Crystal structure of a bioactive sesquiterpene isolated from Artemisia reticulata

The sesquiterpene molecule has been isolated from Indian herb A. reticulata by column chromatography over silica gel with a mixture of binary solvent ethyl acetate and hexane by gradient elution. It was recrystallized at room temperature by slow evaporation to afford suitable crystal for X-ray diffraction study. Antiproliferative bioassay of this molecule has been conducted against human ovarian cancer cell line A 2780.


Chemical context
The title compound is a natural product, which has been isolated from the Indian herb A. reticulata by column chromatography over silica gel. A. reticulata (family: Asteraceae) is a traditional herb which has many applications in folklore medicine for conventional therapy against several diseases such as malaria (Klayman et al., 1984;Malagon et al., 1997;Newton & White, 1999), cancer (Efferth et al., 2001;Lai et al., 1995), cardiovascular (Guantai et al., 1999), vasodilatory (Walker, 1996), hepatitis (Aniya et al., 2000) and diabetes (Iriadam et al., 2006). It is found as a constituent in many ayurvedic or herbal drug preparations such as forkolin and Afsanteen in Indian traditional medicinal systems (Nadkarni, 1954;Satyavati et al., 1987;Subramoniam et al., 1996;Drury, 1978). The Artemisia species are a rich source of bioactive sesquiterpenenoids (Klayman et al., 1984) such as artemisinin, artemisin etc. Artimisinin and artemisin are secondary metabolites isolated from herbs of the species A. annua (Klayman, 1985) belonging to the sesquiterpene class. The title molecule possesses antiplasmodial activity and it is now under clinical trial for the treatment of malaria. Our group are currently searching for artemisin, artemisinin or their analogues from other varieties of Artemisia species and as part of these studies, the structure of the title compound is now reported.

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The compound comprises fused cyclohexane and cyclopentane rings. It has been substantiated by a positive LB test (Liebermann Burchard Test), which indicates that it ISSN 2056-9890 belongs to the sesquiterpene class. The compound is soluble in chloroform but has poor solubility in methanol.

Supramolecular features
In the crystal, molecules are linked by O-HÁ Á ÁO hydrogen bonds, forming chains along [010] (Table 1 and Fig. 2). These chains are cross-linked by weak C-HÁ Á ÁO hydrogen bonds.

Database survey
A search of Cambridge Structural Database (CSD, Version 5.36, last update May 2015; Groom & Allen, 2015) found only one molecule, Pulioplopane A (15-hydroxy-10 (14)-oplopen-4-one; Triana et al., 2005) that has a similar structural skeleton to the title sesquitertene although it is is unrelated in a biochemical sense.

Synthesis and crystallization
The title sesquiterpene was isolated as colourless solid from the methanol extract of A. reticulata by chromatography over silica gel with a mixture of ethyl acetate and hexane with a gradient elution followed by preparative thin layer chromatography. Crystals were obtained after recrystallization three times from ethyl acetate:hexane (1:4) at room temperature by the slow evaporation method. Bioassay of this molecule has been conducted against human ovarian cancer cell line A 2780 and revealed that it possessed significant antiproliferative activity (unpublished results).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were placed in calculated positions with C-H = 0.93-0.98 Å and O-H = 0.82 Å and refined in a riding-motion approximation with U iso (U) = 1.2U eq (C,O). No Friedel pairs were collected therefore the absolute configuration could not be determined from the X-ray data and the assignment is arbitrary.

Figure 2
Part of the crystal structure of the title compound, with hydrogen bonds shown as dashed lines.
burg, Virginia 24061, USA, for their kind co-operation to measure diffraction data for the title compound and to carry out an antiproliferative bioassay against cancer cell lines.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'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 &gt; 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.