ChemInform Abstract: Octamethylcuneane and Octamethylcubane: The First X-Ray Crystal Structure of a Cuneane

In the crystal of octamethylcuneane (1,2,3,4,5,6,7,8-octamethylpentacyclo[3.3.0.0 2,4 .0 3,7 .0 6,8 ]octane), C 16 H 24 , there are two independent molecules in the asymmetric unit. Octamethylcubane(1,2,3,4,5,6,7,8-octamethylpentacyclo[4.2.0.0 2,5 .0 3,8 .0 4,7 ]octane), C 16 H 24 , exhibits crystallographic 3m symmetry. Due to the eclipsed orientation of the methyl groups in octamethylcubane, the average bond length is extended to 1.563 (3) A compared with cubane itself. In the cuneane derivative, the cage bond lengths vary between 1.517(2) and 1.573(2)A in accordance with their bicyclic character and the repulsive forces between the methyl groups. The exocyclic bonds to the methyl C atoms in both molecules are shortened due to the increased s character of the cage C atoms.