Computer-Aided Molecular Design - ACS Publications - American

Chapter 13

Design and Synthesis of 5,6-Dihydro-4H-l,3,4oxadiazines as Potential Octopaminergic Pesticides 1

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Mark A. Dekeyser , W. Ashley Harrison , Paul T. McDonald , G. W. Angle, Jr. , Saad M . M . Ismail , and Roger G. H. Downer Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: March 31, 1995 | doi: 10.1021/bk-1995-0589.ch013

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Research Laboratories, Uniroyal Chemical Ltd., Guelph, Ontario N1H 6N3, Canada New Product Research, Uniroyal Chemical Inc., Bethany, CT 06525 Biology Department, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

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The application of Computer Aided Molecular Design ( C A M D ) tools to the process of insecticide discovery is examined as a key component in a biorational design of octopaminergic pesticides. A study of novel, conformationally restricted analogs of octopamine resulted in die selection of dihydrooxadiazines as candidates for octopaminergic pesticides. A series of dihydrooxadiazines containing N - H , N-methyl and N-fluoroethyl substituents were synthesized and tested for useful pestiddal properties. Certain dihydrooxadiazines showed activity against economically important agricultural pests and interacted with their octopaminergic system.

One biorational approach in the development of safer and more selective pesticides is to select targets which are vital and specific to pest species, thereby minimizing toxicity to non-target organisms (1). Much attention has been directed at the biogenic amine, octopamine (p-hydroxyphenylethanolamine), as a valid target in the search for novel pesticides (2,3). Octopamine is one of the most abundant biogenic amines in the insect and mite nervous system (4,5). Compounds that stimulate octopaminergic systems in insects and mites have the potential to cause physiological, behavioral and lethal effects that often are highly compatible with integrated pest management (EPM) systems. The octopamine receptor has been 0097H5156/95/0589-0183$12.00/0 © 1995 American Chemical Society

Reynolds et al.; Computer-Aided Molecular Design ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

COMPUTER-AIDED M O L E C U L A R DESIGN

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: March 31, 1995 | doi: 10.1021/bk-1995-0589.ch013

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recognized as a useful target since the discovery of the insecticidal and miticidal properties of formamidines (6-9), which function as octopamine agonists by stimulating cyclic adenosine monophosphate (cyclic AMP) (10-24). Twofonnamidines,chlordimeform and amitraz, are œmmercial pesticides (25). Known octopaminergic pesticides were not discovered through structural modifications of octopamine, but were largely the result of random and biochemical screening efforts. We reasoned that it should be possible to design agonists which would avoid the problems of low bioavailability and rapid metabolic degradation associated with octopamine (26), thereby becoming suitable pesticide candidates. It was hypothesized that these problems were largely due to an urtoptimized lipophilic profile of the molecule. A parabolic relationship often exists between pesticidal activity of a compound and its lipid solubility. When the lipophilicity of the compound is low, this implies that it is poorly absorbed by a biological membrane and when its lipophilicity is high, it becomes trapped in the membrane and does not pass on to the site of action. Our strategy for avoiding these problems involved modulation of the physical properties of octopamine by modifying the ethanolamine portion of the molecule and introducing suitable substituents in the phenyl portion. These modifications were designed to enhance penetration of die cuticle and CNS of pest species as well as to afford resistance to oxidative degradation. Towards this end, we used Computer Aided Molecular Design (CAMD) tools to identify novel octopamine mimics as potential pesticide candidates. The candidates were then synthesized and evaluated for pesticidal and octopaminergic activities. We recently reported selected results from this work (27-29). Molecular Modeling Molecules were constructed using the SYBYL software package (Tripos Associates, St Louis, MO). The minimum energy conformations were determined from the MAXIMIN program. An X-ray crystallographic study of octopamine hydrochloride (20) showed that die molecule adopts a conformation whereby the ethanolamine portion is extended away from the phenyl ring. A minimum energy conformation of octopamine which closely resembled die X-ray conformation was used for this study. It is reasonable that low-energy conformers of potential octopamine mimics should be capable of superimposition on a low-energy conformer of octopamine, providing a match between corresponding phenyl rings and nitrogen atoms. The superimposed energy-minimized structures of octopamine and chlordimeform are shown in Figure 1. A close match between these two structures was


Synthesis of 5,6-Dihydro-4R-l,3,4-oxadiazines

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13. DEKEYSER ET AL.

Figure 1. Superimposition of computer-generated low-energy conformers of octopamine and the octopaminergic pesticide chlordimeform (orthogonal views). H atoms have been omitted for clarity.


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observed. Minimum energy conformers of additional proposed octopaminergic pesticides, imidazoline (NC-5) and oxazoline (AC-6), overlayed well with octopamine (not shown). It has been shown that the hydroxy group in the side-chain portion is not critical to oct^amine agonist activity in phenylethanolamine analogs (22). Therefore, it appears that die dimethylformamidine side-chain [N=CH-N(CH3) ] of chlordimeform mimics die ethanolamine side-chain [CH(OH)Cr^NH ] of octopamine, while the 4-chloro-2-methylphenyl group of chlordimeform mimics the 4-hydroxyphenyl group of octopamine. Obviously, these modifications to the octopamine molecule must result in dramatically altered physicochemical properties. A comparison of calculated nitrogen-to-phenyl distances in low-energy conformers and relative lipophilic properties (calculated Log Ρ values) of octopamine and some known octopaminergic pesticides are shown in Table I. On the basis of these values, a minimum pharmacophoric binding model was proposed for octopaminergic pesticide candidates. The model included a phenyl ring and a basic nitrogen atom separated by a distance of 3.4-3.7 angstroms, an angle of 75-100° between the phenyl ring and side-chain and a calculated log Ρ value >1. The objective was to design an isostere with increased lipophilicity while retaining the functionality essential for agonist action. To investigate the potential of nitrogen-containing heterocycles to act as novel octopamine mimics related to NC-5 and AC-6 (Table I), we evaluated die structural similarities between octopamine and several 5,6-dihydro^4H-lA4-oxadiazines. Figure 2 shows the superimposed enei^-minimized structures of octopamine and a proposed dihydrooxadiazine analog (adapted from ref. 17). In this compound, a dihydrooxadiazine ring replaces the ethanolamine portion of octopamine while a bromophenyl group replaces the hydroxyphenyl group of octopamine. We reasoned that this type of compound would be a conformationally restricted lipophilic analog. The use of conformationally restricted analogs of octopamine represents an important approach towards understanding the molecular recognition requirements of the octopamine receptor. Additional analogs that are substituted on the dihydrooxadiazine nitrogen by a methyl and smallfluorine-containinggroups were expected to further enhance lipophilic properties without affecting the nitrogen-to-phenyl distance. Since chlordimeform possesses N-methyl groups andfluorine-containingcompounds are noted for their high level of biological activity (22), these substituents are likely to increase the potential pesticidal activity of dihydrooxadiazines. The number of polar groups in dihydrooxadiazine analogs is less than in octopamine. Therefore, there arefewersites for potential enzymatic degradation in these compounds compared to octopamine. 2


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13. DEKEYSER ET AL.

Synthesis of5 6-Dihydro-4Yl-l 3,4-oxadiazines y

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y

Table L Nitrogen-to-Phenyl Distances and LogP Values for Octopamine and Octopaminergic Pesticides. Compound

N-Phenyl (Angstroms)

LogP


OH

Octopamine CH

3

C I A ^ C H , Chlordimeform

3.5

N=< ^2

n

5

NC-5 Ν

1

Ace

SOURCE: Adapted fromref.17. * Estimated Using PrologP (CompuDrug, Rochester, NY)


3·

2



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H Octopamine

(··-)

Dihydrooxadiazine

(

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Figure 2. Superimposition of computer-generated low-energy conformers of octopamine and a proposed dihydrooxadiazine analog (orthogonal views). H atoms have been omitted for clarity. (Adapted from ref. 17.)


13. DEKEYSER ET AL.

Synthesis of 5,6-Dihydro-4K-l,3,4-oxadiazines

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A comparison of calculated nitrogen-to-phenyl distances in low-energy conformers and lipophilic properties (calculated Log P) of octopamine and severed dihydrooxadiazines are shown in Table Π. The proposed octopaminergic pesticide candidates satisfied the criteria for the binding model, possessing a nitrogen-to-phenyl distance of 3.6 angstroms, a dihedral angle between the phenyl ring and dihydrooxadiazine ring of 94°, a good overlay between one of its low-energy conformers and that of octopamine and also a calculated LogP>l.


Synthesis Dihydrooxadiazines are known in the literature (23). Using a modification of the methods of (23), a series of N-H , N-methyl and N-fluoroethyl dihydrooxadiazines, having various substituents in the phenyl ring, were prepared as shown in Figure 3, in 10-60% overall yields (17,18). The N-H and N-fluoroethyl derivatives were prepared from the appropriate substituted benzoic acids which were converted, via benzoyl halides, to the substituted benzhydrazides then reacted with bromofluoroethane. The N-methyl derivatives were prepared from the appropriate substituted benzoic acids which were first converted to the substituted bertzoates, then to the substituted 2-methyIbenzhydrazides and finally reacted with bromofluoroethane. The types of substituents on the phenyl ring were lipophilic groups, such as halogen and alkyl, rather than the polar hydroxy group found in octopamine. Pesticidal Properties Several dihydrooxadiazines were shown to possess potent ovicidal toxicity to two-spotted spider mites, Tetranychus urticae Koch, and tobacco budworms, Helicoverpa virescens (F.) in greenhouse screening following contact treatment (see Table ΙΠ). Adult female mites were allowed to deposit eggs on cowpea leavesforone day before treatment when they were removed, leaving the eggs. Plants were sprayed to run-off with three rates of the test compound in acetone and distilled water containing a wetting agent. Nine days following treatment, the number of hatched eggs were counted with an estimated percent mortality based on the number of eggs hatched on the check plants. Budworm eggs were immersed on cheesecloth for one minute with three rates of the test compound in acetone and distilled water containing a wetting agent. The cheesecloth samples were set on moist filter paper for five days when the numbers of hatched eggs were counted and an adjusted percent control determined based on the number of eggs hatched in the checks.


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COMPUTER-AIDED MOLECULAR DESIGN

Table Π. Nitrogen-to-Phenyl Distancée and LogP Values for Octopamine and Dihydrooxadiazine Analogs. Compound

N-Phenyl (Angstroms)

LogP *


O H

0^"

H O

3.7

-0.1

3.6

1.6

3.6

2.3

3.6

2.5

Octopamine

Br

I H

N-H Analog

Ν ι

*r 3 N-Methyl Analog C

H

CHJCHJF

N-Fluoroethyl Analog

* Estimated Using Prolog? (CompuDrug, Rochester, NY)



13. DEKEYSER ET Al»

Synthesis of 5,6-DihydrO-4K-l,3,4-oxadiazJnes

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Reagents: [a] SOCI ;NH NH 2

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[b] C H 3 O H ,

H

2

S 0

4

;

2

NH NHCH 2

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[c] FCH CH Br NaOH 2

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[d] xs FCH CH Br, NaOH 2

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Figure 3. Synthesis of N-H (I), N-Methyl (Π) and N-Fluoroethyl (ΙΠ) Dihydrooxadiazines. (Adaptedfromrefs. 17,18.)


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Table WL Pesticidal Activity of Dihydrooxadiazines on Mite (MIOVO) and Budworm (TBOVO) Eggs. % Control® MIOVO


Compound

100ppm TBOVO

88

97

80

14

ι H

N-H Analog

N-Methyl Analog

100 CH CH F 2

2

N-Fluoroethyl Analog CH, ι

3

50

Chlordimeform

SOURCE: Adapted from refs. 18,19.


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13. DEKEYSER ET AL.

Synthesis of 5,6-Dihydro-4H-l,3,4-oxadiazines

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Pesticidal activity appeared to be most enhanced by dihydrooxadiazines substituted in the phenyl ring by halogens, especially bromine, and on the dihydrooxadiazine nitrogen by a fluoroethyl group. The 4-bromophenyl N-fluoroethyl dihydrooxadiazine showed the greatest ovicidal activity on both mites and budworms and was more potent than the formamidine, chlordimeform. A similar spectrum of biological activity to the dihydrooxadiazines was reportedforthe formamidines (24).


Octopaminergic Action Using homogenates of the two-spotted spider mite and the American cockroach nervous system, selected pesticidal dihydrooxadiazines were shown to interact with octopamine receptors by stimulating adenylate cyclase activity (Table IV). All compounds tested caused elevation of cyclic AMP levels at 1 χ ΙΟ" M. This effect was concentration-dependent (not shown). The 4-bromophenyl N-H dihydrooxadiazine (I) showed the greatest octopamine agonist activity. The possibility that dihydrooxadiazine pesticides may interact with the same binding site as octopamine in mite homogenates and cockroach nervous system was investigated by examining the additive effects of (I) and octopamine in elevating cyclic AMP production at maximally effective concentrations. The results shown in Table IV indicate that the level of cyclic AMP production due to a combination of octopamine and (I) was not significantly different from that caused by octopamine alone in either preparation. Thus, it is possible that dihydrooxadiazines and octopamine affect the same receptor and elevate cyclic AMP levels through interaction with an octopaminesensitive receptor coupled to adenylate cyclase. However, other sites associated with the complex may also be important to the pesticidal action of dihydrooxadiazines, for example, inhibition of N-acetyltransferase activity, which is the main pathway for biogenic amine degradation (25). 5

Conclusions Using CAMD, we identified dihydrooxadiazines as a potential new class of octopaminergic pesticides by evaluating the common structural features, lipophilic profiles and biochemical potentdes of established octopaminergic pesticides, includingformamidines(26), imidazolines (27) and aminooxazolines (28). Octopamine and several octopaminergic pesticides have been shown to have certain structural similarities, including the superimposability of essential functionality



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Table IV. Octopaminergic Activity of Dihydrooxadiazines in Mite Homogenates and the Cockroach Nervous System. Cyclic AMP Production


Compound

Mites

@10μΜ*

Cockroach

287

1111

184

768

206

805

Octopamine

371

2366

Octopamine + (I)

386

2397

Control

159

440

I

Η N-H Analog (!)

N-Methyi Analog

CH CH F 2

2

N-Fluoroethyl Analog

pmol/min/mg protein


13. DEKEYSER ET AL·

Synthesis of 5 6-Dihydro-4R-l 3 4-oxadiazines 9

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(phenyl group and a basic nitrogen atom) which were separated by a distance range of 3.4-3.7 angstroms and a dihedral angle between the phenyl ring and the side-chain of 75-100°. Additionally, octopaminergic pesticides possessed enhanced lipophilic values (Log P>1) compared to octopamine. 5,6-Dihydix>-4H-l^ 4

Computer-Aided Molecular Design - ACS Publications - American

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