Pestic. Sci. 1973,4,677-681

The Pyrethrins and Related Compounds XVII. Preparation of the Insecticide Bioresmethrin (5-Benzyl-3- furylmethyl ( + )-trans-Chrysanthemate) and Related Compounds Labelled with Deuterium or Tritium on the Furan Ringu

Michael Elliott, Norman F. Janes and Jack A. Spannerb

Rothamsted Experimental Station, Harpenden, Herts. AL5 2JQ (Manuscript received 2 February 1973 and accepted 14 February 1973)

2-'H-5-Benzyl-3-furylmethyl esters exclusively are obtained from an exchange reaction of the parent esters with deuterium oxide and a trace of acid (n.m.r. and mass spectrometric control). Similarly, with tritiated water, 6 insecticidal esters of 5-benzyl-3-furylmethyl alcohol give the corresponding [2-3H]esters, each with specific activity of approx. 500 mCi/mmol.

1. Introduction

Natural and synthetic pyrethroids, although relatively expensive and non-persistent, have many advantages as insecticides, including safety to man, but recently closely related compounds in the group have been found to have surprisingly different mammal- ian toxicities. For example, the approximate oral toxicities to rats of the natural pyrethrins, of 5-benzyl-3-furylmethyl (+)-trans-chrysanthemate (bioresmethrin) and of 5-benzyl-3-furylmethyl (+)-cis-chrysanthemate are, respectively, 800, 8000 and 80 mg/kg.l>*

To establish conditions for safe use, therefore, metabolismin insects and mammals and persistence must be examined, for which purpose radiolabelled compounds (especially 14C and 3H) are valuable. Tritiated compounds are relatively easy to prepare, but results with them are of limited value unless the stability in vivo of the radiolabel is known, for example by observing analogous deuterated systems by n.m.r.3-5 How- ever, the structures of metabolites of pyrethrins I and I1 and of allethrin in rats were determined successfully4 using tritiated compounds prepared under mildly basic conditions via the enolic forms of the keto-esters. For similar studies on the very potent insecticidal esters of 5-benzyl-3-furylmethyl alcoh01,~*~ we report here a simple and convenient procedure by which the furan nucleus can be labelled with tritium by an exchange reaction under mildly acidic conditions. Only one proton, that adjacent to the furan oxygen, is involved and the labelled insecticidal esters (of chrysanthemic and related acids) are obtained directly.

a Part XIII: Pestic. Sci. 1971,2,243. The papers cited in references 3,4 and 5 are considered to be Parts XIV, XV and XVI, respectively.

* The Radiochemical Centre, Amersham, Bucks. 677

678 M. Elliott, N. F. Janes and J. A. Spanner

2. Experimental 2.1. Spectroscopic and chromatographic methods Nuclear magnetic resonance (n.m.r.) spectra were recorded in carbon tetrachloride with tetramethylsilane as internal standard except where stated otherwise, on a Perkin- Elmer R10 spectrometer at 60 MHz. Mass spectra were determined at 80 eV on a Perkin Elmer Hitachi RMU 6E spectrometer. Plates (Merck Silica gel F254) for t.1.c. were developed in ether : hexane (2: 1). For g.l.c., a stainless steel column (5 ft XQ in) packed with 5 % QFl on chromosorb W was used at 175".

2.2. Compounds used Before reaction, esters were examined by g.1.c. and n.m.r. and if necessary, alcoholic and acid impurities were removed by passage through a column of alumina (Hopkin and Williams, neutral; 5 g/g of ester) in benzene. The samples of the following compounds were more than 90% pure, the major impurity being the ester of the stereoisomeric acid (approx. 5 %) : (1) NRDC 107, 5-benzyl-3-furylmethyl (+)-trans-chrysanthemate (bi~resmethrin),".~~' (2) NRDC 1 19,5-benzyl-3-furylmethyl (+)-cis-chrysanthemate,". (3) RU 11,679, 5-benzyl-3-furylmethyl (+)-trans-ethanochrysanthemate,". (4) RU 1 1,934, 5-benzyl-3-furylmethyl (-)-trans-chrysanthemate," (5) RU 12,065, 5-benzyl-3- furylmethyl (-)-cis-chrysanthemate,a (6) NRDC 108, 5-benzyl-3-furylmethyl 2,2,3,3- tetramethylcyclopropane carboxylate. lo

n

A. a Sample supplied by Roussel-Uclaf, S.A.

679 Preparation of bioresrnethrin labelled radiochemically

2.3. Procedure

2.3.1. Deuteration Bioresmethrin (1 .O g) was heated with deuterium oxide (2.5 ml; 99.8 % D) in dioxan (12.5 ml) and concentrated hydrochloric acid (3 drops) at 100" for 40 min; reaction was then complete (direct observation of reaction mixture by n.m.r., at z 2.75). After evaporation of solvents at 0.1 mm, the residue had an n.m.r. spectrum (Figure 1)

I I I I I I I I 2 3 4 5 6 7 8 9

r

Figure 1. N.m.r. spectra of bioresmethrin (A) before (B) after exchange with DzO, showing exchange of the furan 2-H.

identical to that of bioresmethrin [z 2.75 (s, furan 2-H), 2.85 (m, phenyl) 4.08 (s, furan 4-H), 5.2 (m, --CH=CMe,) 5.20 (s, CH20) 6.13 (s, PhCH,) 8.04 (4, cyclopropyl 3-H) 8.33 (s, CH=CMG) 8.71 (d, cyclopropyl 1-H) 8.79 and 831 (2 x s, cyclopropyl Me's)] except that the resonance at T 2.75 was much smaller. Accurate integration showed 80 % 2H, 20 % 'Hat this position. This was confirmed by the relative intensities of the strong peaks in the mass spectrum at m/e 339 (M+ for monodeutero ester) and 172 (5-benzyl-3- furylmethyl+ with one 'H) and the corresponding weaker ones at 338 and 171 (undeuter- ated species) indicating an 81 : 19 mixture of ,H and IH compounds. T.1.c. showed only one spot (RF 0.69) (detected under u.v.) and, byg.1.c. the same proportion (6 %) of cis- chrysanthemate was present before and after exchange. The optical rotation, [a]Z,O was -7.8" before exchange and -8.1" after (c, 4.8 in acetone). Even after heating for 24 h (section 2.3.2.) there was still no detectable loss of ester.

The 2-protons in NRDC 11 9 and RU 11,679 were similarly rapidly exchanged and no decomposition detected (g.1.c. and n.m.r.).

680

2.3.2. Tritiation

2.3.2.1 Incorporation of 3H

M. Elliott, N. F. Janes and J. A. Spanner

Bioresmethrin (1.0 g) dioxan (12.5 ml) concentrated hydrochloric acid (50 pl) and tritiated water (2.5 ml; 200 Ci; 720 mCi/mA of exchangeable hydrogen) were heated in a sealed tube at 100" for 16 h. After cooling, potassium carbonate (75 mg) was added and volaiile solvents were evaporated into a trap cooled in liquid nitrogen. The residue was dissolved in benzene (25 ml), sealed and stored at -20".

This procedure was repeated in succession with NRDC 119, RU 11,679, NRDC 108, RU 11,934 and RU 12,065, the contents of the trap being acidified before re-use in the next exchange reaction. Six different labelled esters were thus obtained from one portion of tritiated water, all, even the later samples, with adequate specific activities (see Table 1).

TABLE 1. Tritium-labelled esters of 5-benzyl-3-furylmethyl alcohol

Ester

Yield from 100 mg of unlabelled

ester (md

Total activity

(mCi)

Specific activity

(mCi/mmoI)

Radiochemical purity ( %)

(+)-trans-chr ysanthemate 71 (+)-cis-chr ysanthemate 32

(-)-trans-chr ysanthemate 29

2,2,3,3-tetramethylcyclopropane

(+)-trans-ethanochry santhemate 35

(-)-cis-chrysanthemate 39

carboxylate 48

135 51 72 28 27

78

641 537 740 325 230

551

98 95 98 96 96

92

2.3.2.2. Isolation of 3H-esters

A portion (2.5 ml) of the benzene solution of the product was added to a dry column of aluminaa (8 mm i.d. x 50 mm; 4 g; Hopkin and Williams, neutral, for chromato- graphy) and eluted with benzene. The first eluate (10 ml) was collected in a weighed tube, concentrated in dry nitrogen (oxygen-free) and then evaporated to constant weight at 0.05 mm/20". The residues were dissolved in benzene (100 ml for bioresmethrin, 50 ml for the other esters) and specific activities estimated by scintillation counting (Beckmann LSS-250) of aliquots (100 pl) suitably diluted. Radiochemical purities were determined by scanning (Panax RTLS-1A) of t.1.c. plates, which showed that the purification by column chromatography removed substantial amounts of more polar radioactive impurities. After storage, the product was examined by t.1.c. and, if necessary, repurified.

a Some samples of alumina were found to be unsuitable for the chromatographic purification. Each batch was checked by confirming that unlabelled ester (100 mg) was eluted readily by benzene (10 ml) from a portion (approx. 4 g).

Preparation of bioresmethrin labelled radiochemically 681

3. Results and discussion Kankaanpera and Kleemolall showed that in acidified aqueous dioxan the rate of exchange of the proton at the 5-position of 2-methyl-furan was at least 100 times greater than the rate of cleavage of the ring. This suggested that the proton in the 2-position of the furan ring in 5-benzyl-3-furylmethyl alcohol and its derivatives would exchange correspondingly easily. As with 2-methylfuranY this was readily confirmed by observing the disappearance of the n.m.r. signal at z 2.75 directly on the reaction mixture. Exchange of the 2-proton of bioresmethrin with deuterium was complete after 40 min at 100" but even after 24 h no hydrolysis or other decomposition or racemisation was detected and no other protons were replaced. The structures of the esters were confirmed by the mass spectra of the deuterated products which gave an estimate of the deuterium incorporation that agreed well with the result from n.m.r.

To incorporate tritium under similar conditions with tritiated water (1.4 Ci/mmol; 4.8 % TOH) reaction was assumed to be complete after 12 h, for there was no convenient method for monitoring tritium, equivalent to n.m.r. for deuterium. Scanning thin layer chromatograms showed that the product from tritium exchange contained some polar and highly radioactive impurities, but they were all removed by passing the product in benzene through neutral alumina. The procedure therefore provided a simple and direct route to radiolabelled esters with a very high specific activity, that involved very little manipulation of the radioactive products.

Six esters were prepared with high radioactive content and purity, as shown in Table 1. The levels of radioactivity indicate that equilibration with the tritiated water was nearly complete in each case. These should be suitable for investigating metabolism in vivo and to estimate persistence and residues; the stability of the tritium label can be judged from the behaviour of the deuterated esters examined by n.m.r.

Miyamoto, Nishida and Ueda12 synthesised resmethrin labelled with 14C at C-2 of the furan ring and successfully identified metabolites produced in the rat. The easily synthesised compounds described in this paper should complement the 14C material of Miyamoto and make possible studies needing a wider range of highly radioactive esters.

Acknowledgement We thank Dr J. Martel, Roussel Uclaf S.A., for generous gifts of the samples indicated.

References 1. Elliott, M. Bull. Wld Hlth Org. 1971,44, 315. 2. Verschoyle, R. D.; Barnes, J. M. Pestic. Biochem. Physiol. 1972,2,308. 3. Elliott, M.; Casida, J. E. J. ugric. Fd Chem. 1972,20, 295. 4. Elliott, M.; Janes, N. F.; Kimmel, E. C.; Casida, J. E. J . ugric. Fd Chem. 1972,20,300. 5 . Elliott, M.; Gaughan, L. C.; Casida, J. E. J. ugric. Fd Chem. 1972,20, 731. 6. Elliott, M.; Farnham, A. W.; Janes, N. F.; Needham, P. H.; Pearson, B. C. Nature, Lond.

1967,213,493. 7. Elliott, M.; Janes, N. F.; Pearson, B. C. Pestic. Sci. 1971,2,243. 8 . Lhoste, J.; Martel, J.; Rauch, F. Meded. Fuc. Lundbouwwetensh Rijksuniv. Genr. 1971,36, 978;

Chem. Abstr. 1972,77, 3032111. 9. Velluz, L.; Martel, J.; Nomine, G. C.r. hebd. Skunc. Acud. Sci. Paris 1969,268,2199.

10. Barlow, F.; Elliott, M.; Farnham, A. W.; Hadaway, A. B.; Janes, N. F.; Needham, P. H.; Wickham, J. C. Pestic. Sci. 1971,2, 115.

11. Kankaanpera, A.; Kleemola, S . Acru chem. scund. 1969,23,3707. 12. Miyamoto, J.; Nishida, T.; Ueda, K. Pesticide Biochem. Physiol. 1971,1,293.