Cytospolides A-E, New Nonanolides from an Endophytic Fungus, Cytospora sp.

Cytospolides A-E (1-5), five new nonanolides with an unprecedented 15-carbon skeleton with the unique chemical feature of a C-2 methyl group, were isolated from the endophytic fungus Cytospora sp. The structures were elucidated by spectroscopic analysis, chemical interconversion, and single-crystal XRD. The solution and solid state conformers were compared by experimental methods (X-ray, NMR) and solvent and gas phase DFT calculations. The absolute configurations were assigned by time dependent (TD)-DFT calculations of CD spectra, including solution and solid state CD/TD-DFT approaches. The cytotoxicity assay of C-2 epimers showed significantly different activity against A549, suggesting that the C-2 methyl group has an important role in growth inhibition towards the tumor line. The existence of two conformers of the bioactive epimer in solution was proven by the soft pulse transfer NMR technique and further supported by DFT calculations. The discovery of the new metabolites not only extends the nonanolide family with cytospolide-type skeletons but also gives insight into the biosynthetic process of these fungal metabolites.