(1S*,2S*,5R*,8S*,11S*,14R*,17S*,20R*)-14-Methyl-6-methylene-10,16,18-trioxahexacyclo[12.5.1.15,8.01,11.02,8.017,20]henicosane-7,9-dione: natural diterpenoid macrocalyxoformin B

The title compound, C20H24O5, isolated from Rabdosia var. lophanthoides Hara, is built up from six fused rings. Cyclohexane ring A adopts a chair conformation, ring B exists in a screw-boat conformation and ring C adopts a boat conformation; the three five-membered rings D, E and F adopt envelope conformations.

The title compound, C 20 H 24 O 5 , isolated from Rabdosia var. lophanthoides Hara, is built up from six fused rings. Cyclohexane ring A adopts a chair conformation, ring B exists in a screw-boat conformation and ring C adopts a boat conformation; the three five-membered rings D, E and F adopt envelope conformations.

S1. Comment
The diterpenoid macrocalyxoformin B, (I), has been previously isolated from Rabdosia macrocalyx Hara (Wang et al., 1986) and its structure was established from the spectroscopic and chemical evidence. Recently, it was for the first time isolated from Rabdosia var lophanthoides Hara, and its structure was confirmed by an X-ray diffraction study.
The molecule is built up from six fused rings, three six membered and three five membered rings (Fig. 1). Some geometrical features of these rings were investigated using PLATON (Spek, 2003).
All the three five-membered rings adopt envelope conformation.

S2. Experimental
The title compound (I) was isolated from Rabdosia var lophanthoides Hara and crystals suitable for X-ray structure analysis were obtained by slow evaporation from a solution of methanol at room temperature.
In the absence of significant anomalous scattering, the absolute configuration could not be reliably determined from the X-ray analyses and the Friedel pairs were merged, relative stereochemistry is shown in the Scheme and figures.   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.27 e Å −3 Δρ min = −0.23 e Å −3

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.