Agnatasterone A and B, Unusual Pregnane Steroids Isolated from the North‐East Atlantic Sponge Axinella agnata

Two novel pregnatrienolones isolated in very small amounts from the North‐East‐Atlantic demosponge Axinella agnata (Tetractinomorpha, Axinellida) are unique in having C(2)C(3) (or C(3)C(4)), C(7)C(8), and C(16)C(17) bonds and a 12β‐OH group which, being strongly H‐bonded to a 20‐keto group, resists acylation. 1 H‐ and 13 C‐NMR spectroscopy of the steroids and of products of their selective epoxidation or reduction allow us to propose the structures (+)‐12β‐hydroxy‐5α‐pregna‐2,7,16‐trien‐20‐one ( = agnatasterone A, (+)‐ 1 ), and (+)‐12β‐hydroxy‐5α‐pregna‐3,7,16‐trien‐20‐one ( = agnatasterone B. (+)‐ 5 ), for the two steroids with minimal recourse to model compounds.