α-Pyrone derivatives, tetra/hexahydroxanthones, and cyclodepsipeptides from two freshwater fungi

Eighteen (1–18) and seven (1, 4, 6–8, 17 and 18) compounds were isolated from organic extracts of axenic cultures of two freshwater fungi Clohesyomyces sp. and Clohesyomyces aquaticus (Dothideomycetes, Ascomycota), respectively. Compounds 1–12 belong to the α-pyrone class of natural products, compounds 13 and 14 were tetrahydroxanthones, compounds 15 and 16 were hexahydroxanthones, while compounds 17 and 18 were cyclodepsipeptides. The structures were elucidated using a set of spectroscopic and spectrometric techniques. The absolute configurations of compounds 2, 3, 6, and 7 were assigned via a modified Mosher’s ester method using 1H NMR data. The relative configurations of compounds 14–16 were determined through NOE data. Compounds 1, 2, 6, 8, 13, 14, and 15 were found to inhibit the essential enzyme bacterial peptidyl-tRNA hydrolase (Pth1), with (13; secalonic acid A) being the most potent. Compounds 1 and 4–18 were also evaluated for antimicrobial activity against an array of bacteria and fungi but were found to be inactive.