Potential proinsecticides of fluorinated carboxylic acids III. Evaluation of the N-acylaziridine structure by F-19 NMR monitoring of the in vitro behaviour in insect tissues

To determine the reversible masking potential of carboxylic acids afforded by the N-acylaziridine structure, the hydrolysis of the fluorinated compoun d I was studied in buffered solutions at different pH, in insect tissues du ring in vitro and ex vivo assays, and in the presence of a-chymotrypsin. F- 19[H-1] NMR monitoring was used directly with the reacting media, quantifyi ng the fluorinated entities relative to an internal standard. Slightly basi c pH (7.8) results in both the hydrolysis of the N-acyl function into 2-met hylaziridine II and the fluorinated carboxylate III (pathway a), and the re giospecific cleavage of the heterocycle leading exclusively to the alpha-O- substituted regioisomer Nb of the corresponding beta-hydroxyamide (pathway b). Under slightly acidic conditions (pH 6.3) the N-acyl function is preser ved, but the hydrolysis of the heterocycle is more pronounced and now only regioselective since the minor product, hydroxyamide IVc (pathway c), is al so obtained as well as the predominant regioisomer IVb. In the presence of diluted haemolymph, caterpillar or locust, the hydrolysis pathway a is acco mpanied by the hydrolysis pathway b. On the other hand, pathway a becomes t he predominant hydrolysis mode when triggered by alpha-chymotrypsin, and th e very efficient and nearly exclusive one, in the presence of locust fat bo dy and mesenteron. Thus it must be concluded that, in contrast with the bet a-hydroxyamide Nh, the N-acylaziridine I acts as a proinsecticide by virtue of being a precursor of carboxylate III, which is confirmed by several ex vivo experiments with locusts.