Asteltoxins I–T: Polyketides from a Plant Endophytic Strain of Pochonia bulbillosa

Asteltoxins I–T (1–12),
12 asteltoxin-type polyketides, together with four known analogues
(13–16), were isolated from an endophytic
fungus Pochonia bulbillosa KNI755.
Their structures, including absolute configurations, were determined
by extensive spectroscopic analysis, single-crystal X-ray diffraction,
and electronic circular dichroism (ECD) calculations. Compound 1 featured a tetrahydro-2H-pyran-linked asteltoxin
heterodimer incorporating a clavatol unit. Compounds 2 and 3 demonstrated unique tetrahydro-1,4-oxazine-linked
asteltoxin heterodimers comprising a diphenyl ether unit. Compounds 4–7 presented unusual 1,4-dioxane-linked
asteltoxin heterodimers also containing a diphenyl ether unit. Compounds 1–7 were immunosuppressive against concanavalin
A (ConA)-induced T cell proliferation and lipopolysaccharide (LPS)-induced
B cell proliferation with EC50 values ranging from 9.2
to 26 μM and from 6.3 to 27 μM, respectively. Compound 1 exhibited inhibitory activity against the HCT116 human cancer
cell line with an IC50 value of 10 μM.