Pentacyclo[9.3.1.12,6.14,8.19,13]octadeca-1(2),8(9)-diene

The title compound, C18H24, was the main product of thermolysis of noradamantene dimer (heptacyclo[9.3.1.12,6.14,8.19,13.01,9.02,8]octadecane). The crystal structure was determined to prove that the thermolysis product of noradamantene dimer is favored by stretch release due to ring opening of the four-membered ring. The bond length of the quaternary C atoms of the starting material was calculated as 1.6 Å, enlarged in comparison to other single bonds. After the rearrangement, the stretch release of the above carbons leads to an increase of the distance between them (2.824 Å) with respect to the crystallographic data.

The title compound, C 18 H 24 , was the main product of thermolysis of noradamantene dimer (heptacyclo-[9.3.1.1 2,6 .1 4,8 .1 9,13 .0 1,9 .0 2,8 ]octadecane). The crystal structure was determined to prove that the thermolysis product of noradamantene dimer is favored by stretch release due to ring opening of the four-membered ring. The bond length of the quaternary C atoms of the starting material was calculated as 1.6 Å , enlarged in comparison to other single bonds. After the rearrangement, the stretch release of the above carbons leads to an increase of the distance between them (2.824 Å ) with respect to the crystallographic data.

Related literature
For reviews on noradamantene and analogous pyramidalized alkenes, see: Borden (1989Borden ( , 1996; Vá zquez & Camps (2005). For the syntheses of noradamantene dimer, see: Renzoni et al. (1986) and for related analogs, see: Camps et al. (1996a,b). For the synthesis of the precursor diiodide (3,7-diiodo-tricyclo-[3.3.1.0 3,7 ]nonane), an important intermediate in the synthetic route towards the generation of noradamantene, see: Ioannou & Nicolaides (2009 Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis CCD; data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999) and Mercury (Macrae et al., 2006); software used to prepare material for publication: WinGX (Farrugia, 1999) and publCIF (Westrip (2010). We are grateful to the Research Promotion Foundation (IÅE) of Cyprus and the European Structural Funds for grant ANABAÂ/ÅAÀIO/0308/12 which allowed the purchase of the XRD instrument, NEKÅ/0308/03 enabling the purchase of a 500 MHz NMR spectrometer, of the RSC journal archive and for access to Reaxys and financial support to SI (ÅENEK/ ENIAEX/0308/01). Partial financial support (SI) was also provided by the SRP "Interesting Divalent Carbon Compounds" granted by UCY. The A. G. Leventis Foundation is gratefully acknowledged for a generous donation to the University of Cyprus enabling the purchase of the 300 MHz NMR spectrometer. Dr Athanassios Nicolaides and Dr Anastasios Tasiopoulosor are thanked for their illuminating comments. category (figure 3) and it can serve as a building block for the formation of larger polycyclic hydrocarbons like the title compound. The most pyramidallized alkene (n=0) of the same homologous series is rearranged spontaneously to the corresponding diene once the dimer is formed (Camps et al. 1996a,b) (figure 3). This is attributed to its grater stretch due to the smaller carbon side chain. The title compound is the main product of thermolysis of noradamantene dimer and its formation depends on the reaction conditions. At different reaction conditions (higher temperatures, reaction time) [2]diadamantane (McKervey 1980, Graham et al. 1973) and another asymmetric diene were identified among the products.

Experimental
Synthesis of pentacyclo [9.3.1.1 2,6 .1 4,8 .1 9,13 ] octadeca-di-1(2),8(9)-ene. Heptacyclo [9.3.1.1 2,6 .1 4,8 .1 9,13 .0 1,9 .0 2,8 ] octadecane-(10 mg,0.042 mmol) was placed in a cylindrical glass container with small diameter (~5 mm suitable for glass workshops) sealed at the bottom edge, while the other edge was connected at the vacuum line. The glass cylinder was washed three times with argon and placed under vacuum for 5 minutes after which the opened edge was sealed as well with the use of a flamethrower, encapsulating the reactant under vacuum. The capsule was placed in a controlled temperature oven at 350 o C for 5 minutes. Crystals of the product and the reactant were formed when the capsule cooled down to room temperature. The starting material was removed by breaking carefully the glass of the one edge and washing the solid with hexane 3x1 ml. The residue was mostly product which was recrystallized by sealing the capsule again under vacuum and reheating it at 350°C for another 5 minutes. Colorless crystals of pure product were formed when the capsule cooled down to room temperature.

Refinement
The H atoms are positioned with idealized geometry and refined using a riding model with U iso (H) = 1.2 of U eq (C).

Figure 4
The formation of the title compound. 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 > 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.