Versispiroketal A, an unusual tetracyclic bridged spiroketal from the sponge-associated fungus Aspergillus versicolor SCSIO 41013.

Versispiroketal A (1), an unprecedented 6/5/5/6 tetracyclic polyketide featuring a rarely encountered bridge-fused spiroketal skeleton, was isolated from the sponge-associated fungus Aspergillus versicolor SCSIO 41013. The structure and absolute configuration of 1 were unequivocally determined by comprehensive spectroscopic analysis, single-crystal X-ray diffraction analysis and quantum chemical ECD calculations. Compound 1 showed weak cytotoxicity against four cancer cell lines. A plausible biosynthetic pathway for 1 was also postulated.