6,8-Dihydroxy-8a-methyl-3,5-dimethylidenedecahydronaphtho[2,3-b]furan-2(3H)-one

The title compound, C15H20O4, is a eudesmanolide isolated from the Chinese medicinal plant Carpesium triste Maxim. The molecule contains three rings, viz. two fused six-membered rings in chair conformations and a five-membered ring in a flattened envelope conformation. In the crystal, two hydroxy groups are involved in the formation of intra- and intermolecular O—H⋯O hydrogen bonds. The H atoms in these groups are split, with site-occupation factors of 0.5. The intermolecular hydrogen bonds link molecules into chains propagating in [010].


Ting-ting Liu, Hai-Bo Wu and Wen-Shu Wang Comment
Carpesium triste Maxim grows in the northeast and southwest area of mainland China. It is used as traditional Chinese medicine having effects on detoxification and antibacterial activities. As a part of our research on biological resource by ethnic minorities in Guizhou province, the title compound was isolated form Carpesium triste Maxim. Its structure was identified by NMR spectra data and compared with the previous reports (Masao & Fumiko, 1975). Herewith we present its molecular structure. The molecule of the title compound contains a three-ring system A/B/C (Fig. 1). Ring A and B are both in chair conformations and there is a trans-junction between ring A (C1-C5/C10) and ring B (C5-C7/C8-C10).
Furthermore, the methyl group at C14 sites is in the opposite orientation with the two hydroxyl groups at C1 and C3 sites.
The furan ring C (C8-C12/O1) is in an envelope-like conformation. Two hydroxy groups contribute to the formation of intra-and inter-molecular O-H···O hydrogen bonds (Table 1). The latter ones link molecules into chain propagated in direction [0 1 0].

Refinement
The hydrogen atoms which bonded with C were placed in calculated positions with C-H = 0.95-1.00Å. The hydroxyl H atoms were located in Fourier difference map. The H atoms in these droups are splitted with s.o.f. = 0.5. The positions of hydroxyl H atoms were refined freely. All H atoms were refined with U iso (H) = 1.2U eq (C, O).  View of the title molecule showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are presented as a small spheres of arbitrary radius. In the both hydroxy groups only one H atom is presented.

Special details
Experimental. Since the skeleton methyl groups in eudesmanolide are biogenic 8a position, we draw the relative stereochemistry of the title eudesmanolide, by reference to the structures of related eudesmanolide in (Masao & Fumiko, 1975) although the absolute configuration could not be reliably determined from anomalous dispersion effects, if Mo radiation is used in experiment. Furthermore, the relative stereochemistry in the title compound was confirmed by NMR data. 13 C NMR (125 MHz, CDCl 3 , δ,p.p.m.): 178.1 (C12), 149.2 (C4), 141.4 (C11), 120.5 (C13), 110.9 (C15), 77.6 (C8), 75.2 (C3), 74.7 (C1), 63.8 (C10), 40.3 (C7), 33.9 (C5), 33.6 (C9), 33.5 (C2), 26.8 (C6), 17.7 (C14). Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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.