Electronic absorption and circular dichroism spectra of the perturbed coplanar cis-diene chromophore in deuterium- and methyl-substituted 7,7-dimethylbicyclo[4.1.1]octa-2,4-dienes

The optically active monodeuterated 7,7-dimethylbicyclo[4.1.1]octa-2,4-dienes have been prepared with known absolute configuration from (+)-nopinone. Access to their methyl congeners has been achieved from (+)-α-pinene and (+)-nopinone, respectively. The chirality is due solely to isotopic substitution. The contributions of the C-D and C-CH3 groups to the observed absorption and circular dichroism spectra are analyzed. In particular, attention is directed to the planar cis-1,3-diene unit, the resultant zero dihedral angle between the C2-C3 and C4-C5 bonds at equilibrium, and the consequences of this unique fixed geometry