Conformational studies on the bicyclo[3.3.2]decane system

The spectral properties of a series of bicyclo[3.3.2]decane derivatives have been examined, and the occurrence, in certain cases, of abnormal δ(CH) and ν(CH) i.r. absorptions attributed to a C-3, C-7transannular methylene interaction in a twin-chair conformation. There is some evidence for a twin-chair ⇌ boat-chair equilibrium, and data from reduction of the 3-one by dissolving metals reveals essentially no free energy difference between these conformers. In cases where the two-carbon bridge is constrained by a double bond, or its equivalent, such that C-1, C-9, C-10, and C-5 are coplanar, the preferred conformation is boat-chair as confirmed by an X-ray crystal analysis of 7,8,9,10-tetrahydro-6,10-propano-6H-cyclohepta[b]quinoxaline. When this ring system carries an ethano-bridge, the question of whether there is an eclipsed or staggered arrangement around C-9, C-10 is discussed in the light of u.v. and n.m.r. studies and the corresponding situation in homoadamantane, where an X-ray structure of the 1,8-bismethoxycarbonyl derivative has revealed a virtually eclipsed arrangement.