Alternatone A, an Unusual Perylenequinone-Related Compound from a Soft-Coral-Derived Strain of the Fungus Alternaria alternata

A novel perylenequinone-related compound, alternatone A (1), with an unprecedented tricyclo[6.3.1.0(2,7)] dodecane skeleton, together with three known perylenequinones, altertoxin I (2), stemphyperylenol (3), and alterperylenol (4), was isolated from the soft-coral-derived fungus Alternaria alternata L3111'. Their structures including absolute configurations were elucidated on the basis of comprehensive spectroscopic analysis, electronic circular dichroism calculations, and X-ray diffraction data. Compound 4 showed cytotoxicity against A-549, HCT-116, and HeLa cell lines with IC50 values of 2.6, 2.4, and 3.1 mu M, respectively. A possible biosynthetic pathway of 1 was proposed.