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

Abstract

To determine the reversible masking potential of carboxylic acids afforded by the N-acylaziridine structure, the hydrolysis of the fluorinated compound I was studied in buffered solutions at different pH, in insect tissues during invitro and exvivo assays, and in the presence of α-chymotrypsin. ¹⁹F[¹H] NMR monitoring was used directly with the reacting media, quantifying the fluorinated entities relative to an internal standard. Slightly basic pH (7.8) results in both the hydrolysis of the N-acyl function into 2-methylaziridine II and the fluorinated carboxylate III (pathway a), and the regiospecific cleavage of the heterocycle leading exclusively to the α-O-substituted regioisomer IVb of the corresponding β-hydroxyamide (pathway b). Under slightly acidic conditions (pH 6.3) the N-acyl function is preserved, but the hydrolysis of the heterocycle is more pronounced and now only regioselective since the minor product, hydroxyamide IVc (pathway c), is also obtained as well as the predominant regioisomer IVb. In the presence of diluted haemolymph, caterpillar or locust, the hydrolysis pathway a is accompanied by the hydrolysis pathway b. On the other hand, pathway a becomes the predominant hydrolysis mode when triggered by α-chymotrypsin, and the very efficient and nearly exclusive one, in the presence of locust fat body and mesenteron. Thus it must be concluded that, in contrast with the β-hydroxyamide IVb, the N-acylaziridine I acts as a proinsecticide by virtue of being a precursor of carboxylate III, which is confirmed by several exvivo experiments with locusts. Proinsecticides potentiels d'acides carboxyliques fluore′s III. Evaluation de la structure N-acylaziridine par suivi en RMN ¹⁹F du comportement invitro dans les tissus d'insectes. Pour de′terminer les possibilite′s de masquage d'acides carboxyliques offertes par la structure N-acylaziridine, l'hydrolyse du compose′ fluore′ I de cette se′rie est e′tudie′e à diffe′rents pH et dans des tissus d'insectes au cours d'expe′riences invitro et exvivo, ou en pre′sence d'α-chymotrypsine. La RMN ¹⁹F[¹H] a e′te′ utilise′e comme technique de suivi direct des milieux re′actionnels, en quantifiant les diffe′rentes entite′s fluore′es relativement à un standard interne. Un milieu faiblement basique (pH 7,8) provoque à la fois l'hydrolyse de la fonction acyle en me′thyle-2 aziridine II et en carboxylate fluore′ III (voie a) et la rupture re′giospe′cifique de l'he′te′rocycle conduisant exclusivement au re′gioisomère α-O-substitue′ du β-hydroxyamide correspondant (voie b). En milieu le′gèrement acide (pH 6,3) la fonction acyle est pre′serve′e, cependant que la rupture de l'he′re′rocycle est seulement re′giose′lective puisque l'hydroxyamide IVc (voie c) est e′galement obtenu à côte′ du re′gioisomère pre′ponde′rant IVb. En pre′sence d'he′molymphe dilue′e de chenille ou de criquet, la voie a est concurrence′e par la voie d'hydrolyse b. En revanche, la voie a devient le mode d'hydrolyse très pre′ponde′rant avec l'α-chymotrypsine, et le mode exclusif et extrèmement rapide en pre′sence des tissus corps gras ou me′sente′ron du criquet. On doit donc conclure que la N-acylaziridine I se comporte bien comme un proinsecticide potentiel en masquant re′versiblement le carboxylate III, contrairement au β-hydroxyamide IVb, ce qui a e′te′ confirme′ par des expe′riences exvivo conduites avec le criquet.