New Polyketide Congeners with Antibacterial Activities from an Endophytic Fungus Stemphylium globuliferum 17035 (China General Microbiological Culture Collection Center No. 40666).

Four new polyketides, heterocornol Y (1), stemphyindan (2), pestalospirane C (3), and stemphyspyrane (4), along with five known ones (5-9) were isolated from the endophytic fungus Stemphylium globuliferum 17035 (SG17035) based on the One Strain Many Compounds (OSMAC) strategy allied with an LC-MS approach. These structures were elucidated through extensive spectroscopic analyses, single-crystal X-ray diffraction, and ¹³C NMR-DP4 analysis. Pestalospirane C (3) and stemphyspyrane (4) featured unprecedented spiroketal skeletons. In addition, the putative biosynthetic logic for compounds 1-4 was proposed. Antibacterial and cytotoxic activities of compounds 1-9 were evaluated. Stemphyspyrane (4) displayed promising antibacterial activity against different pathogens, especially against Staphylococcus aureus, Porphyromonas gingivalis, and methicillin-resistant Staphylococcus aureus (MRSA) with MIC values of 3.125 μM, 6.25 μM, and 12.5 μM, respectively. It is promising as an antibacterial agent for further optimization.