A second polymorph of β-arteether

The crystal structure of the title compound, C17H28O5, reported here is a polymorph of the structure first reported by El-Feraly, Al-Yahya, Orabi, McPhail & McPhail [J. Nat. Prod. (1992). 55, 878–883]. It is a derivative of the antimalaria compound artemisinin and consists primarily of three substituted ring systems fused together. A cyclohexane ring (distorted chair conformation) fused to a tetrahydropyran group (distorted chair) is adjacent to an oxacycloheptane unit containing an endo-peroxide bridge, giving the molecule its particular three-dimensional arrangement. The crystal packing is stabilized by intermolecular C—H⋯O interactions between an O atom from the endo-peroxide bridge and H atoms from both the cyclohexane and seven-membered oxacycloheptane fused rings, as well as between an O atom and H atom from adjacent tetrahydropyran rings. The two polymorphs have the same space group and similar cell parameters for the a and b axes, but significantly different values for the c axis.

The crystal structure of the title compound, C 17 H 28 O 5 , reported here is a polymorph of the structure first reported by El-Feraly, Al-Yahya, Orabi, McPhail & McPhail [J. Nat. Prod. (1992). 55, 878-883]. It is a derivative of the antimalaria compound artemisinin and consists primarily of three substituted ring systems fused together. A cyclohexane ring (distorted chair conformation) fused to a tetrahydropyran group (distorted chair) is adjacent to an oxacycloheptane unit containing an endo-peroxide bridge, giving the molecule its particular three-dimensional arrangement. The crystal packing is stabilized by intermolecular C-HÁ Á ÁO interactions between an O atom from the endo-peroxide bridge and H atoms from both the cyclohexane and seven-membered oxacycloheptane fused rings, as well as between an O atom and H atom from adjacent tetrahydropyran rings. The two polymorphs have the same space group and similar cell parameters for the a and b axes, but significantly different values for the c axis.
RJB acknowledges the Laboratory for the Structure of Matter at the Naval Research Laboratory, Washington DC, USA, for access to their diffractometers. BN thanks Strides Arco Labs, Mangalore, India, for a gift sample of the title compound.

Comment
Artemisinin and its derivatives, dihydroartemisinin, artemether, arteether and artesunate, are antimalarial drugs which possess bioactivity with reduced toxicity (Wu & Li, 1995). Artemisinin is isolated from the leaves of the plant Artemisia annua (Qinghao). It is a sesquiterpene lactone with an endo-peroxide linkage. Artemisinin derivatives are more potent than artemisinin and are active by virtue of the endo-peroxide. Because of their activity against strains of the parasite that had become resistant to conventional chloroquine therapy and the ability, due to the lipophilic structure, to cross the blood brain barrier, it was particularly effective for the deadly cerebral malaria (Shen & Zhuang, 1984). Because of their shorter lifetime and decreasing activity, they are used in combination with other antimalarial drugs. The notable activity of artemesinin derivatives in vitro and in vivo has been reported in the literature (Li et al. 2001& Yang et al. 1997. However, some derivatives of artimisinine showed moderate cytotoxicity in vitro. The electronegativity and bulk of the substituents that are attached to the aryl group play an insignificant role in cytotoxicity. The antimalarial activity and cytotoxicity of some sesquiterpenoids has been reported in the literature (Venugopalan et al., 1995;Wu et al., 2001;Saxena et al., 2003). The endoperoxide group present in these compounds plays an important role in antimalarial activity. Its 1,2,4-trioxane ring is unique in nature. After being opened in the plasmodium it liberates singlet oxygen and forms free radicals, which in turn produce oxidative damage of the parasite's membrane. Artemisinin is hydrophobic in nature and is partitioned into the membrane of the plasmodium. The crystal structure of an ether dimer of deoxydihydroqinghaosu, a potential metabolite of the antimalarial arteether, has been reported (Flippen-Anderson et al., 1989). The correlation of the crystal structures of diastereomeric artemisinin derivatives with their proton NMR spectra in CDCl 3 has been reported (Karle & Lin, 1995). The crystal structure of artemisinin has been reported (Lisgarten et al., 1998). The crystal structure of a dimer of α-and β-dihydroartemisinin (Yue et al., 2006) and that of 9,10-dehydro-deoxyartemisin has recently been reported (Li et al., 2006). The synthesis of artemisinin and its derivatives have been described (Lui et al., 1979;Liu, 1980;Robert et al., 2001). β-Arteether (AE) is an endo-peroxide sesquiterpene lactone derivative currently being developed for the treatment of severe, complicated malaria caused by multidrug-resistant Plasmodium falciparum (Grace et al., 1998). β-Artemether (AM), the O-methyl ether prodrug of dihydroartemisinin (DHA), is an endo-peroxide antimalarial (Maggs et al., 2000). In view of the importance of the title compound, C 17 H 28 O 5 (I), β-arteether, as an antimalarial drug, this paper describes a new polymorphic form of the crystal structure first reported by El-Feraly et al. (1992), from data measured at 103 (2) K.

Experimental
The title compound (C 17 H 28 O 5 ) was obtained in the pure form from Strides Arco Labs, Mangalore, India. X-ray diffraction quality crystals were grown from acetone [m.p.: 353 K]).

Refinement
All H atoms were initially located in a difference Fourier map. The methyl H atoms were then constrained to an ideal geometry with C-H distances of 0.98Å and U iso (H) = 1.5U eq (C), but each group was allowed to rotate freely about its C-C bond. All other H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with C-H distances in the range 0.90-1.00 Å and U iso (H) = 1.17-1.22U eq (C). Fig. 1. The molecular structure of (I), showing the atom numbering scheme and 50% probability displacement ellipsoids.  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.