Structurally diverse metabolites from a soil-derived fungus Aspergillus calidoustus

Fifteen secondary metabolites, including five new sesquiterpenoids (1-5), one new benzofuranoid (10), one new ophiobolin sesterterpenoid (11), and one new 3,5-dimethylorsellinic acid (DMOA)-based meroterpenoid (15), as well as seven known analogues (6-9 and 12-14), were isolated and characterized from fungus Aspergillus calidoustus, which was separated from the wetland soil collected at Dianchi Lake, Yunnan Province. Compound 5 featured an unusual dioxolane moiety, and compound 15 was a rare austin meroterpenoid analogue with the opened A ring, and also featured an undescribed oxygen bridge between C-3 and C-16 to construct an unexpected tetrahydrofuran ring. Their structures were established by widespread spectroscopic methods, single-crystal X-ray diffraction experiments, and ECD calculation. All the isolated drimane sesquiterpenoids were evaluated for the in vitro cytotoxicity against five tumor cell lines, including SW480 (colon cancer), IOMM-Lee (meningioma), HeLa (cervical cancer), FARAGE (diffuse large B-cell lymphoma), and SU-DHL-4 (diffuse large B-cell lymphoma). Compound 9 exhibited significant cytotoxicity against FARAGE and SU-DHL-4 tumor cell lines with IC₅₀ values of 5.54 and 9.78 μM, respectively. Further mechanism study showed that 9 could significantly promote apoptosis in FARAGE and SU-DHL-4 cell lines by interfering with mitochondrial function.