cis-9,10-Bis(bromomethyl)-1,4,5,8-tetraoxadecalin

The title compound, C8H12Br2O4, is a bicyclic ketal in which the two six-membered rings are cis to one another and assume a double-chair conformation. A crystallographic twofold axis bisects the molecule.

The title compound, C 8 H 12 Br 2 O 4 , is a bicyclic ketal in which the two six-membered rings are cis to one another and assume a double-chair conformation. A crystallographic twofold axis bisects the molecule.

Related literature
A determination of this structure has been previously attempted (Fuchs et al., 1972), but the authors did not publish or deposit any atomic coordinates. The same paper gives detailed information for a related structure which has H atoms in place of the bromomethyl groups of the title compound. A search of the Cambridge Structural Database [Version 5.29; (Allen, 2002); CONQUEST (Bruno et al., 2002)] did not yield any other closely related structures. See also : Fuchs (1970 Data collection: CAD-4 EXPRESS (Enraf-Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: DIRDIF (Beurskens et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Comment
The title compound is a bicyclic ketal in which the two six-membered rings are cis to one another and assume a doublechair conformation. A crystallographic twofold axis relates one of the fused rings to the other.
A determination of this structure has been previously attempted (Fuchs et al., 1972), but the authors did not publish or deposit any atomic coordinates. They halted their study after three cycles of isotropic refinement because "···the accuracy of the atomic parameters was relatively low." They did present the Fourier map resulting from the isotropic refinement, and it shows an overall structure in agreement with that presented here. The space group and unit-cell parameters which they obtained are also in agreement with those of the present study.
The authors of this prior study attributed the "relatively low" accuracy of their atomic parameters to the domination of the scattering by the Br atoms. Our present study suggests that the importance of absorption corrections probably played a part as well.
The same paper (Fuchs et al., 1972) gives detailed structural information for a related compound which has hydrogen atoms in place of the bromomethyl groups of the title compound. That structure is similar to the one reported here, in that the two six-membered rings are cis to one another and assume a doublechair conformation.

Experimental
The title compound 3 has been described as the major reaction product in the acid-catalyzed reaction of diketone 1 with ethylene glycol (Fuchs, 1970). Since the dispiro derivative 2 was needed for one of our projects, literature procedures (Fuchs et al., 1972) were followed to yield a product mixture from which only the bicyclic title compound 3 could be isolated, as colorless crystals. The NMR data obtained for 3 deviated from the published data (Fuchs, 1970), and the X-ray structure was therefore determined to ultimately confirm the formation of 3.

Refinement
All H atoms were constrained using a riding model. C -H bond lengths were fixed at 0.97 Å, with U iso (H) = 1.2 U eq. (C).