New Austalides from the Sponge-Associated Fungus Aspergillus sp.

Chromatographic separation of a crude extract obtained from the fungus Aspergillus sp., isolated from the Mediterranean sponge Tethya aurantium, yielded five new meroterpenoid metabolites, austalides M-Q (1-5), together with nine known compounds (6-13). The structures of the new compounds were unambiguously elucidated on the basis of extensive 1D and 2D NMR methods and by mass spectral analysis. Furthermore, the absolute configurations of 1 and 4 were determined by time-dependent density functional theory electronic circular dichroism (TDDFT ECD) calculations, allowing the assignment of the absolute configuration of analogous compounds 2, 3, and 5. The calculations revealed that the conformation of the benzene-fused phthalide chromophore, which is sensitive to even minor changes in its proximity, is decisive for the ECD parameters, rendering a simple ECD comparison of related homochiral austalides difficult. All compounds were evaluated for their cytotoxic activity against murine cancer cell line L5178Y by using the MTT method. Compounds 8 and 11 exhibited moderate to pronounced cytotoxicity, with IC50 values of 39.4 and 0.2 mu M, respectively, whereas the remaining investigated compounds showed either weak or no activity in this assay.