(20S*,24S*)-25-Hydroxy-20,24-epoxy-A-homo-4-oxadammaran-3-one (Chrysura) isolated from the leaves of Walsura chrysogyne

The title dammarane triterpenoid, C30H50O4, assigned the name chrysura, was isolated from an ethyl acetate extract of Walsura chrysogyne leaves (Meliaceae). It has 20S*,24S* relative stereochemistry and an oxepanone ring with two methyl groups at position 4. The two cyclohexane rings adopt chair conformations. The cyclopentane and tetrahydrofuran rings have envelope conformations; their mean planes make a dihedral angle of 13.1 (3)°, indicating that the rings are only slightly tilted with respect to each other. There is an intramolecular C—H⋯O hydrogen bond in the molecule, which forms S(6) and S(7) ring motifs. In the crystal, molecules are linked via O—H⋯O and C—H⋯O hydrogen bonds, forming chains propagating along [001] which stack along the b-axis direction.

The title dammarane triterpenoid, C 30 H 50 O 4 , assigned the name chrysura, was isolated from an ethyl acetate extract of Walsura chrysogyne leaves (Meliaceae). It has 20S*,24S* relative stereochemistry and an oxepanone ring with two methyl groups at position 4. The two cyclohexane rings adopt chair conformations. The cyclopentane and tetrahydrofuran rings have envelope conformations; their mean planes make a dihedral angle of 13.1 (3) , indicating that the rings are only slightly tilted with respect to each other. There is an intramolecular C-HÁ Á ÁO hydrogen bond in the molecule, which forms S(6) and S(7) ring motifs. In the crystal, molecules are linked via O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, forming chains propagating along [001] which stack along the b-axis direction.
The title dammarane triterpenoid, namely chrysura (1), has been isolated for the first time from the ethyl acetate extract of the leaves of Walsura chrysogyne (Meliaceae). Recently, the same compound was reported to have been obtained from Aglaia foveolata, but in resin form (compound 5 in reference Pan et al., 2010). They determined its relative stereochemistry by Nuclear Magnetic Resonance (NMR) spectroscopy. Herein, we describe the crystal structure of the title compound, chrysura (1), whose relative configuration was also obtained by two-dimensional NMR spectroscopy. By a close comparison of the 13 C NMR signals at C-20, C-21, C-22, C-23 and C-24 reported for compound 5 (δ 86.5, 27.2, 34.8, 26.3 and 86.4;Pan et al., 2010) and those obtained for the title compound, chrysura (1) (δ 86.5, 27.2, 35.0, 26.4 and 86.5), it was shown that these two compounds are identical. This is substantiated by the 1 H NMR signal at H-24 of chrysura (1), which is a doublet of doublet with J values of 10 and 5.5 Hz, comparable to the values observed for compound 5, that is 9.9 and 5.6 Hz. Hence, the relative configuration at C20 and C24 of chrysura (1), was determined by NMR to be the same as that of compound 5 [Pan et al., 2010].
The molecular structure of the title molecule, chrysura (1), is shown in Fig. 1. The two cyclohexane rings, B (C5-C10) and C (C8,C9,C11-C14), adopt chair conformations. The cyclopentane ring D (C13-C17) and the tetrahydrofuran ring E (O33,C20, C22-C24) have envelope conformations, with atoms C14 and C23 at the flap of rings D and E, respectively. The mean planes through rings D and E make a dihedral angle of 13.1 (3)°, indicating that they are only slightly twisted with respect to each other. As shown in Fig. 1, the structure of the molecule is stabilized by an intramolecular C-H···O hydrogen bond (Table 1), which forms S(6) and S(7) ring motifs (Bernstein et al., 1995).
In the crystal of chrysura (1), molecules are linked via intermolecular O-H···O and C-H···O hydrogen bonds (Table   1), forming chains propagating along [001]. These chains stack along the b-axis, as shown in Fig. 2.
Hence, in the title compound, chrysura (1), the relative configurations at C20 and C24 of the epoxy unit (ring E) have been confirmed to be S-methyl configurations.

Refinement
All the H atoms were positioned geometrically and refined using a riding model: O-H = 0.82 Å and C-H = 0.93 -0.98 Å with U iso~( H) = 1.5U eq (O, C methyl ), and = 1.2U eq (C) for all other C-bound H atoms. A rotating-group model was applied for the methyl groups. The anomalous dispersion effects of the atoms in the molecule are not sufficient to determine the absolute structure of the molecule in the crystal [Flack parameter = 0.1 (5)]. Fig. 1. The molecular structure of the title molecule, chrysura (1), showing 50% probability displacement ellipsoids and the atom-numbering scheme. The intramolecular C-H···O hydrogen bond is shown as a dashed red line.  Table 1 for details). H atoms not involved in the hydrogen bonds (dashed lines) have been omitted for clarity.

Special details
Experimental. The needle-shape crystal was placed in the cold stream of an Oxford Cyrosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.