity
IMI
25
contact toxicity
oral toxicity
II
0.555−0.858
F
II
contact toxicity
oral toxicity
IV
F
F
>27
IV
a
b
F, failed to measure, which means that some of the bee in the test group escaped. The grade of the bee toxicity, very toxicity: I; high toxicity: II;
moderate toxicity: III; low toxicity: IV.
1H NMR and 13C NMR spectral data of compounds 1−
33 (PDF)
effectiveness, a series of eight-membered oxabridged neon-
icotinoid analogues were synthesized. Biological evaluation
revealed that all of these compounds possess significant
synergistic effects on imidacloprid toxicity against cowpea
aphid (A. craccivora) under the concentration of 4 and 2 mg/L.
The structure−activity relationships (SARs) showed that
compound 25 having an electron-donating group (EDG) t-
Bu at position 4 of the phenyl ring exhibited excellent
synergistic effects for imidacloprid activity. Interestingly,
compound 25 presented a higher degree of synergism than
IPPA08, along with a notable selectivity in its effects on
imidacloprid toxicity against the honey bee (A. mellifera L.).
These results indicate that replacement of the pyridyl ring by a
phenyl ring was a successful strategy to obtain a novel synergist
with improved effectiveness. Further studies on field-testing of
the title compounds are ongoing.
AUTHOR INFORMATION
■
Corresponding Author
Zhong Li − Shanghai Key Laboratory of Chemical Biology,
School of Pharmacy, East China University of Science and
Technology, Shanghai 200237, China; orcid.org/0000-
0001-6665-5040; Phone: +86-21-64253540;
Email: lizhong@ecust.edu.cn; Fax: +86-21-64252603
Authors
Xiao Zhang − Shanghai Key Laboratory of Chemical Biology,
School of Pharmacy, East China University of Science and
Technology, Shanghai 200237, China
Our previous investigation found that at low concentrations,
IPPA08 can act as a positive allosteric modulator of
noncanonical interfaces and likely slow the decay of currents
through stabilizing the open-channel state caused by the action
of imidacloprid on canonical interfaces.16 Based on our present
results, we propose two hypotheses. First, the origin of the
differential synergistic activity of compound 25 and IPPA08
toward A. craccivora and A. mellifera L. likely resides in the
target itself. Second, it is possible that the synergistic
mechanism of compound 25 is different from that of
IPPA08 considering their structural difference, i.e., phenyl
ring and pyridyl ring. We are currently working to gain
experimental evidence to further support such hypotheses. In
this work, we only provided some implicit cues for such
hypotheses. At the same time, our present study provides a
unique strategy to alleviate the dilemma from neonicotinoids.
When imidacloprid or thiamethoxam are co-applied with
compound 25, at the same insecticidal activity level, the
amount of imidacloprid or thiamethoxam was reduced. The
reduction in the use of neonicotinoids indirectly improves the
safety of honey bees as well as decreases the risk of
environmental residue by neonicotinoids. Moreover, com-
pound 25 is expected to be developed as a novel synergist,
which is selective and safe for honey bee.
Yiping Wang − Shanghai Key Laboratory of Chemical
Biology, School of Pharmacy, East China University of
Science and Technology, Shanghai 200237, China
Zhiping Xu − Shanghai Key Laboratory of Chemical Biology,
School of Pharmacy, East China University of Science and
Technology, Shanghai 200237, China
Xusheng Shao − Shanghai Key Laboratory of Chemical
Biology, School of Pharmacy, East China University of
Science and Technology, Shanghai 200237, China;
orcid.org/0000-0002-2579-7533
Zewen Liu − Key Laboratory of Integrated Management of
Crop Diseases and Pests (Ministry of Education), College of
Plant Protection, Nanjing Agricultural University, Nanjing,
Jiangsu 210095, China; orcid.org/0000-0003-1084-
9449
Xiaoyong Xu − Shanghai Key Laboratory of Chemical
Biology, School of Pharmacy, East China University of
Science and Technology, Shanghai 200237, China;
orcid.org/0000-0001-9679-3153
Peter Maienfisch − Shanghai Key Laboratory of Chemical
Biology, School of Pharmacy, East China University of
Science and Technology, Shanghai 200237, China; Research
Portfolio Manager Insecticides and Seedcare, Syngenta Crop
Protection AG, Basel CH-4002, Switzerland
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.jafc.0c04786
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jafc.0c04786.
Notes
The authors declare no competing financial interest.
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Guidelines for the Testing of Chemicals; OECD: Paris, France, 2004;
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ACKNOWLEDGMENTS
■
This work was financial supported by National Key Research
and Development Program of China (2017YFD0200500) and
Innovation Program of Shanghai Municipal Education
Commission (2017-01-07-00-02-E00037).
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