Redetermination and absolute configuration of 6-hydroxysalvinolone

The crystal structure of the title compound [systematic name: 5,6,10-trihydroxy-7-isopropyl-1,1,4a-trimethyl-2,3,4,4a-tetrahydrophenanthren-9(1H)-one], C20H26O4, has been reported previously [Salae et al. (2009 ▶). Acta Cryst. E65, o2379–o2380], but the absolute configuration could not be determined as there was no significant anomalous dispersion using data collected with Mo radiation. The absolute configuration has now been determined by refinement of the Flack parameter with data collected using Cu radiation. The absolute configuration at position 4a of the diterpenoid is (R)-methyl; other features of the molecule and its crystal packing are similar to those previously described.

The crystal structure of the title compound [systematic name: 5,6,10-trihydroxy-7-isopropyl-1,1,4a-trimethyl-2,3,4,4a-tetra hydrophenanthren-9(1H)-one], C 20 H 26 O 4 , has been reported previously [Salae et al. (2009). Acta Cryst. E65, o2379-o2380], but the absolute configuration could not be determined as there was no significant anomalous dispersion using data collected with Mo radiation. The absolute configuration has now been determined by refinement of the Flack parameter with data collected using Cu radiation. The absolute configuration at position 4a of the diterpenoid is (R)-methyl; other features of the molecule and its crystal packing are similar to those previously described.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: SJ2711).

Comment
The title diterpenoid compound (I) known as 6-hydroxysalvinolone (Topcu & Ulubelen, 1996) or 14-deoxycoleon U (Fraga et al., 2005;Hueso-Rodríguez et al., 1983), was isolated from the roots of Premna obtusifolia, a Thai manglove plant which was collected from Satun province in the Southern part of Thailand. Its crystal structure has been reported (Salae et al., 2009) but the absolute configuration could not be determined due to no large anomalous dispersion using a data set collected with Mo radiation. Data on the same sample was recollected using Cu radiation with our newly-installed Bruker Apex-Duo CCD diffractometer and the absolute configuration at atom C10 (or the 4a position) was determined as (R)-methyl making use of the large anomalous scattering of Cu Kα X-radiation with the Flack parameter being refined to 0.06 (17). We report herein the crystal structure of (I) determined from the Cu data. Fig. 1 shows the molecular structure of (I); the ring conformations, bond lengths and angles are almost identical to those previously described (Salae et al., 2009).

Experimental
The compound was isolated and the crystal grown as reported by Salae et al. (2009).

Refinement
The H atom attached to C15 was placed in a calculated position with d(C-H) = 0.96 Å and the U iso values was constrained to be 1.2U eq of the carrier atom. The remaining H atoms were located from the difference map and isotropically refined.
The highest residual electron density peak is located at 0.73 Å from C13 and the deepest hole is located at 0.50 Å from O1.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems 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.
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 > 2sigma(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.