Studies on Biologically Active Organophosphorus Compounds

Abstract

Two types of metabolic activation of triaryl phosphate to cause delayed neurotoxicity were disclosed: 1) tri-o-tolyl phosphate (TOCP) was converted into a highly active saligenin cyclic phosphate, 2-(o-tolyoxy)-4H-1, 3, 2-benzodioxaphosphorin 2-oxide, by the microsomal hydroxylation of a methyl group followed by cyclization, which was accelerated by plasma albumin; 2) tri-p-ethylphenyl phosphate (TPEP) was activated by similar hydroxylation at α-carbon atom followed by dehydrogenation to form α-oxo metabolites. Studies on structure-activity relationships by modifying the structure of the neurotoxic metabolites showed that the selective toxicity is governed greatly by the size of non-leaving groups (selectophore) and that the combination of the selectophore and phosphorylating activity, which is due to the cyclic ester structure in TOCP series and to the electron-withdrawing leaving group in TPEP series, decides the final biological activity of phosphorus compounds. Thus, the metabolite homologs carrying small alkyl group (s) as the non-leaving group in both series did not cause ataxia in hens, but exerted insecticidal activity. Salithion (2-methoxy-4H-1, 3, 2-benzodioxaphosphorin 2-sulfide) was therefore developed into a commercial insecticide. The phosphorylating property of saligenin cyclic phosphate esters was also utilized for the syntheses of biologically interesting natural phosphate esters by developing the thiolate isomer of salithion, MTBO. This has the advantage of oxidatively activatable thiolate ester group, in addition to the highly reactive cyclic ester structure of saligenin which also acts as a removable protective group. Some potent synergists of malathion were also found in the series of cyclic phosphorus esters related to the TOCP-metabolite. They were the potent inhibitors of carboxyesterase. Another synergist found was a thioquinol phosphate, which was obtained by modifying the metabolite of triphenyl phosphate, a malathion synergist. Another thiolate-type synergist IBP inhibited the degradation of malathion by inhibiting carboxyesterase and also demethylation in vivo. Finding L-leucine to be a neuroactive substance in the blood of silkworm larvae, we combined it and related amino acids with an extremely reactive five-membered cyclic phosphate and obtained novel insecticidal 4-isobutyl-2-methoxy-1, 3, 2-oxazaphospholidine 2-sulfide and the 4-isopropyl homolog. These were particularly effective to organophosphate resistant strains of houseflies. The structuretoxicity relationship of bicyclic phosphate esters, 4-substituted 2, 6, 7-trioxa-1-phospha-bicyclo [2.2.2] octanes, was characterized by the importance of the steric and hydrophobic properties of the bridge-head substituent for the high toxicity, which appeared to be due to their anti-GABA activity.