(1RS,4SR)-3-Dichloromethylene-1,4-dimethyl-2-oxabicyclo[2.2.2]oct-5-ene

X-ray crystallography was used to confirm the structure of the enantio-enriched title compound, C10H12Cl2O, a bicylic enol ether. A bridged boat-like structure is adopted and the dichloromethylene C atom is positioned significantly removed from the core bicyclic unit. In the crystal structure, molecules pack to form sheets approximately perpendicular to the a and c axes.

X-ray crystallography was used to confirm the structure of the enantio-enriched title compound, C 10 H 12 Cl 2 O, a bicylic enol ether. A bridged boat-like structure is adopted and the dichloromethylene C atom is positioned significantly removed from the core bicyclic unit. In the crystal structure, molecules pack to form sheets approximately perpendicular to the a and c axes.

Comment
The reaction between dienes and ketenes to produce cyclobutanones was long considered to be a textbook example of a [2 + 2] cycloaddition that could be understood in terms of a π2 s + π2 a Woodward-Hoffmann formalism. More recently, evidence has been presented for a stepwise hetero-Diels-Alder/Claisen rearrangement pathway (Yamabe et al., 1996) and it was reported that the periselectivity of these cycloadditions is responsive to the nature of the diene (Machiguchi et al., 1999). The situation is, however, more complex and a combined theoretical and experimental study of the reaction of cyclopentadiene with either dichloro-or diphenylketene revealed that both [4 + 2] and [2 + 2] adducts may be produced directly through parallel reaction pathways traversing a bifurcating energy surface (Ussing et al. 2006). Our studies sought to address certain mechanistic aspects of the Claisen rearrangement of bicyclic enol ethers structurally analogous to those produced in diene/ketene [4 + 2] cycloadditions (Robertson & Fowler, 2006); within this study, although crystals were obtained as a racemate, the title compound was prepared in an enantioenriched form in order to determine if access to non-racemic cyclobutanones could be achieved.
The relationship between computed distances of reacting termini and activation energies has been discussed for structurally similar Claisen precursors in the context of the mechanism of chorismate mutase (Khanjin et al., 1999). The molecular stucture ( Fig. 1) shows the dichloromethylene carbon to be significantly removed from the carbon at C5 (3.5523 Å) and yet the title compound can be induced to undergo the Claisen rearrangement under mild thermal conditions to yield (1RS, 6SR)-8,8-dichloro-3,6-dimethylbicyclo[4.2.0]oct-3-en-7-one. Also of note are the sheets of molecules which form approximatedly perpendicular to the a-and c-axes as shown in Fig. 2 and Fig. 3.

Experimental
The title compound was crystallized by concentration of a sample dissolved in petroleum ether.

Refinement
Changes in illuminated volume were kept to a minimum, and were taken into account (Görbitz, 1999) by the multi-scan inter-frame scaling (DENZO/SCALEPACK, Otwinowski & Minor, 1997).
The H atoms were all located in a difference map, but those attached to carbon atoms were repositioned geometrically.