Pestic. Sci. 1997, 50, 291È296

A New Group of Plant-Derived NaphthoquinonePesticides*

Bhupinder P. S. Khambay,” David Batty, David G. Beddie, Ian Denholm &Matthew R. CahillBiological and Ecological Chemistry Department, IACR-Rothamsted, Harpenden, Herts, AL5 2JQ, UK

(Received 5 November 1996 ; revised version received 22 April 1997 ; accepted 2 May 1997)

Abstract : A series of compounds with structures based on insecticidal/acaricidalnaphthoquinones isolated from Calceolaria andina has been synthesised. Afeature of the series is the lack of resistance shown by strains resistant to estab-lished classes of pesticides. The importance for activity of the tetra-substitutedcarbon atom in the side-chain, as observed in the natural products, has beeninvestigated. In analogues with acyclic side-chains the position of the tetra-substituted carbon for optimum activity was dependent on the length of the side-chain. With cyclic side-chains, activity was dependent on the size of the ring, thenumber and position of the substituents therein. Activity of the compoundsexamined was particularly high against Bemisia tabaci and T etranychus urticae indirect-contact tests, but was much lower than expected in leaf-dip tests. A partialimprovement based on formulation has been demonstrated.

Pestic Sci., 50, 291È296, 1997No. of Figures : 3. No. of Tables : 7. No. of Refs : 15

Key words : naphthoquinones, insecticides, acaricides, structureÈactivity relation-ships, natural products

1 INTRODUCTION

From a collaborative programme involving the Uni-versity of Chile, the Agrochemical Evaluation Unit atthe University of Southampton and the Jodrell Labor-atory at Kew, the insecticidal/acaricidal and fungicidalactivity in extracts of Calceolaria andina was recognisedand shown to be due to the presence in the processedextract of two naphthoquinones1 (1 and 2, Fig. 1).Activities of both compounds were of a similar order tothose of some established insecticides against thesucking pests Bemisia tabaci Genn (whiteÑy) and T etra-nychus urticae Koch (two-spotted spider mite) (Table 1).Both compounds exhibited a particularly favourable

* Based on a paper presented at the meeting “Advances in theChemistry of Crop ProtectionÏ, organised by P. J. Crowley, G.Mitchell, G. Keen, J. Pickett and P. D. Riordan on behalf ofthe SCI Pesticides Group and the RSC Biological & Medici-nal Chemistry Group and held on 9È11 September 1996 atChurchill College, Cambridge.” To whom correspondence should be addressed.Contract grant sponsor : British Technology Group; Biotech-nology and Biological Sciences Research Council.

property with regard to resistant insects as exempliÐedin Table 2. Strains of B. tabaci and T . urticae that arestrongly resistant to established chemical classes (e.g.pyrethroids and organophosphates)2,3 showed negligi-ble resistance to the new compounds.

TABLE 1Relative Activities of Known Compounds against WhiteÑies

(Bemisia tabaci) and Mites (T etranychus urticae)

Relative activitiesCompound

B. tabaci T. urticae

1 100a 100b2 150 1803 7 134 7 805 39 110

Bifenthrin 1400 170Cypermethrin 44 ÈDicofol È 240

7 mg litre~1, glass vial.a LC5051 mg litre~1, micro-immersion.b LC50

2911997 SCI. Pestic. Sci. 0031-613X/97/$17.50. Printed in Great Britain(

292 B. P. S. Khambay et al.

Fig. 1. Structures of compounds discussed.

Previous publications on the pesticidal activity ofnaphthoquinones cover compound 3,4 compound 45and compound 5.6 In bioassay procedures reportedhere, these compounds were generally less active thanthe natural products (see Table 1). A distinctive featurein the structure of the two natural products (1 & 2) isthe quaternary (tetra-substituted) carbon at C-1 in theside-chain at position 3 of the parent naphthoquinone.The aim of the present study was to investigate theimportance of this feature on the spectrum and levels ofarthropodicidal activity by removing it from the naturalproduct (1) and by introducing it into the three types ofpreviously reported compounds (3, 4 & 5).

TABLE 2Resistance Factorsa of Natural Naphthoquinones and Other

Pesticides

Compound T. urticae B. tabaci

Cypermethrin È 400Bifenthrinb 190 ÈChlorpyrifosb 360 ÈProfenofosc È 63

1b 0É9 1É92b 1É6 0É7

a As deÐned in Section 2.2.b Direct (micro-immersion or vial) bioassays.c Leaf-dip bioassays.

2 EXPERIMENTAL

2.1 Synthesis

The compounds synthesised comprised three series, cor-responding to the general formulae shown in Fig. 2.

The synthetic routes used for these series are sum-marised in the schemes shown in Fig. 3. Compounds inseries 1 were synthesised either by reacting 2-hydroxy-1,4-naphthoquinone (Lawsone) with the appropriateallylic alcohol under Mitsunobu reaction conditionswith subsequent Claisen rearrangement7 (see Fig. 3,scheme 1), or by reacting acylated Lawsone with thecorresponding carboxylic acid in a peroxysulfate-mediated radical decarboxylation reaction (see Fig. 3,scheme 2).8 All the compounds in series 2 and 3 weresynthesised by the latter method. In some cases, theHooker oxidation with alkaline permanganate9 orhydrogen peroxide10 was used to provide homologueswith one less carbon at C-1 in the side chain (Fig. 3,scheme 3).

2.2 Biological testing

The susceptible and multi-resistant strains of T . urticae(GSS and NYRBIF) and B. tabaci (SUDS and NED3)have been described previously.3,11 The strain of Myzuspersicae (peach-potato aphid)12 tested (USIL) was a

Fig. 2. General structures of compounds synthesised.

A new group of plant-derived naphthoquinone pesticides 293

Fig. 3. Synthetic routes used.

laboratory reference clone susceptible to all aphicideclasses.

Bioassay protocols used in the present study havebeen reported in full elsewhere. The micro-immersionassay for T . urticae13 involved immersing adult mitesbrieÑy in an aqueous suspension of the test compound.In the glass-vial assay for B. tabaci,14 adult whiteÑieswere exposed in a vial which had been precoated withthe test compounds. The topical bioassay for M. per-sicae entailed applying a 0É25 kg droplet of chemical inacetone to the dorsal surface of apterous adults. In allcases, treated individuals were transferred to fresh leafdiscs of either broad bean (for T . urticae), cotton (for B.tabaci) or chinese cabbage (for M. persicae) and mortal-ity assessed after 72, 48 and 72 h respectively. Leaf-dipassays2 were conducted by dipping leaf discs from theabove plants into serial dilutions of formulated testmaterial and air drying before transferring testorganisms to them. Mortality was assessed after thesame holding times as before.

For leaf-dip assays, the formulation (A) was preparedby dissolving the test compound (4 parts by weight) inSolvessoR (Multisol Ltd, UK) (14 parts) containing thesurfactants (Atlox 4851B (1 part) and Atlox 3400B (1part)) (ICI Surfactants, UK).

All dose-response assays encompassed at least fourtest concentrations with a minimum of two replicates of15 individuals per concentration. Data were subjectedto probit analysis to obtain estimates, whose Ðdu-LC50cial limits typically spanned a three-fold range of con-centrations tested. Resistance factors are expressed as

relative to that of the corresponding susceptibleLC50strain.

3 RESULTS AND DISCUSSION

The results from the various bioassays are presented inTables 3 to 7. Where indicated, activities are quoted asrelative to compound 1 (\100).

For the compounds in series 1 (see Table 3), theimportance of the gem-dimethyl group at C-1 in theside-chain of compound 1 for activity was conÐrmed byprogressive loss of activity in compounds with only one(7) or no (8) groups there. Insertion of aCH3 CH2between the ring and the gem-dimethyl group (9) orafter the latter (10) also led to loss in activity. Intro-duction of one methyl at C-2 in 7 (to give 11) or twomethyls at C-3 of 8 (to give 12), changes which main-tained the molecular weight at that of 1, also led toreduction of activity. Replacing the vinyl group with a

group gave a compound (13) more active thanwCH3the natural product 1 against B. tabaci, but furtherextension to (14), (15), or (16)wC2H5 wC3H7 wC4H10led to progressive loss in activity against both species.The hydroxy analogue (17) of compound 4 was particu-larly e†ective against T . urticae, but when a gem-dimethyl group was introduced at C-1 (compound 18)substantial loss of activity against both T . urticae andB. tabaci was incurred.

As in the case of 1, stepwise insertion of groupsCH2at C-1 in 13 (to give series 2, 19È29) (See Table 4) alsoled to progressive loss of activity against B. tabaci. Incontrast, the activity against T . urticae peaked sharplyat n \ 9 (compound 27). A similar optimum (near

has been observed for the corresponding(CH2)11)terminated series (see compound 4) for activitywCH3

against T . urticae.5

294 B. P. S. Khambay et al.

TABLE 3Relative Activities of Compounds in Series 1

Relative activityaCompound

number R B. tabaci T. urticae

7 50 61

8 39 È

9 44 77

10 NAb 17

11 27 10

12 42 24

13 120 62

14 110 38

15 70 55

16 67 50

17 \10 5500

18 NA \10

a Relative activities are based on compound 1\ 100 (seeTable 1).b NA\ not active at highest concentration tested (1000 mglitre~1).

The investigation was next extended to compounds inwhich the quaternary carbon in the side-chain formedpart of a ring system (series 3). The results for threeselected examples (30, 31, 32) (Table 5) showed thatoptimum activity was associated with a cyclohexyl ringsystem (compound 31). Removing the methyl group atC-1 or replacing it with orwC2H5 , wCF3

(compounds 33 to 36) or changing itswCHxCH2position of substitution in 31 (compounds 37 to 39),

TABLE 4Relative Activities of Compounds in Series 2

Relative activityaCompound

number n B. tabaci T. urticae

13 0 120 6219 1 64 120 2 100 14021 3 44 170022 4 61 150023 5 35 52024 6 54 10025 7 56 140026 8 10 94027 9 4 11 00028 10 1 360029 11 1 500

a Relative activities are based on compound 1\ 100.

generally lowered activity. Introducing an additionalgroup in the cyclohexyl ring (compounds 40 towCH342) also appeared to reduce activity. However, these

compounds were mixtures of isomers (up to 2 : 1 ratio ofsyn : anti isomers, except compound 37, which was10 : 1). Further studies are under way to isolate and testthe individual diastereomers. Consistently, when a CH3group was introduced at C-1 to create a quaternarycarbon in a Bayer type compound (5) to give 43, activityalso decreased substantially. Preliminary results withexamples of the synthetic naphthoquinones reportedhere indicate that their resistance factors, assessed in thesame way as for the natural compounds (Table 2) areequally low, i.e. there is no cross-resistance to estab-lished classes.

Having identiÐed a range of highly active naphtho-quinones using direct contact assays, evaluation ofselected compounds was extended to leaf-dip assays (seeSection 2.2) which more closely resemble Ðeld condi-tions. Formulation A, based on SolvessoR (commonlyused for lipophilic compounds, e.g. pyrethroids) and apair of suitable surfactants, was developed (see Section2.2). With the present compounds, emulsions stable forthe duration of the bioassay were obtained. Using thisformulation, compounds 1 and 2 are compared withcommercially formulated samples of bifenthrin andchlorpyrifos in Table 6, using dose-transfer factors(DTFs) (see footnote to Table 6), as an empiricalmeasure of the e†ectiveness of the formulation inmaking the compound available for pick-up by the pest.As can be seen, the DTFs for compounds 1 and 2 are

A new group of plant-derived naphthoquinone pesticides 295

TABLE 5Relative Activities of Compounds in Series 3

Relative activityaCompound

number R B. tabaci T. urticae

30 23 104

31 7000 180

32 7 100

33 XxH 110 17034 1-Et 241 NA35 1-CF3 29 13936 1-CHxCH2 780 6337 2-Me 640 13038 3-Me 180 5039 4-Me 160 68040 1-Me, 2-Me 250 17041 1-Me, 3-Me 220 5042 1-Me, 4-Me 35 NA43 1-Me, 4-tBu 7 NA

a Relative activities are based on compound 1\ 100.

close to those for the standards for T . urticae but aresigniÐcantly lower for B. tabaci. Monitoring over timethe vial (contact) and leaf-dip assays showed that, in thelatter test, the poisoning e†ect was substantially slowerand attenuated in comparison with the vial assay, sodose-transfer was less efficient.

TABLE 6Performance of Naphthoquinones in Leaf-Dip Bioassays,using Formulation A, in Comparison with Direct Bioassays

B. tabaci T. urticae

Compound L C50 DT Fa L C50 DT F

1 7 14 51 1302 4É8 17 29 75

Bifenthrin 0É3 100 86 120Chlorpyrifos È È 100 71

(ng per insect) or (mg litre~1) in vial ora DTF\ (LD50 LC50micro-immersion in leaf-dip assay.assay] 100)/LC50

The problem of dose-transfer was much more pro-nounced with the more active synthetic analogues thanwith the natural products. Results for one extensivelystudied synthetic analogue, BTG 509 (compound 6, Fig.1),15 against both the test species and M. persicae areshown in Table 7. As can be seen, with formulation A,DTF against B. tabaci is similar to that observed for thenatural products (1 and 2) (Table 6), but in contrastDTFs against T . urticae and M. persicae are muchlower. A signiÐcant increase in DTFs against the lattertwo species, especially M. persicae, was observed whenanother formulation (currently under patentconsideration), based on a formulant of a di†erentmolecular weight, was used (Table 7). Clearly, formula-tion is an important consideration in the developmentof these compounds.

In summary, synthesis and assay of a series of pesti-cidal naphthoquinones has clariÐed the signiÐcance of aquaternary carbon atom as a structural feature in thenatural compounds, and has led to the discovery of arange of synthetic compounds highly active againstboth susceptible and resistant strains. Their e†ectivenessin leaf-dip assays is lower than expected, but can beimproved to some extent by using a more suitableformulation.

ACKNOWLEDGEMENTS

The authors thank Stuart Cameron and Carlos Escobarfor synthesis of some of the compounds, Elizabeth

TABLE 7Performance of Two Formulations of BTG 509 (6) in Leaf-Dip in Comparison with Direct

Bioassays using Formulations A and B

B. tabaci T. urticae M. persicae

L C50 L C50 L D50Formulation (ng per insect) DT F (ng per insect) DT F (ng per insect) DT F

A 20 25 0É7 0É6 29 \0É5B 25 2É2 44

296 B. P. S. Khambay et al.

Cook, Elizabeth Simms, Mark Hedges and FionaGuthrie for technical assistance and Norman Janes foradvice and help in preparing the manuscript. This workwas supported by the British Technology GroupLimited. IACR-Rothamsted receives grant-aidedsupport from the Biotechnology and Biological SciencesResearch Council of the UK.

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