Mepronil

Metabolic pathway

Mepronil is an analogue of flutolanil containing a 2-methyl group as opposed to the 2-trifluoromethyl group. The compounds have the same mode of action and are metabolised via similar routes except that the methyl group of mepronil provides an extra site for metabolic attack. The compound is metabolised by dealkylation and hydroxylation at both the methyl and isopropyl groups and by aryl hydroxylation. Some hydrolysis occurs in plants and animals.

Degradation

Mepronil is stable to light, heat and air and in solution over a pH range of 3-10. It is hydrolysed under strongly alkaline conditions (PM). Mepronil on a silica gel surface exposed to sunlight between September and December (Japan) was 66% degraded with an estimated half-life of 36 days. This could be shortened by the addition of the photosensitiser xanthone (Yumita and Yamamoto, 1982). Irradiation of an aqueous solution with UV light at 30 °C afforded 32% degradation in 80 hours. ¹⁴C-Aniline and ¹⁴C-carbonyl labelling was used in these experiments. Carbonyl label afforded marpally more metabolites, indicating that some amide bond cleavage occurred. Twelve photo-products were identified and four unknowns were detected. Aqueous photolysis afforded somewhat fewer products than the surface irradiation. Hydroxylation occurred initially at four positions in the molecule and these were followed by further oxidation, hydrolysis, cyclisation or cleavage. The products are shown in Scheme 1. The major products (5-10%) of surface photolysis were compounds 2, 3 and 4 and on aqueous photolysis were 6 and 13 with an unknown derived only from carbonyl labelling.