2023 Volume 48 Issue 4 Pages 202-210
Acynonapyr, discovered by Nippon Soda Co., Ltd., is a novel acaricide with N-pyridyloxy azabicycle as a unique core structure. Acynonapyr exhibits high activity against the spider mite species in the genera Tetranychus and Panonychus, with good efficacy at all life stages. Early in this research, cyclic amines substituted with (hetero)aryl(oxy) moieties were designed as target molecules and diversely synthesized, and 4-[4-(trifluoromethyl)phenoxy]-1-[5-(trifluoromethyl)-2-pyridyl]piperidine was found to show weak acaricidal activity. The structural optimization of this acaricidal active piperidine as the first lead compound led to the discovery of acynonapyr. In this report, our research process that led to the discovery of acynonapyr is described.
Spider mites in genera such as Tetranychus and Panonychus are pests causing severe damage to a broad range of crops including fruits, vegetables, and teas. However, controlling mites has been a difficult and laborious problem since before because mites exhibit rapid resistance development to acaricides, which is caused by extremely high fecundity and a short life cycle. Although the rotational application of several selected acaricides with different modes of action has been used as a common approach to overcome this problem, only a few conventional acaricides have shown high efficacy at most agricultural sites. On the other hand, novel acaricides with new modes of action have been highly desirable, along with the properties of specific activity against the mites and superior safety toward non-target organisms from the viewpoint of integrated pest management (IPM) programs.1) Our discovery of a new lead compound with weak acaricidal activity led us to research and develop a novel acaricide with a highly unique chemical structure, considering the on-site situations described above. Herein, we report the discovery, synthesis, and acaricidal activity of acynonapyr (1; Fig. 1) and related compounds.2,3)
1H NMR spectra were recorded on JEOL ECA-500 and Bruker ADVANCE III HD600 spectrometers in CDCl3 solution with tetramethylsilane as an internal standard. Melting points were uncorrected. 1H NMR spectral data of several key compounds during lead optimization are described in the Supplementary material (Supplemental Table S1).
1.1. Synthesis of acynonapyr (1) via N-hydroxylation of an NH-free azabicycleThe synthetic route to 1 via N-hydroxylation of an NH-free azabicycle is shown in Fig. 2. Bicyclic ketone 2 was obtained by the Mannich reaction using glutaraldehyde, 3-oxopentanedioic acid and benzylamine, and the subsequent ketone reduction gave 3-endo-ol 3 stereoselectively.4) O-arylated azabicycle 5 was obtained by coupling 3 with fluorobenzene 4, and debenzylation of 5 gave NH-free azabicycle 6. Cyanoethylamine 7 was obtained by reacting 6 with acrylonitrile, and oxidation of 7 gave hydroxylamine 8. Finally, coupling of 8 with chloropyridine 9 provided 1.
To a solution of 9-benzyl-9-azabicyclo[3.3.1]nonane-3-endo-ol 34) (3.12 g, 13.5 mmol) in DMF (30 mL) was added 60% NaH (0.81 g, 20.25 mmol) at 0°C. The mixture was warmed to room temperature and stirred for 30 min. Then 4-fluoro-3-propoxybenzotrifluoride 4 (3.00 g, 13.5 mmol) was added. The mixture was heated to 80°C and stirred for 2 hr. After cooling, it was worked up in the following fashion (procedure A): the mixture was poured into water and AcOEt, and the phases were separated. The aqueous phase was extracted with AcOEt, and the combined organic phases were washed with brine, dried over MgSO4, filtered, and concentrated. The residue was purified by silica gel chromatography (hexane/AcOEt, 7/3) to give compound 5 (4.70 g, 80%): viscous oil; 1H-NMR (CDCl3, 600 MHz): δ 1.06 (t, J=6 Hz, 3H), 1.21–1.24 (m, 2H), 1.50–1.52 (m, 1H), 1.66–1.70 (m, 2H), 1.83–1.87 (m, 2H), 1.95–2.00 (m, 2H), 2.43–2.54 (m, 3H), 3.03 (brs, 2H), 3.82 (s, 2H), 3.97 (t, J=6 Hz, 2H), 4.74–4.78 (m, 1H), 6.94 (d, J=12 Hz, 1H), 7.08 (s, 1H), 7.16 (d, J=12 Hz, 1H), 7.23 (t, J=9 Hz, 1H), 7.31 (t, J=6 Hz, 1H), 7.35 (t, J=6 Hz, 2H) ppm.
1.1.2. 3-endo-[2-Propoxy-4-(trifluoromethyl)phenoxy]-9-hydroxy-9-azabicyclo[3.3.1]nonane 8To a solution of compound 5 (4.70 g, 10.84 mmol) in EtOH (100 mL) was added 20% Pd(OH)2/C (0.94 g), and the mixture was stirred under a hydrogen atmosphere at 50°C for 5 hr. After cooling to room temperature, the mixture was filtered through Celite, and the filtrate was concentrated under reduced pressure. The crude debenzylated compound was prepared, purified, and stored as an HCl salt (6·HCl) on a large scale. To a solution of NH-free azabicycle 6 (HCl salt free; 3.69 g, 10.75 mmol) in MeOH (60 mL) was added acrylonitrile (1.43 g, 27 mmol) at room temperature, and the mixture was stirred overnight. The solvent was removed under reduced pressure, and the residue was purified by silica gel chromatography (hexane/ AcOEt, 4/1 → 6/4) to give cyanoethylamine 7 (2.68 g, 63%): mp 38–39°C; 1H-NMR (CDCl3, 600 MHz): δ 1.06 (t, J=6 Hz, 3H), 1.26–1.29 (m, 2H), 1.45–1.49 (m, 1H), 1.70–1.74 (m, 2H), 1.82–1.88 (m, 4H), 2.42–2.53 (m, 5H), 2.91 (t, J=6 Hz, 2H), 3.06 (brs, 2H), 3,97 (t, J=6 Hz, 2H), 4.65–4.68 (m, 1H), 6.92 (d, J=12 Hz, 1H), 7.08 (s, 1H), 7.15 (d, J=12 Hz, 1H) ppm.
To a solution of cyanoethylamine 7 (2.68 g, 6.76 mmol) in CH2Cl2 (50 mL) were added m-CPBA (ca.70% purity, 2.11 g, 8.6 mmol) and K2CO3 (1.40 g, 10.1 mmol) at room temperature. After the mixture was stirred for 2 hr, MgSO4 (ca.17 g) was added. The mixture was filtered, and the filtrate was concentrated under reduced pressure. The residue was diluted with CH2Cl2 and the organic phase was washed with L-Ascorbic acid aq. and water, dried over MgSO4, filtered, and concentrated. The residue was purified by silica gel chromatography (hexane/AcOEt, 1/1 → 1/2) to give 9-hydroxy-9-azabicycle 8 (2.28 g, 94%): white solid, mp 94–100°C; 1H-NMR (CDCl3, 25°C, 600 MHz): δ 1.06 (t, J=6 Hz, 3H), 1.19–1.22 (m, 0.37H), 1.28–1.32 (m, 0.86H), 1.45–1.47 (m, 0.26H), 1.53–1.57 (m, 1.87H), 1.66–1.69 (m, 1.89H), 1.82–1.87 (m, 3.71H), 1.97–2.00 (m, 0.36H), 2.29–2.36 (m, 1.18H), 2.47–2.52 (m, 0.37H), 2.57–2.64 (m, 1.82H), [3.35 (brs, 0.34H), 3.45 (brs, 1.66H)], 3.95–3.98 (m, 2H), [4.52–4.54 (m, 0.17H), 4.95–4.99 (m, 0.83H)], [6.88 (d, J=6 Hz, 0.17H), 6.94 (d, J=12 Hz, 0.83H)], [7.07 (s), 7.08 (s), total 1H], [7.15 (s), 7.16 (s), total 1H], 7.99 (brs, 1H) ppm. The broad signal at δ 7.99 ppm corresponds to the proton of the 9-OH group of 8, which disappeared after D2O exchange. Also, 8 is presumed to exist as a mixture of two conformers around room temperature owing to a slow equilibrium between the axial and equatorial N-OH groups in a structural event similar to that of acynonapyr 1 as described in Section 1.1.3.
1.1.3. 3-endo-[2-Propoxy-4-(trifluoromethyl)phenoxy]-9-[5-(trifluoromethyl)-2-pyridyloxy]-9-azabicyclo[3.3.1]nonane 1 (Acynonapyr)To a solution of 8 (0.50 g, 1.39 mmol) and chloropyridine 9 (0.26 g, 1.39 mmol) in THF (5 mL) was added t-BuOK (1 M in THF, 1.4 mL, 1.4 mmol) at 0°C under nitrogen atmosphere. The mixture was warmed to room temperature and stirred for 2 hr. The mixture was worked up as in procedure A, and the residue was purified by silica gel chromatography (hexane/AcOEt, 98/2 → 95/5) to give acynonapyr 1 (0.44 g, 63%). mp 77.2–78.8°C; 1H-NMR (CDCl3, 25°C, 500 MHz): δ 1.05–1.10 (q-like, 3H), 1.34–1.42 (m, 1.22H), 1.70–1.74 (m, 1.60H), 1.78–1.86 (m, 3.61H), 2.06–2.13 (m, 2.03H), [2.38–2.43 (m), 2.57–2.77 (m), total 3.52H], 3.56–3.59 (m, 2H), 3.96–4.00 (m, 2H), [4.64–4.68 (m), 4.91–4.95 (m), total 1H], 6.93 (d, J=12 Hz, 1H), 7.09–7.10 (m, 1H), 7.17 (d, J=12 Hz, 1H), [7.26 (d, J=6 Hz, 0.22H), 7.39 (d, J=12 Hz, 0.78H)], 7.85–7.89 (m, 1H), 8.49 (s, 1H) ppm.
Variable-temperature 1H-NMR measurements in DMSO-d6 show that 1 exists as a mixture of two conformers in a ratio of ca. 4 : 1 around room temperature. This observation is presumably attributable to a slow equilibrium between axial and equatorial N-oxy moieties at the 9-position. In the NMR spectra of 1 at 25°C, the 3-exo-proton of the azabicyclic ring is observed as two signals at δ 4.93 and 4.66 ppm (each multiplet); these signals change to a broad one at δ 4.90 for measurements at 150°C, then appear again at 25°C in the same pattern as described above.
1.2. Synthesis of acynonapyr (1) via oxidation of an N-pyridyl azabicycleAnother synthetic route to 1 via oxidation of azabicycle 10 is shown in Fig. 3. To a solution of 6·HCl salt (15.0 g, 39.6 mmol) in CH3CN (120 mL) were added chloropyridine 9 (21.50 g, 0.118 mol) and K2CO3 (21.9 g, 0.16 mol). The mixture was then heated under reflux and stirred overnight. After cooling, it was worked up as in procedure A. The residue was purified by silica gel chromatography (hexane/AcOEt, 98/2) to give azabicycle 10 (2.43 g, 13%): viscous oil; 1H-NMR (CDCl3, 600 MHz): δ 1.03 (t, J=6 Hz, 3H), 1.54–1.57 (m, 1H), 1.63–1.66 (m, 2H), 1.75–1.85 (m, 4H), 1.87–1.91 (m, 2H), 2.44–2.48 (m, 2H), 2.61–2.69 (m, 1H), 3.95 (t, J=6 Hz, 2H), 4.36–4.40 (m, 1H), 4.82 (brs, 2H), 6.62 (d, J=12 Hz, 1H), 6.88 (d, J=12 Hz, 1H), 7.08 (s, 1H), 7.13 (d, J=6 Hz, 1H), 7.60 (d, J=12 Hz, 1H), 8.37 (s, 1H) ppm.
To a solution of azabicycle 10 (1.00 g, 2 mmol) in CH2Cl2 (10 mL) was added m-CPBA (ca.65% purity, 0.81 g, 3 mmol) at room temperature. The mixture was then stirred overnight and diluted with AcOEt. The organic phase was washed with saturated Na2S2O4 aq., NaHCO3 aq., and water, and dried over MgSO4. The solvent was removed under reduced pressure, and the residue was purified by silica gel chromatography (hexane/AcOEt, 98/2 → 95/5) to give acynonapyr 1 (0.37 g, 37%).
2. Bioassays2.1. Acaricidal activity against Tetranychus urticaeThe test compound was dissolved in DMF to give the 5% of emulsifiable concentrate (EC) formulation and diluted to several concentrations (125, 31, 8, 2 and 0.5 ppm) with water containing the surfactant (RABIDEN 3S®, 0.2 mL/L) depending on the activity evaluation. Seventeen adult female spider mites were released on the first true leaf of a kidney bean plant 7–10 days after germination. The plant and spider mites were sprayed with the above test solutions and placed in a thermostatic chamber at 25°C and 65% humidity. Mortality of adult mites was assessed by the number of surviving mites 3 days after treatment.
2.2. Acaricidal activity against Tetranychus kanzawaiTen adult female spider mites were released on a kidney bean leaf disk in a Petri dish. The leaf disks and spider mites were sprayed with the test solutions using a rotary distributing sprayer. The following procedures were conducted similarly to those described in Section 2.1.
2.3. Acaricidal activity against Panonychus citriTen adult female spider mites were released on a citrus leaf disk in a Petri dish. The leaf disks and spider mites were sprayed with the test solutions using a rotary distributing sprayer. The following procedures were conducted similarly to those described in Section 2.1.
2.4. Activity against eggs, larvae and nymphs of T. urticae and P. citriThe test solutions were prepared by diluting the suspension concentrate (SC) formulation containing 20% acynonapyr to water added the surfactant (RABIDEN 3S®, 0.2 mL/L). 10–15 adult females of T. urticae or P. citri were released on the leaf disk of a kidney bean or citrus plant in a Petri dish and allowed to oviposit for 24 hr. The leaf disks were sprayed with the test solutions using a rotary distributing sprayer and then placed in a thermostatic chamber at 25°C and 65% humidity. The ovicidal activity was assessed by counting the number of surviving mites and unhatched eggs 7 days after treatment. 10–30 spider mite larvae or nymphs were released on the leaf disk of a kidney bean or citrus plant respectively in a Petri dish. The leaf disks were sprayed with the test solutions using a rotary distributing sprayer and then placed in a thermostatic chamber at 25°C and 65% humidity. The acaricidal activity against larvae and nymphs was assessed by counting the number of surviving mites 3 days after treatment. All experiments were performed in triplicate.
In the early 2000s, we conducted research to discover a novel insecticide with a new mode of action, specifically a targeted pest-selective neural action. From the viewpoint of chemical structure in the course of this research, we focused on alicyclic amine cores substituted with a hydrophobic moiety such as benzyl or diarylmethyl group, which are common to some biologically active compounds with specific neural action, for example, the insecticides reported in a patent literature5) and ion-channel-targeted pharmaceuticals6) (Fig. 4). While applying these amine cores to our target insecticides, we became interested in the possibility of six-membered alicyclic amines substituted with functionalized (hetero)aryl moieties, which were rarely found in the structures of conventional pesticides. Thus, we designed N-(hetero)aryl cyclic amines with another (hetero)aryl(oxy) moiety and planned that these compounds could be diversely prepared in short steps using commercially available reagents, conventional agrochemical core moieties (e.g., fluorine-containing heterocycles), and our chemical library of synthetic intermediates (Fig. 5a). Our main synthetic approach to the targeted amines was as follows: (1) 1,4-bis[(hetero)aryl] piperazines A and 1-[(hetero)aryl]-4-[(hetero)aryloxy]piperidines B were prepared by the reaction of piperazine and 4-hydroxypiperidine with various functionalized (hetero)aryl halides and/or phenols, respectively, via nucleophilic substitution or condensation; (2) the subsequent modifications of amines obtained by the above approach (1) were conducted, such as reduction, halogenation, (de-)protection, and/or alkylation.
The amines obtained by the above approaches (1) and (2) were evaluated for acaricidal and insecticidal activities. The results showed that piperazines A and piperidines B had no insecticidal activity. However, one type of piperidines B, 4-[4-(trifluoromethyl)phenoxy]-1-[5-(trifluoromethyl)-2-pyridyl]piperidine 11 and its 4-(trifluoromethoxy)phenoxy analog 12 (Fig. 5. b) exhibited weak activity against both Tetranychus urticae and Panonychus citri (Table 1), and piperidine 11 was slightly more active than 12 against P. citri. Based on these findings, we evaluated the acaricidal activity affected by structural modifications of the pyridyl moiety and the connection (the oxygen atom shown here) between the benzene ring and the 4-position of piperidines 11 or 12 (Table 1; compounds 13–20). N-[4-(trifluoromethyl)phenyl] piperidine 13, N-[2-chloro-4-(trifluoromethyl)phenyl] piperidine 14, and N-[3-chloro-5-(trifluoromethyl)-2-pyridyl] piperidine 15 were inactive against T. urticae. Also, structural modifications of the connection showed that oxygen atom was more suitable for the acaricidal activity than the other moieties NH, N(Me), C(O), CH2, and CF2 (in compounds 16–20, respectively). Furthermore, T. urticae treated with piperidine 11 showed excitatory neurotoxic symptoms that were not previously observed. These interesting symptoms suggested that 11 might have a new mode of action and encouraged us to conduct discovery research on a novel acaricide. Thus, we selected piperidine 11 as the first lead compound and started its structural optimization as described below.
 | ||||||
|---|---|---|---|---|---|---|
| Compound No. | X | Y | R1 | R2 | Acaricidal activity (Aa)a) | |
| T.u.b) | P.c.c) | |||||
| 11 | O | N | CF3 | H | + | ++ |
| 12 | O | N | OCF3 | H | + | + |
| 13 | O | CH | CF3 | H | − | NDd) |
| 14 | O | CH | CF3 | Cl | − | ND |
| 15 | O | N | CF3 | Cl | − | ND |
| 16 | NH | N | CF3 | H | + | + |
| 17 | N(Me) | N | CF3 | H | + | − |
| 18 | C(O) | N | OCF3 | H | − | ND |
| 19 | CH2 | N | OCF3 | H | − | ND |
| 20 | CF2 | N | OCF3 | H | − | ND |
a) LC90 (ppm): ++++; Aa≦2, +++; 2<Aa≦8, ++; 8<Aa≦31, +; 31<Aa≦125, −; Aa>125. b) Tetranychus urticae. c) Panonychus citri. d) ND: No data.
The structural optimization of 11 was conducted on the [4-(trifluoromethyl)phenyl] moiety (Table 2; compounds 21–26). Compound 21 with a [5-(trifluoromethyl)-2-pyridyl] moiety was inactive, but compound 22, with a chlorine atom at the 2-position of the benzene ring, was about four-fold more active against T. urticae than piperidine 11. We focused on the effect of the chlorine atom and conducted the introduction of various substituents to this position. Acaricidal evaluations of the compounds synthesized in this manner revealed that compound 24 with an ethoxy group and compound 25 with an n-propoxy group were even four-fold more active against both T. urticae and P. citri than compound 22; 25 was slightly more active than 24. Also, the activities of compound 23 with a methoxy group and compound 26 with an n-butoxy group were much lower than those of compounds 24 and 25. Furthermore, compounds 27–35, with substituents similar in size to the n-propoxy group, showed high activity comparable to that of 25 (Fig. 6).
 | ||||
|---|---|---|---|---|
| Compound No. | X | Acaricidal activity (Aa)a) | ||
| T.u.b) | P.c.c) | T.k.d) | ||
| 11 | CH | + | ++ | NDe) |
| 21 | N | − | ND | ND |
| 22 | C–Cl | ++ | ++ | ND |
| 23 | C–OMe | + | ++ | ND |
| 24 | C–OEt | +++ | +++ | ND |
| 25 | C–OnPr | +++ | +++ | + |
| 26 | C–OnBu | − | ND | ND |
a) See the footnote of Table 1. b) Tetranychus urticae. c) Panonychus citri. d) Tetranychus kanzawai. e) ND: No data.
However, the active piperidines 24, 25, and 27–35 were found to be far inferior to major conventional acaricides in practical efficacy because they showed short-term efficacy against both T. urticae and P. citri, and very weak activity against Tetranychus kanzawai. We speculated that this weak activity was due in part to the high metabolic detoxification ability of T. kanzawai, which could cause some structural decomposition of the piperidine moieties, such as elimination of the phenoxy moieties and/or oxidation at the 1 (nitrogen atom)- or 2-position of the piperidine rings. Therefore, to solve the biological problems described above, we conducted structural optimization of piperidine 25 as the second lead compound.
2.2. The conversion of the piperidine ring into azabicyclesWe planned the conversion of the central piperidine ring of compound 25 into sterically rigid azabicycles that could be expected to prevent the above metabolic reactions.7) First, we selected nortropane, 8-azabicyclo[3.2.1]octane that was used as a core structure of the known insecticides8) and pharmaceuticals.9) The two stereoisomers 3-endo-36 and 3-exo-37, which are named in the stereo relationship between the ethylene (C2) bridge and the 3-substituent (the phenoxy moiety shown here) of a tropane ring, were synthesized respectively using commercially available tropine as a starting material. Interestingly, acaricidal evaluations revealed that 3-exo-37 was inactive, but 3-endo-36 was two to four times more active and longer lasting than compound 25 (Table 3).
 | |||||
|---|---|---|---|---|---|
| Compound No. | azabicycle | stereob) | Acaricidal activity (Aa)a) | ||
| T.u.c) | P.c.d) | T.k.e) | |||
| 36 |  | endo | ++++ | +++ | ++ |
| 37 |  | exo | − | NDf) | ND |
| 38 |  | exo | ++++ | ++++ | +++ |
| 39 |  | endo | − | ND | ND |
| 40 |  | exo | ++++ | ++++ | ND |
| 41 |  | exo | +++ | +++ | ++ |
| 25 |  | +++ | +++ | + | |
a) See the footnote of Table 1. b) The stereo relationship between the phenoxy moiety and the C2, C3, or CH2OCH2 bridges of each azabicycle. c) Tetranychus urticae. d) Panonychus citri. e) Tetranychus kanzawai. f) ND: No data.
Next, we selected isotropane, 3-azabicyclo[3.2.1]octane that is structurally different only in the location of the C2 bridge from that of a tropane ring.10) Regarding this selection, we speculated that a double bond generated by metabolic elimination of the phenoxy moiety of the isotropane derivative would be extremely unfavorable at the bridgehead carbon of its isotropane ring due to Bredt’s rule.11) The two stereoisomers 8-exo-38 and 8-endo-39 were synthesized respectively using endo-3-benzyl-3-azabicyclo[3.2.1]octane-8-ol.10) Interestingly, their acaricidal evaluations showed that 8-endo-39 was inactive, but 8-exo-38 was about four-fold more active and fast-acting than compound 25 (Table 3). Furthermore, we selected an azabicyclo[3.3.1]nonane ring system in which the C2 bridge of an isotropane ring is structurally extended by one atom. Several 9-exo-analogs of 38 were synthesized by the procedure similar to 38; for example, compound 40 with a propylene (C3) bridge and compound 41 with a CH2OCH2 bridge in a 3-oxa-7-azabicyclo[3.3.1]nonane ring system. As a result of their evaluations, it was found that 40 exhibited high activity comparable to that of 38, but the activity of 41 was lower than that of 38 (Table 3).
All our considerations for the conversion into azabicycles in the optimization of compound 25 revealed that the above active isotropane derivatives including 38 and 40 had two main problems that were difficult to solve for our development. First, these compounds did not generally show long-term efficacy against both T. urticae and P. citri, compared with the active tropane derivatives such as 3-endo-36. Second, the industrial production of the isotropane derivatives would not be cost-effective, especially due to the great difficulty in the scalable and highly stereoselective synthesis of their 8- or 9-exo-intermediates. Therefore, we decided to conduct further structural optimization of 3-endo-tropane 36 as a new lead compound.
2.3. Further optimization of tropane 36Initially, no significant difference was observed in acaricidal activity affected by changing the n-propoxy of tropane 36 to an isobutoxy group (in 27) or a cyclopropylmethoxy group (in 28), which were selected from 2-substituents of the benzene rings of the compounds shown in Fig. 6. Next, we conducted on the optimization of the tropane ring, by focusing on the nitrogen atom and C2 bridge. As a result, it was found that oxidation of tropane C (R=i-Bu, Fig. 7) by m-chloroperbenzoic acid (m-CPBA) unexpectedly gave none of amine oxide D, but N-pyridyloxyamine E that showed a low polarity completely different from the high polarities of typical aliphatic N-oxides. This result was presumed to indicate that oxidation of tropane C gave N-oxide D, which readily rearranged to E via [1,2]-the pyridyl moiety migration to the oxygen atom (known as the Meisenheimer rearrangement,12) Fig. 7). The structure of pyridyloxyamine E was confirmed by the following synthetic procedure using NH-free tropane F as the starting material (Fig. 8): hydroxylamine (N-OH) I was obtained by oxidation of 2-cyanoethylamine G via the Cope elimination of in situ-generated N-oxide H13); then, I and chloropyridine 9 were coupled in the presence of a base to give E. We also confirmed that the oxidative rearrangement shown in Fig. 7 occurred not only in the reactions of tropane C (R=n-Pr; 36 or cyclopropylmethyl), but also in azabicycle 10 with C3 bridges (Fig. 3).
On the other hand, E exhibited good long-term efficacy and photostability under sunlight, although the activity of E was slightly lower than that of tropane 36. For further optimization of E, considering valuable findings on the effects of the C3 bridge of compound 40 on the acaricidal activity, we designed a structural extension of the C2 of E (R=n-Pr; 42 in Table 4) to C3 and butylene (C4) bridges. These pyridyloxyamines were synthesized by the same procedure as that of E shown in Fig. 8, except for the synthesis of the endo-phenoxy-substituted NH-free azabicyclic intermediates. Acaricidal evaluations of the two pyridyloxyamines obtained above revealed that compound 1 (acynonapyr) with C3 bridges was more active than tropane 36, pyridyloxyamine 42 with a C2 bridge, N-pyridylamine 10 with C3 bridges and pyridyloxyamine 43 with a C4 bridge against four spider mite species in the genera Tetranychus and Panonychus (Table 4); 1 also showed long-term residual efficacy and good activity at all life stages including eggs (Table 5). Also, neither the exo-stereoisomer opposite 1 nor the N-pyridylthio analog of 1 was active (data not shown). Based on these findings, we next confirmed the substituent effects of the phenoxy and pyridyl moieties of 1, together with the effects of several CF3-substituted 5- or 6-membered (hetero)aryl cycles other than the pyridyl moiety, such as 1,3,4-thiadiazole, phenyl, and pyridazine. The results showed that 1 was the most active of all the above compounds. Finally, we selected N-pyridyloxy azabicycle 1 (acynonapyr) as the developed compound, through various evaluations including field efficacy, ecobiological, and toxicological properties.14)
 | |||||
|---|---|---|---|---|---|
| Compound No. | azabicycle | Acaricidal activity (Aa)a) | |||
| T.u.b) | P.c.c) | T.k.d) | P.u.e) | ||
| 42 |  | +++ | +++ | +++ | ++ |
| 1(Acynonapyr) |  | ++++ | ++++ | +++ | +++ |
| 43 |  | ++ | ++ | + | NDf) |
| 36 |  | ++++ | +++ | ++ | +++ |
| 10 |  | ++ | ND | ND | ND |
a) See the footnote of Table 1. b) Tetranychus urticae. c) Panonychus citri. d) Tetranychus kanzawai. e) Panonychus ulmi. f) ND: No data.
| LC50 (ppm) | ||||||
|---|---|---|---|---|---|---|
| Adult females | Eggs (0–1 day old) | Larvae | Proto-nymphs | Deuto-nymphs | ||
| Death at egg stage | Including death after hatching | |||||
| T. urticae | 0.74 | 27.20 | 0.35 | 0.46 | 0.73 | 0.81 |
| P. citri | 0.73 | 11.60 | 4.80 | 0.41 | 0.70 | 0.77 |
We started research on the discovery of a novel insecticide, in which cyclic amines substituted with (hetero)aryl(oxy) moieties were designed as target molecules. In this research, we found that piperidine 11 showed weak acaricidal activity and produced excitatory neurotoxic symptoms suggesting a new mode of action. We were interested in this finding and decided to research this novel acaricide. The structural optimization of piperidine 11 as the first lead compound revealed that the introduction of two moieties enhanced the acaricidal activity of 11: an n-propoxy group or substituents similar in size to that at the 2-position of the benzene ring; either stereoisomer of each aryloxy-substituted azabicycle with C2–3 bridge(s), which has [3.2.1]octane or [3.3.1]nonane ring systems. Next, considering the chemical and biological aspects of the azabicyclic moieties for our development, further structural optimization of 3-endo-tropane 36 was conducted. The oxidative transformation gave pyridyloxyamine E with good photostability; the extension of the C2 bridge of E to C3 led to highly active N-pyridyloxy azabicycle 1 (acynonapyr) with long-term residual efficacy and good activity at all life stages against the spider mites (Fig. 9).
More recently, we elucidated that acynonapyr is a calcium-activated potassium channel modulator,15) and the Insecticide Resistance Action Committee (IRAC) classified it as a member of a new group (Group 33) in 2021. The results of the mode-of-action studies will be reported in due course. Acynonapyr (Cynazet™) was marketed in October 2020 in Japan and Korea, and is expected to contribute as a key acaricide that is highly compatible with IPM programs in controlling spider mites on agricultural sites worldwide.
We would like to thank Nippon Soda Co., Ltd. colleagues Tomomi Kobayashi for his long-term synthetic work on discovering acynonapyr, Daisuke Hanai and Takao Iwasa for their efforts with bioassays in the early stages of this research, and Koichi Hirata for his efforts in the elucidation of acynonapyr’s mode of action. Nisso Chemical Analysis Service Co., Ltd. staff Shinpei Tsushima is acknowledged for his detailed structural analysis of acynonapyr and for providing its spectral data. Finally, we would like to thank all staff members who contributed to this research and development.
The online version of this article contains supplementary material (Supplemental Table S1), which is available at https://www.jstage.jst.go.jp/browse/jpestics/
