(1S,3S,8R,9S,11R)-10,10-Dichloro-3,7,7,11-tetramethyltetracyclo[6.5.0.01,3.09,11]tridecane

The title compound, C17H26Cl2, was synthesized from β-himachalene (3,5,5,9-tetramethyl-2,4a,5,6,7,8-hexahydro-1H-benzocycloheptene), which was isolated from the essential oil of the Atlas cedar (Cedrus Atlantica). The asymmetric unit contains two independent molecules with similar conformations. Each molecule is built up from fused six- and seven-membered rings and two three-membered rings from the reaction of β-himachalene with dichlorocarbene. In both molecules, the six-membered ring has a half-chair conformation, whereas the seven-membered ring displays a boat conformation. The absolute configuration was established from anomalous dispersion effects.

The title compound, C 17 H 26 Cl 2 , was synthesized fromhimachalene (3,5,5,4a,5,6,7,, which was isolated from the essential oil of the Atlas cedar (Cedrus Atlantica). The asymmetric unit contains two independent molecules with similar conformations. Each molecule is built up from fused six-and sevenmembered rings and two three-membered rings from the reaction of -himachalene with dichlorocarbene. In both molecules, the six-membered ring has a half-chair conformation, whereas the seven-membered ring displays a boat conformation. The absolute configuration was established from anomalous dispersion effects.  tetracyclo-[6.5.0.0 1,3 .0 9,11 ]tridecane.

Experimental
In a three-necked flask equipped with a dropping funnel, a condenser and a magnetic stirrer, maintained at 0°C, 2 g of ( 1S,3R,8R)-2,2-dichloro-3,7,7,10 tetramethyltricyclo[6.4.0.0 1 , 3 ]dodec-9-ene (El Jamili et al., 2002) were introduced in 50 ml of ether. Thereafter and simultaneously 7 g of sodium were added by small portions during one hour, and dropwise 65 ml of a methanol solution 2.5% of water. The reaction mixture was stirred for 12 h. After hydrolysis with 20 ml of water, the two phases were separated and the aqueous phase extracted three times with 20 ml of ether. The organic phases were combined and dried over sodium sulfate and concentrated. The residue obtained was chromatographed on a silica column with hexane as eluent to give the sesquiterpene hydrocarbon (1S,3S,8R,9S,11R)-3,7,7,11-tetramethyltricyclo-[6.5.0. 0 1,3]tridec-9-ene) with a yield of 90%. The treatment of this sesquiterpene with two equivalents of N-bromosuccinimide (NBS) (Auhmani et al., 2002) gave the title compound with a yield of 80%. Crystals of the title compound suitable for X-ray analysis were recrystallized from pentane.

Refinement
All H atoms were fixed geometrically and treated as riding, with C-H = 0.96-0.98 Å and with U iso (H) = 1.2U eq (C) or U iso (H) = 1.5U eq (C) for methyl H atoms.

Figure 1
The molecular structure of the title compound with displacement ellipsoids drawn at the 30% probability level. H atoms are represented as small spheres of arbitrary radii.

Data collection
Bruker APEXII CCD diffractometer Radiation source: fine-focus sealed tube Graphite monochromator ω and φ scans 10017 measured reflections 5832 independent reflections 5438 reflections with I > 2σ(I)  (6) Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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.