Dehydrobrachylaenolide: an eudesmane-type sesquiterpene lactone

The three-ring eudesmanolide, C15H16O3, is a natural product isolated from Dicoma anomala Sond. (Asteraceae). The compound contains an endo–exo cross conjugated methylenecyclohexenone ring with an envelope conformation trans-fused with cyclohexane and trans-annelated with an α-methylene 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.

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 methylenecyclohexenone ring with an envelope conformation transfused with cyclohexane and trans-annelated with anmethylene -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-HÁ Á ÁO interactions.

Related literature
For NMR studies of this compound, see: Bohlmann & Zdero, (1982); Grass et al. (2004). For the chemical synthesis and confirmation of the absolute structure, see: Higuchi et al. (2003).
The molecular geometry and labelling scheme are shown in Fig. 1. The methylenecyclohexenone ring adopts an envelope conformation, with the C5 atom out of the plane of the ring by approximately 0.7 Å. The γ-lactone ring is twisted on C6-C7, while the cyclohexane ring adopts a chair conformation. An axial position is occupied by methyl group C14, and the methylene carbon atom C15 is in the equatorial position. A weak intramolecular interaction is formed between C15-H15B···O2. Fig. 2 illustrates the molecular packing viewed down the c axis. Weak intermolecular hydrogen bonds are present between atoms C6-H6···O1 i and and C14-H14···O1 i [symmetry code (i): 1/2 -x, 1 -y, 1/2

S2. Experimental
The compound was isolated from Dicoma anomala Sond (Asteraceae), and recrystallized from propanol at room temperature.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.