Cyclohexadepsipeptides of the Isaridin Class from the Marine-Derived Fungus Beauveria felina EN-135

Three new cyclohexadepsipeptides of the isaridin class including isaridin G (1), desmethylisaridin G (2), and desmethylisaridin C1 (3), along with three related known metabolites (4–6), were isolated and identified from the marine bryozoan-derived fungus Beauveria felina EN-135. The structures of these compounds were elucidated on the basis of extensive spectroscopic analysis, and the structures and absolute configurations of compounds 1–3 were confirmed by single-crystal X-ray diffraction analysis. The crystal structures showed the presence of β-turns for the Tyr³/N-Me-Val⁴ and Phe³/N-Me-Val⁴ amide bonds in compounds 2 and 3, respectively, in the cis conformations, which were opposite other reported isaridins. The conformations of the HMPA¹-Pro² amide bond in compound 2 are different in the solution and in the crystal structures, which showed trans and cis geometries, respectively, while compounds 1 and 3 do not exhibit this phenomenon. Each of the isolated compounds was evaluated for antimicrobial activity and brine shrimp lethality. Compound 3 exhibited antibacterial activity against E. coli with an MIC value of 8 μg/mL.