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13 Flubendiamide, a New Insecticide Characterized by Its Novel Chemistry and Biology

Akira Seo, Masanori Tohnishi, Hayami Nakao, Takashi Furuya, Hiroki Kodama, Kenji Tsubata, Shinsuke Fujioka, Hiroshi Kodama, Tetsuyoshi Nishimatsu, Takashi Hirooka

13.1 Introduction

Resistance has often been a problem or a potential problem for insecticides and this is one of the most important reasons why the insecticides with a new mode of action have been always desired, though it is quite a diffi cult task to fi nd such molecules. Flubendiamide, discovered by Nihon Nohyaku (NNC), is a novel insecticide belonging to the new chemical class of 1,2-benzenedicarboxamides or phthalic diamides, having a unique chemical structure (Figure 1) [1–3]. Flubendiamide is co-developed by NNC and Bayer CropScience globally [4]. The structure-activity relationships, the chemistry, including topics in process research, the mode of action and the biological profi les are described.

O

HNS

O

NH

I

CF3

CF3

F

O O

Figure 1. The chemical structure of fl ubendiamide, 3-iodo-N′-(2-mesyl-1,1-dimethylethyl)-N-{4-[1,2,2,2-tetrafl uoro-1-(trifl uoromethyl)ethyl]-o-tolyl}phthalamide.

Pesticide Chemistry. Crop Protection, Public Health, Environmental SafetyEdited by Hideo Ohkawa, Hisashi Miyagawa, and Philip W. LeeCopyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimISBN: 978-3-527-31663-2

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13.2 Structure-Activity Relationship

13.2.1 Lead Generation

Figure 2 shows the early phase of research for fl ubendiamide. In 1989, Dr. T. Tsuda, at Osaka Prefecture University in Japan, reported that some pyrazinedicarbox-amide derivatives showed moderate herbicidal activity [5]. From 1990, the research for herbicide discovery was conducted at NNC Research Center. In the course of this research, a lead compound for an insecticide was discovered in 1993 from the class of benzenedicarboxamides as shown in Figure 2. This compound provided insecticidal activity on lepidoptera at the relatively high dose of 50–500 mg a.i./L. Moreover, it did not show activity against other species such as Hemiptera or Aca-rina. Although the level of activity was not satisfactory, this compound attracted the attention of researchers for both the novelty of its chemical structure and the char-acteristic insecticidal symptoms such as gradual contractions of the insect body. We therefore started the study for further optimization of this lead compound.

N

NNH

O

O

NH

R

Xn

NH

O

O

NH

Cl

NO2

Ym

NNH

O

O

NH

R

Xn

Research for herbicide (from '90)

Herbicidal derivatives(Tsuda et al, '89)

JP patent 1997-323974

The lead compound for insecticide

Figure 2. Optimization history of fl ubendiamide.

13.2.2 Lead Optimization

The weak insecticidal activity was found in the lead compound; its structure was quite new as an insecticide. However, there were various points to be improved for practical use such as increased insecticidal activity, reduced phytotoxicity to crops and instability of the compound. Two thousand derivatives were synthesized with the general formula shown in Figure 3. Many studies on the improvement of the activity were conducted, and fl ubendiamide was fi nally discovered in 1998.

The chemical structure of benzenedicarboxamides can be divided into three parts as shown in Figure 3. These are characterized by (A) the phthaloyl moiety, (B) the aliphatic amide moiety and (C) the aromatic amide moiety. A brief description of the structure-activity relationships for each part is described below.

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Table I shows the insecticidal activity of 1,2-benzenedicarboxamides. Insecticidal activities are shown as EC50 values against common cutworm (Spodoptera litura) and diamondback moth (Plutella xylostella).

In order to improve the activity of the lead compound 1, the nitro group was changed to other groups. Although the non-substituted derivative 3 showed similar or slightly higher activity, we found that the chloro-derivative 4 showed much stronger activity. Optimization of substituent X with chloro-derivatives at positions 3–6 resulted in the fi nding that the 3-position was clearly the best (Table II, compounds 4 to 7). As for substituents at the 3-position of the phthaloyl moiety, various groups such as halogen atoms, aryl groups, haloalkyl groups, haloalkoxy groups, and haloalkylthio groups were evaluated. Among these substituents, lipophilic and bulky substituents tended to show good activity, and the iodine atom was found to be the best substituent for X. It should be noted that compounds having an iodine atom are very rare among commercial agrochemicals.

As for substituents on the aniline ring, the ortho-methyl group was fi xed, because it was essential for keeping the stability of the diamide structure. The optimization of the best position with a chlorine atom as substituent Y showed that the 4-position was the best by comparison of compounds 10–12. Other groups were introduced as substituent Y onto the aniline ring, and the results showed the tendency for a more lipophilic substituent to be preferable. Notably, the fl uoroalkyl group was highly effective as exemplifi ed with the heptafl uoroisopropyl compound 15. The heptafl uoroisopropyl group has never been reported as a substituent in a commercial pesticide and is seldom used in pesticide research.

The last section shows the effect of substituents (R1, R2) on the aliphatic amide moiety. As for the aliphatic side chain, it was found that the alpha-branched alkyl side chain was essential for stabilizing the diamide structure. In the case of non-branched alkyl, the diamide derivatives tend to decompose to the corresponding phthalimides. A variety of substituents were examined to improve the activity. As shown in Table I, the introduction of a heteroatom or a functional group increased the insecticidal activity; especially a sulfur atom within the alkyl side chain markedly increased the activity. This sulfonylalkylamine is also novel as an amine residue in pesticide chemistry. In summary, fl ubendiamide has unique substituents as essential parts of the structure in three adjacent positions on the benzene ring, which characterizes the chemical structure of fl ubendiamide as totally novel.

N

N

O

OAr

R3

R2

R1

Ym

Xn

(A)

(B)

(C)

NH

O

O

NH

Cl

NO2

O

HNS

O

NH

I

CF3

CF3

F

O O

1Lead compound General formula

2Flubendiamide

Figure 3. Lead optimization of benzenedicarboxamide derivatives.

13.2 Structure-Activity Relationship

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Table I. Insecticidal activities of 1,2-benzenedicarboxamides against Spodoptera litura and Plutella xylostella.

NH

O

NH

O

R2R1

X 3

4

56 Y

6

5

43

2

No. X Y R1 R2 EC50 value (mg a.i./L)

S. litura P. xylostella

1 3-NO2 4-Cl H H 10–100 10–100

3 H 4-Cl H H 10–100 3–10

4 3-Cl 4-Cl H H 10 1–3

5 4-Cl 4-Cl H H > 500 5

6 5-Cl 4-Cl H H > 500 50

7 6-Cl 4-Cl H H > 500 10

8 3-F 4-Cl H H > 100 1–3

9 3-Br 4-Cl H H 10 1

10 3-I 4-Cl H H 3–10 0.3–1

11 3-I 3-Cl H H 10 3

12 3-I 5-Cl H H 10–100 3–10

13 3-I 4-OCH3 H H 30–100 10–30

14 3-I 4-OCF3 H H 1–3 0.3–1

15 3-I 4-CF(CF3) 2 H H 0.3–1 0.1–0.3

16 3-I 4-CF(CF3) 2 CH3 H 0.3–1 0.3–1

17 3-I 4-CF(CF3) 2 CH3 NHCOCH3 0.1 not tested

2 3-I 4-CF(CF3) 2 CH3 SO2CH3 0.03–0.1 0.001–0.003

13.3 Chemistry

Figure 4 shows the synthetic pathway to fl ubendiamide employed at the early stages of discovery. 3-Iodophthalic anhydride 21 was the important intermediate, which was prepared from commercially available 3-nitrophthalic acid according to known methods via a diazonium intermediate. Phthalamic acid 22 was obtained by the reaction of 21 with thioalkylamine with high regioselectivity. Phthalamic acid 22 was treated with methyl chloroformate to give isoimide 23, which was reacted with the corresponding aniline to afford diamide 24. Finally fl ubendiamide was obtained by the oxidation of diamide 24 with hydrogen peroxide. It seemed that there was no alternative route, since a practical iodination was very limited.

13 Flubendiamide, a New Insecticide Characterized by Its Novel Chemistry and Biology

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O

I O

O

COOHNO2

COOH

H2NCF3

CF3F

H2

H2NS

Et3N

O

HN

O

NH

IS

CF3

CF3F

COONaNH2

COONa

O

HNIS

COOH

HCOOH

H2O2

ClCO2CH3

K2CO3

COOHI

COOH

O

HN

O

NH

IS

CF3

CF3F

O O

O

I N

O

S

1) NaNO2 TsOH (cat.)

18 19 20

Flubendiamide

Pd/CNaOH

2) KI

3) H+

21 22 23

24

H2SO4 (cat.)

Figure 4. Synthetic pathway of fl ubendiamide at the early optimization stage.

Process investigation was started at a very early stage of the optimization studies before fl ubendiamide had been discovered, nevertheless extensive research to resolve the issues such as cost, quality, safety, and environment performance were conducted on related analogs. Since fl ubendiamide consisted of three characteristic building blocks, it was necessary that a regioselective introduction of the three components be found, and also an inexpensive manufacturing method for each component be established. Figure 5 shows the newly developed regioselective introduction of an iodine atom. Iodine was introduced at the ortho-position of the benzamide by a Pd(II) catalyzed reaction, which was quite novel in the area of palladium chemistry. This reaction realized direct and regioselective introduction of iodine onto the benzene ring in one step. Both the reaction yield and regioselectivity were excellent, and furthermore, the reaction itself is practical from the view point of manufacturing. It is noteworthy that the waste volume could be extremely reduced compared with the case of the former diazonium method. With these characteristic points established, this reaction may be classifi ed as “Green chemistry”.

O

HN

OS

H3C CH3

CH3

O

NH

Y

O

HN

OS

H3C CH3

CH3

O

NH

I

Y

cat. Pd(II)Iodinating reagent

Figure 5. Regioselective introduction of an iodine atom.

13.3 Chemistry

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13.4 Mode of Action

Flubendiamide is most effective on larvae followed by adults, but it has no ovicidal activity. In the course of extensive research on the mode of action of fl ubendiamide, it was determined that fl ubendiamide was a ryanodine receptor modulator. Flubendiamide fi xes the Ca-channel of insect ryanodine receptors (RyR) in the open state, and subsequently induces calcium release from the membrane vesicle [6]. In parallel, the RyR activation by fl ubendiamide induces the stimulation of the Ca-pump via functional connection between these two components [7]. It is suggested that the effect of fl ubendiamide on intracellular calcium regulation is essential for the insecticidal activity. Furthermore, fl ubendiamide shows very little effect on the mammalian RyR isoform. This comparative study concluded that fl ubendiamide specifi cally activates insect RyR. By the binding assay using 3H-fl ubendiamide, it was confi rmed that the binding site was specifi c to insect RyR, and its binding site was different from those of other RyR modulators such as ryanodine. Finally, it is known that the binding site of ryanodine is located at a pore region of the RyR. Thus, we conclude that the selective action of fl ubendiamide is due to the specifi city of the binding site.

13.5 Biological Profi le

Table II shows the insecticidal activity of fl ubendiamide against major insect and acarina species. Flubendiamide provided high activity on all lepidopterous insect pests, and its EC50 values were between 0.004 and 0.58 mg a.i./L. How-ever, fl ubendiamide did not show activity against other insect species. Thus, the insecticidal spectrum of fl ubendiamide is expected to be broad among lepidoptera pests in agriculture. Against the resistant strain of diamondback moth, fl ubendiamide provided the same level of activity as against the susceptible strain. This result indicates that fl ubendiamide will be useful for insecticide resistance management (IRM) programs.

Table III shows the activity of fl ubendiamide on several species of benefi cial arthropods and natural enemies. Flubendiamide was inactive against benefi cial arthropods (except silkworm) and natural enemies tested. This result indicates that fl ubendiamide should be very safe for natural enemies, and consequently will fi t well into integrated pest management (IPM) programs.

Field evaluations of fl ubendiamide have been conducted in many areas on various crops such as vegetables, top fruits, and cotton. Flubendiamide shows excellent performance on controlling the major lepidopterous pests in the fi eld at the recommended dose and its effi cacy was better than those of standard insecticides. Furthermore, fl ubendiamide (20% WDG) showed no phytotoxicity to vegetables, tea and top-fruits at recommended doses [3–4].

13 Flubendiamide, a New Insecticide Characterized by Its Novel Chemistry and Biology

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Table II. Insecticidal spectrum of fl ubendiamide.

Scientifi c name Common name Tested stage

DAT EC50

(mg a.i./L)

Plutella xylostella Diamondback moth L3 4 0.004

(Resistant strain)* L3 4 0.002

Spodoptera litura Common cutworm L3 4 0.19

Helicoverpa armigera Cotton bollworm L3 4 0.24

Agrotis segetum Turnip moth L2–3 7 0.18

Autographa nigrisgna Beet semi-looper L3 4 0.02

Pieris rapae crucivora Common cabbage worm L2–3 4 0.03

Adoxophyes honmai Smaller tea tortrix L3 5 0.38

Homona magnanima Oriental tea tortrix L4 5 0.58

Hellula undalis Cabbage webworm L3 5 0.01

Chilo suppressalis Rice stem borer L3 7 0.01

Diaphania indica Cotton caterpillar L3 3 0.02

Sitophilus zeamais Maize weevil A 4 > 1000

Nilaparvata lugens Brown rice planthopper L3 4 > 1000

Myzus percicae Green peach aphid All 7 > 1000

Pseudococcus comstocki Comstock mealybug L1 7 > 100

Tetranychus urticae Two-spotted spider mite All 4 > 100

L2, L3, A: second, third and Adult DAT: Day(s) after treatment* resistant strains to pyrethroids, BPUs, Ops, and carbamates

13.5 Biological Profi le

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Table III. Activity of fl ubendiamide on benefi cial arthropods and natural enemies.

Common name Scientifi c name Test stage EC30

(mg a.i./L)

Honeybee Apis mellifera Adult > 200

Horn-faced bee Osmia cornifrons Adult > 200

Bumblebee Bombus ignitus Adult > 200

Lady beetle Harmonia axyridis Coccinella septempunctata bruckii

AdultAdult

> 200> 200

Parasite wasp Encarsia formosa Aphidius colemani Cotesia glomerata

AdultAdultAdult

> 400> 400> 100

Green lacewing Chrysoperla carnea Larva > 100

Predatory bug Orius strigicollis Adult > 100

Predatory midge Aphidoletes aphidimyza Larva > 100

Predatory mite Amblyseius cucumeris Phytoseiulus persimilis

AdultAdult

> 200> 200

Spider Pardosa pseudoannulata Misumenops tricuspidatus

AdultAdult

> 100> 200

Silkworm Bombyx mori Larva < 50

13.6 Toxicological Properties

Flubendiamide shows low acute oral toxicity. The LD50 for male and female rats were both > 2000 mg/kg. The agent is slightly irritating to rabbit eyes, non-irritating to rabbit skin, non-mutagenic in the Ames test, and non-sensitizing to guinea pig skin. The acute oral LD50 for quail was > 2000 mg/kg. The LD50 for carp was > 548 mg/L. These fi ndings suggest that fl ubendiamide is safe for mammals, fi sh, and birds.

13 Flubendiamide, a New Insecticide Characterized by Its Novel Chemistry and Biology

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13.7 Conclusion

Flubendiamide represents a novel class of insecticide having a unique chemical structure, and provides a new mode of action, which acts as a RyR modulator. This activity is highly selective to insect RyR, and no cross-resistance to existing insecticides is observed. Flubendiamide will also be very suitable for Insecticide Resistant Management. Furthermore, fl ubendiamide shows a broad insecticidal spectrum against lepidopterous insect pests, excellent effi cacy in fi eld evaluations, and excellent safety against various benefi cial arthropods and natural enemies. It will be suitable for IPM programs.

13.8 Acknowledgments

The authors would like to express sincere thanks to all their distinguished colleagues involved in the discovery of fl ubendiamide in Nihon Nohyaku Co., Ltd. The authors also wish to acknowledge the many scientists at Bayer CropScience AG and Professor Yasuo Mori at Kyoto University during the course of the mode of action work.

13.9 References

Keywords

Flubendiamide, Benzenedicarboxamide, Insecticide, Ryanodine Receptor Modulator

1 M. Tohnishi, H. Nakao, E. Kohno, T. Nishida, T. Furuya, T. Shimizu, A. Seo, K. Sakata, et al., Eur. Pat. Appl., 2000, EP 1006107.

2 M. Tohnishi, H. Nakao, T. Furuya, A. Seo, Hiroki Kodama, K. Tsubata, S. Fujioka, H. Kodama, et al., J. Pesticide Sci., 2005, 30, 354–360.

3 T. Nishimatsu, T. Hirooka, H. Kodama, M. Tohnishi, A. Seo, Proceedings of the BCPC International Congress – Crop Sci. & Technology, 2005, 2A-3, 57–64.

4 T. Nishimatsu, H. Kodama, K. Kuriyama, M. Tohnishi, D. Ebbinghaus, J. Schneider, Int’l. Conf. on Pesticides, Kuala Lumpur, Malaysia, Book of Abstracts, 2005, 156–161.

5 T. Tsuda, H. Yasui, H. Ueda, J. Pestic. Sci., 1989, 14, 241–243.

6 U. Ebbinghaus-Kintscher, P. Luemmen, N. Lobitz, T. Schulte, C. Funke, R. Fischer, T. Masaki, N. Yasokawa, et al., Cell Calcium, 2006, 39, 21–33.

7 T. Masaki, N. Yasokawa, M. Tohnishi, T. Nishimatsu, K. Tsubata, K. Inoue, K. Motoba, T. Hirooka, Mol. Pharmacol., 2006, 69, 1733–1739.

13.9 References

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