Structure–Activity Relationship of Natural Dihydrochalcones and Chalcones, and Their Respective Oxyalkylated Derivatives as Anti-Saprolegnia Agents

Saprolegnia sp. is a pathogenic oomycete responsible for severe economic losses in aquaculture. To date, there is no treatment for its control that is effective and does not pose a threat to the environment and human health. In this research, two dihydrochalcones 1 and 2, and three chalcones 3–5, isolated from the resinous plant Adesmia balsamica, as well as their synthesized oxyalkylated derivatives 6–29 already reported and a new synthesized series of oxyalkylchalcones 30–35, were evaluated for their anti-saprolegnia activity and structure–activity relationship as potential control and treatment agents for strains of Saprolegnia parasitica and S. australis. Among the molecules tested, natural 2′,4′-dihydroxychalcone (3) and new oxyalkylchalcone 34 were the most potent antisaprolegnia agents against both strains, even with better results than the commercial control bronopol. On the other hand, the structure–activity relationship study indicates that the contributions of steric and electrostatic fields are important to enhance the activity of the compounds, thus the presence of bulky substituents favors the activity.