Discovery of Benzothiazol-2-ylthiophenylpyrazole-4-carboxamides as Novel Succinate Dehydrogenase Inhibitors

Succinate dehydrogenase (SDH) has been considered an ideal target for discovering fungicides. To develop novel SDH inhibitors, in this work, 31 novel benzothiazol-2-ylthiophenylpyrazole-4-carboxamides were designed and synthesized using active fragment exchange and a link approach as promising SDH inhibitors. The findings from the tests on antifungal activity indicated that most of the synthesized compounds displayed remarkable inhibition against the fungi tested. Compound Ig N-(2-(((5-chlorobenzo[d]thiazol-2-yl)thio)methyl)phenyl)-3-(difluoromethyl)-1-methyl-1H-yrazole-4-carboxamide, with EC50 values against four kinds of fungi tested below 10 μg/mL and against Cercospora arachidicola even below 2 μg/mL, showed superior antifungal activity than that of commercial fungicide thifluzamide, and specifically compounds Ig and Im were found to show preventative potency of 90.6% and 81.3% against Rhizoctonia solani Kühn, respectively, similar to the positive fungicide thifluzamide. The molecular simulation studies suggested that hydrophobic interactions were the main driving forces between ligands and SDH. Encouragingly, we found that compound Ig can effectively promote the wheat seedlings and the growth of Arabidopsis thaliana. Our further studies indicated that compound Ig could stimulate nitrate reductase activity in planta and increase the biomass of plants.