Synthesis and biological activity of analogs of the antifungal antibiotic UK-2A. I. Impact of picolinamide ring replacement.

Background: The antifungal antibiotic UK-2A strongly inhibits mitochondrial electron transport at the Q site of the cytochrome bc1 complex. Previous reports have described semi-synthetic modifications of UK-2A to explore the structure-activity relationship (SAR), but efforts to replace the picolinic acid moiety have been limited.

Results: Nineteen UK-2A analogs were prepared and evaluated for Q site (cytochrome c reductase) inhibition and antifungal activity. While the majority are weaker Q site inhibitors than UK-2A (IC , 3.8 nM), compounds 2, 5, 13 and 16 are slightly more active (IC , 3.3, 2.02, 2.89 and 1.55 nM, respectively). Compared to UK-2A, compounds 13 and 16 also inhibit growth of Zymoseptoria tritici and Leptosphaeria nodorum more strongly, while 2 and 13 provide stronger control of Z. tritici and Puccinia triticina in glasshouse tests. The relative activities of compounds 1-19 are rationalized based on a homology model constructed for the Z. tritici Q binding site. Physical properties of compounds 1-19 influence translation of intrinsic activity to antifungal growth inhibition and in planta disease control.

Conclusions: The 3-hydroxy-4-methoxy picolinic acid moiety of UK-2A can be replaced by a variety of o-hydroxy-substituted arylcarboxylic acids that retain strong activity against Z. tritici and other agriculturally relevant fungi. © 2018 Society of Chemical Industry.