Thiophene carboxamide fungicides: Structure-activity relationships with the succinate dehydrogenase complex from wild-type and carboxin-resistant mutant strains of Aspergillus nidulans

Abstract

Carboxin (5,6-dihydro-2-methyl-1,4-oxathiin-3-carboxanilide) and the thiophene compound 3-methylthiophene-2-carboxanilide (I), inhibit succinate oxidation (succinate-ubiquinone reductase; complex II) in mitochondria from a wild-type strain and three mutant, carboxin-resistant strains of Aspergillus nidulans. Studies by White et al. [ Pestic. Biochem. Physiol. 9, 165–182 (1978)] showed that certain oxathiin carboxamide structures were selectively active against particular mutated succinate dehydrogenase complexes (SDCs) of A. nidulans, significantly lowering the level of resistance. Although no oxathiin carboxamides were found to be negatively correlated to carboxin with respect to their effect on SDCs from wild-type and mutant strains of A. nidulans, several could distinguish between moderately and highly carboxin-resistant SDCs and, as shown in the present study, between the two non-allelic highly carboxin-resistant mutants cbx B-28 and cbx C-34. Variation in the molecular structure of thiophene carboxamides can also affect the phenotypic expression of mutations to carboxin resistance in the SDC of A. nidulans, with certain structures being capable of differentiating between moderately and highly carboxin-resistant mutated SDCs. With a moderately carboxin-resistant mutant, cbx A-17, a wide structural variety of thiophene carboxamides, e.g., the 2′-methyl, 2′-benzoyl, 3′-phenoxy, 4′- nbutyl and the N-nhexyl derivatives of (I), did exhibit negative activity correlation to the parent anilide (I). However, with the possible exception of the 4′- nbutyl and 4′- noctyloxy analogs of (I), thiophene carboxamides showed no negative activity correlation to carboxin or (I) for the highly carboxin-resistant mutants cbx B-28 and cbx C-34. As with carboxin-resistant mutants of Ustilago maydis [ Pestic. Biochem. Physiol. 14, 26–40 (1980)], molecular selectivity for mutated carboxin-resistant SDCs of A. nidulans can be markedly influenced by substitution of an oxathiin with a thiophene heterocyclic ring. None of the thiophene carboxamides were considerably toxic to mycelial growth of the wild-type and carboxin-resistant strains of A. nidulans with permeability rather than affinity for the SDC appearing to be the limiting factor. For certain derivatives such as the 5-amino analog of (I), SDC activity and cell growth were inhibited similarly. Several thiophene carboxamides [2′-phenyl, 4′-phenoxy, and N-ndecyl analogs of (I)] showed specificity for the highly carboxin-resistant mutants cbx B-28 and cbx C-34. Thiophene carboxamide structures have been identified which inhibit spore germination of non-Basidiomycete plant pathogens, particularly Phytophthora infestans and Verticillium dahliae. In vivo experiments with late blight ( P. infestans) on tomato plants have shown that a few thiophene carboxamides, e.g., the 3′- nbutyl analog of (I) give satisfactory protectant activity.