6β,15β-Diacetoxy-1β,7β,13α-trihydroxy-7α,20-epoxy-ent-kaur-16-ene

The title compound, C24H34O8, a natural ent-kaurane diterpenoid, is composed of four rings with the expected cis and trans junctions. The crystal structure is stabilized by intermolecular O—H⋯O hydrogen bonds. In addition, an intramolecular O—H⋯O hydrogen bond occurs.


Comment
The title compound (I), 6β, 15β-Diacetoxy-1β, 7β, 13α-trihydroxy-7α, 20-epoxy-ent-kaur-16-ene is a new natural entkaurane diterpenoid isolated from the medicinal plant Isodon japonica. The leaves of this plant has been used as an antibacterial, anti-inflammatory, stomachic, and anthelmintic agent in China, Korean and Japan by local people (Jung et al., 1990;Li & Tian, 2001). The structure of compound (I) was postulated from spectroscopic methods. In order to further confirm the structure and conformation of (I), a crystal structure analysis has been undertaken. The X-ray crystallographic analysis of (I) confirms the molecular structure of (I) proposed by spectroscopic methods. Fig.1 shows its conformation: three hydroxyl groups adopt β, β, α-orientations at C1, C7 and C13, two acetoxyl groups adopt β-orientations at C6 and C15 respectively. There is a trans junction between ring A (C1-C5/C10) and ring B (C5-C10); cis junctions are present between ring B and ring C (C8/C9/C11-C14), and ring C and ring D (C8/C13-C16). The bond lengths and angles are within expected aranges (Allen et al., 1987). Conformation of ring can be seen according to the X-ray diffraction pattern (Fig.1). Ring A adopts chair conformation, with an average torsion angles of 51.43 (18) °. Rings B and C adopt boat conformation because of the formation of the oxygen bridge at C-7 and C-20. Ring D shows an evenlope conformation; the flap atom, C14, lies 0.693 Å from the plane defined by atoms C8, C15, C16 and C13. In addition, the six-membered rings O1/C20/C10/C5-C7 and O1/C7-C10/C20 both adopt boat conformations. Compound (I) contains nine chiral centers at C1(R), C5(R), C6(S), C7(S), C8(S), C9(S), C10(S), C13(S) and C15(R). Although the absolute configuration could not be reliably determined from anomalous dispersion effects, the negative optical rotation showed this compound to be in the ent-kaurane seuies as reported in genus Isodon (Sun et al.,2001), rather than in the kaurane series, and so allowed us to assign the correct configuration.

Experimental
The dried and crushed leaves of Isodon japonica (17 kg, collected from Tongbai Prefecture, Henan Province, China) were extracted four times with Me 2 CO/H 2 O (7:3, v/v) at room temperature over a period of six days. The extract was filtered and the solvent was removed under reduced pressure. The residue was then partitioned between water and AcOEt. After removal of the solvent, the AcOEt residue was separated by repeated silica gel (200-300 mesh) column chromatography and recrystallization from CHCl 3 /CH 3 OH(10:1), giving 70 mg of compound (I) (m.p. 505-507 K. Optical rotation: [α] D 20 -79.6 ° (c 0.45, CH 3 OH). Single crystals suitable for X-ray diffraction were obtained by slow evaporation of a solution of the title compound in methanol at room temperature.
supplementary materials sup-2 Refinement All the Friedel pairs were merged. All H atoms were included in calculated positions and refined as riding atoms, with C-H = 0.98Å (CH 3 ), 0.99Å (CH 2 ), 1.00Å (CH), and O-H = 0.89Å, and with U iso (H) = 1.2 U eq (C). The choice of enantiomer was based on comparison of the optical rotation with that of related compounds with known stereochemistry. Fig. 1. The molecular structure of the title compound with the atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are presented as a small spheres of arbitrary radius.

Special details
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.
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 > 2σ(F 2 ) is used only for calculating Rfactors(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.