11,12-Dihydroxy-10,6,8,11,13-icetexapentan-1-one

The title compound [systematic name: 14,15-dihydroxy-7,7-dimethyl-13-(propan-2-yl)tricyclo[9.4.0.03,8]pentadeca-1(11),3(8),9,12,14-pentaen-4-one], C20H24O3, is a new icetexane diterpenoid which was isolated from the roots of Premna obtusifolia (Verbenaceae). The molecule has three fused rings: a cyclohexenone, a central cycloheptene and a benzene ring. The cyclohexenone ring is in an envelope conformation, whereas the cycloheptene ring is in a twisted boat conformation. Intramolecular O—H⋯O hydrogen bonds generate S(5) and S(8) ring motifs. In the crystal, molecules are linked into dimers through O—H⋯O hydrogen bonds. These dimers are arranged in to sheets parallel to the ac plane. C—H⋯O and weak C—H⋯π interactions are also present.


Comment
The extracts of Verbenaceae plants have been found to possess anti-HIV-1 integrase activity (Bunluepuech & Tewtrakul, 2009). Premna obtusifolia (Verbenaceae), a small tree found in the mangrove forests, is one of the Verbenaceae plants. As part of our research on bioactive compounds from medicinal plants, we previouly reported the crystal structures of diterpenoids from the roots of Premna obtusifolia (Verbenaceae) which was collected from Satun province in the southern of Thailand (Asik et al., 2010;Razak et al., 2010). The title icetexane diterpenoid (I), also named as Obtusin N, is a new compound which was isolated from the same plant. The icetexane diterpenoids encompass a variety of bioactive and structurally interesting compounds (Hymavathi et al., 2009;Simmons & Sarpong, 2009). We herein report the crystal structure of (I).

Experimental
The air-dried roots of premna obtusifolia (4.5 kg) were extracted with hexane (2 x 20 L) at room temperature. The combined extracts were concentrated under reduced pressure to afford a dark yellow extract (40.0 g) which was subjected to quick column chromatography (QCC) over silica gel using solvents of increasing polarity from n-hexane to EtOAc to afford 7 fractions (F1-F7). Fraction F6 was further purified by quick column chromatography (QCC) using n-hexane-ETOAc (9:1), yielding the title compound (87.3 mg). Yellow needle-shaped single crystals of the title compound suitable for x-ray structure determination were recrystallized from n-hexane after several days.

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.