Recent Progress in the Synthesis of Succinate Dehydrogenase Inhibitors: From Pyrazole-Coupled Carboxamides and Beyond

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Abstract
A growing global population and increasingly stringent food-security demands, together with evolving challenges such as crop diseases, resistant weeds, and agrochemical-resistant pathogenic microorganisms, require new strategies to safeguard agricultural productivity. Succinate dehydrogenase (SDH, also known as complex II) participates in both the tricarboxylic acid cycle and the mitochondrial respiratory chain in pathogenic fungi, thereby affecting fungal metabolism and mitochondrial respiration. This enzyme has therefore been widely explored as a target in the search for new fungicides. Currently, 25 commercially available succinate dehydrogenase inhibitors (SDHIs) are used to protect economically important crops. The discovery of new agrochemicals is driven by the need to mitigate annual losses in agricultural productivity, estimated at approximately US$220 billion per year. This review summarizes advances in the synthesis of potent SDHIs over the last decade, including semisynthetic approaches and strategies combining carboxamide scaffolds with heterocyclic moieties based on bioisosterism, scaffold hopping, and molecular hybridization. The studies summarized here show that numerous hit compounds have recently been synthesized with EC50 values against several fungal strains and IC50 values for SDH inhibition that compare favorably with those of reference agrochemicals such as boscalid, thifluzamide, fluxapyroxad, fenfuram, and fluopyram. Finally, both established and emerging synthetic methods aimed at accelerating the generation of large compound libraries are increasingly supported by robust computational approaches, including 3D-QSAR and molecular dynamics simulations, to predict binding modes and provide structural insights for future investigations.