syn-Dispiro[1,3-dioxolane-2,17′-pentacyclo[12.2.1.16,9.02,13.05,10]octadecane-18′,2′′-[1,3]dioxolane]-7′,15′-diene

The title compound, C22H28O4, is composed of a central octadecane ring and two spiro[bicyclo[2.2.1]hept[2]ene-7,2′-[1,3]dioxolane] units. This polycycle has pseudo twofold symmetry and the central cyclooctane ring has a distorted boat configuration.


Comment
The title compound is of interest as a non-chlorinated tether unit for syn-bisquinoxaline molecular tweezers. The non-chlorinated compounds are anticipated to display higher solubility in common organic solvents, thus facilitating the quantitative investigation of host-guest chemistry in solution. The title polycyclic molecule, 3, presented here was obtained by a twofold Diels-Alder reaction of cyclooctadiene and a cyclopentadieneone derivative, 1, followed by subsequent dehalogenation (Fig.   1). Larger molecular frameworks that incorporate scaffold 2a can be found in syn-bisquinoxalines that have previously been investigated for their luminescent properties (Chou et al., 2005) and for their behavior as molecular tweezers . Compound 3 stems from the chlorinated derivative 2a, which was separated from its anti-isomer 2b via repeated recrystallization from diethyl ether, i.e., the ether solution becomes more enriched in syn-isomer 2a. To improve the solubility of any molecular framework that is derived from scaffold 2a, we reduced the latter with sodium metal in ethanol and liquid ammonia to furnish 3 in good yield. The fully dechlorinated compound 3 did indeed show improved solubility in common organic solvents and, upon further functionalization to tweezer scaffolds, is expected to improve overall solubility.
A literature search revealed three related crystal structures. The first (Garcia et al., 1991a) is similar to 3, but has the anti-orientation and an open ketal structure on each of the bridgehead carbon atoms. The second (Tenbusch et al., 2010) is an octachloro derivative with the anti-orientation. The third (Garcia et al., 1991b) is an octachloro syn-structure with an open ketal arrangement on each of the bridgehead carbon atoms; this structure assumes the same distorted boat configuration as does compound 3.

Experimental
The synthesis of the title compound, 3, is described in Fig. 1. A mixture of cycloocatdiene (3 g, 29 mmol) and spiroketal (1) (15 g, 57 mmol) was refluxed in toluene (3 ml) for three hours. The off-white paste was filtered, washed with methylene chloride (75 ml), dried, and washed again with cold methanol (15 ml) to remove traces of the mono-Diels-Alder adduct. The remaining colorless solid (14.5 g, 83%) was composed of a mixture of 2a and 2 b in a 1:4 ratio, respectively. After repeated recrystallization from diethyl ether, the pure syn-isomer 2a was obtained as colorless platelets (3.7 g, 21%).
A solution of 2a (1 g, 1.58 mmol) and THF (20 ml) was added to a mixture of liquid ammonia (250 ml) and ethanol (1.5 ml). Pieces of sodium metal (0.8 g, 29.6 mmol) were slowly added over two hours; the reaction mixture was then stirred for an additional hour. The reaction was quenched with solid ammonium chloride, and the ammonia was allowed to evaporate.

Refinement
The H-atoms were included in calculated positions and constrained using a riding model: C-H = 0.97 Å for methylene, 0.98 Å for methine, and 0.93 Å for olefinic H-atoms, with U iso (H) = 1.2U eq (C).