Dehydro-brachylaenolide: an eudesmane-type sesquiterpene lactone.

The three-ring eudesmanolide, C(15)H(16)O(3), is a natural product isolated from Dicoma anomala Sond. (Asteraceae). The compound contains an endo-exo cross conjugated methyl-enecyclo-hexenone ring with an envelope conformation trans-fused with cyclo-hexane and trans-annelated with an α-methyl-ene γ-lactone. The absolute structure was assigned by optical rotation measurements compared to those from the synthetic compound with known stereochemistry. The crystal packing is consolidated by C-H⋯O interactions.