Asteltoxins from the Entomopathogenic Fungus Pochonia bulbillosa 8-H-28

New asteltoxins C (3) and D (4) were found in the extract of the entomopathogenic fungus Pochonia bulbillosa 8-H-28. Compound 2, which was spectroscopically identical with the known asteltoxin B, was isolated, and structural analysis led to a revision of the structure of asteltoxin B. Compounds 2 and 4 have a novel tricyclic ring system connected to a dienyl α-pyrone structure. Compound 3 has a 2,8-dioxabicyclo[3.3.0]octane ring similar to that of asteltoxin (1). Compound 3 showed potent antiproliferative activity against NIAS-SL64 cells derived from the fat body of Spodoptera litura larvae, while 2 and 4 were inactive.