Batrachopolyenes: Volatile Norsteroids from Femoral Scent Glands of Frogs

Steroid hormones are C₁₈-C₂₁-sterane derivatives, featuring the typical 6-6-6-5 ring system. Here we report on a novel C₁₈-steroid ring system named batrachane with a contracted A-ring resulting in a 5-6-6-5 ring arrangement. The isolation, structural elucidation, and total synthesis of three members of the novel batrachopolyene family occurring in the tropical frog genus Odontobatrachus is reported. The batrachopolyenes represent an entirely new collection of volatile steroidal natural products produced by anuran amphibians. Alongside the contracted A-ring, each member contains a Δ_16-17 double bond but differs in the central belt of the steroidal structure. To create these atypical structural features, syntheses featuring a combination of Breslow radical chain relay chlorination, Favorskii ring contraction, and Clemmensen reduction proved successful. The occurrence of such compounds in another distal anuran group, the Mantellinae, suggests a more widespread distribution of the batrachane-type compounds among frogs. The new structural steroid type raises questions concerning steroid biosynthesis and reception, as well as distribution in frogs in general and the structures of their hormones.