Versilulins A–D, Spiroketals, and Diphenyl Ether Derivatives from Marine-Derived Aspergillus versicolor 347 with Antibacterial Activity

Four unreported compounds, including two pairs of spiroketal racemates, (±)-versilulin A (1a and 1b) and (±)-versilulin B (2a and 2b), and two diphenyl ether derivatives, versilulins C and D (3 and 4), together with four known compounds (5-8), were isolated from the endophytic fungus Aspergillus versicolor 347. The assignment of their structures was accomplished by spectroscopic analysis, including HRESIMS, NMR, chiral analysis, ECD calculation, and X-ray diffraction. Among them, compounds 1 and 2 possess an unprecedented 6/5/6/6 tetracyclic polyketide with a unique 1,4-dioxospiro[4.5]decan-8-one core skeleton. Compound 3 was characterized by an unusual 6/6/6/6/6/6 hexacyclic ring system featuring a rare 9-oxospiro[5.5]undecan-3-one. Compound 4 contains a novel 6/8/6/6/6/6 hexacyclic fused ring system. Biological assays revealed that compounds 3 and 5 displayed modest antibacterial activity against Staphylococcus aureus with an equal MIC value of 32 μg/mL.