Anthosterones A and B, ring A contracted steroids from the sponge Anthoracuata graceae

Four new steroids, anthosterone A (5), anthosterone B (6), and the Δ4-3,6-diketo steroids 7 and 8, have been isolated from the sponge Anthoarcuata graceae. The structures of anthosterones A (5) and B (6) were deduced from their spectral data and verified via single crystal X-ray diffraction analysis on 5. The proposed structures for steroids 7 and 8 were based on spectral analysis and confirmed by synthesis of a model compound 12. Anthosterones A and B represent the first examples of a new type of ring A contraction in a steroid nucleus.