Products Formed in Pyrethrin Concentrates during Storage

Abstract

IN the course of work on the structure of the pyrethrins, concentrates rich in pyrethrins I and II were stored in the dark for some months. It was then found that the concentrates were no longer completely soluble in petroleum ether and, surprisingly, that the insoluble residues showed a high 'apparent' pyrethrin content when assayed both by the Wilcoxon-Holaday and seil methods¹, in spite of their low toxicity to house flies in comparison with freshly prepared pyrethrum extracts at equivalent concentrations. Thus two typical residues showed (Sample A) pyrethrin I, 13.5 percent w/w, pyrethrin II, 64.7 percent w/w (Wilcoxon-Holaday), and pyrethrin I, 15.1 percent, pyrethrin II, 65.1 percent (seil), and (Sample B) pyrethrin I, 43.1 percent, pyrethrin II, 7.5 percent (Wilcoxon-Holaday), and pyrethrin I, 47.5 percent, pyrethrin II, 7.2 percent (seil), yet 1 percent solutions of these residues in acetone were less effective against house flies under exactly comparable conditions than a 0.1 percent solution of pyrethrins in the same solvent made up from concentrates of the same type but freshly prepared. In addition, appreciable absorption of light was observable (in alcoholic solution) at the absorption maximum characteristic of the pyrethrins²; thus (A) showed λ_max. 2310 A., E^1%_1cm. 144, whereas (B) showedλ_max. 2280 A., E^1%_1cm. 495.