endo-endo-anti Diels-Alder diadduct of hexachlorocyclopentadiene with 1,6-dioxacyclodeca-3,8-diene.

There are two independent molecules of endo-endo-anti-1,7,8,9,10,16,17,18,19,19,20,20-dodecachloro-4,13- dioxapentacyclo[14.2.1.1(7,10).0(2,15).0(6,11)]icosa-8,17-di ene, C18H12Cl12O2, in the unit cell with different conformations. In one, the ten-membered ring adopts a chair-chair conformation, and in the other, it adopts a distorted chair-chair conformation. There are near-zero torsion angles at the ring-fusion bonds in both forms [1.5 (6) and 8.1 (5) degrees in the chair-chair molecule, and 2.9 (5) and 2.5 (5) degrees in the distorted chair-chair molecule]. The torsion angles about the bonds comprising the sides of the chair-chair are 158.5 (3), -160.7 (3), -146.8 (4) and 155.7 (3) degrees, and those of the distorted chair-chair are 148.0 (3), -146.8 (3), 175.7 (3) and -52.3 (4) degrees. The norbornene C=C bonds have lengths of 1.314 (6) and 1.305 (6) A in the chair-chair molecule, and 1.309 (6) and 1.318 (6) A in the other. The four dioxolane-system O-C bonds have average lengths of 1.411 (5) and 1.421 (5) A in the two independent molecules.