The Two Conformations of Hindered Photochromic 4a,4b-Dihydrophenanthrenes

We analyze conformational stereoisomerism in the sterically hindered 4a,4b-dihydrophenanthrenes of C2 symmetry. In these labile photointermediates, steric hindrance brought about by disubstitution at the 4 and 5 positions or by benzoannelation at the [c] and [g] bonds allows observation of two interconvertible intermediates denoted as L and S. In such molecules, theoretical analysis suggests two low-energy structures (C and T). These differ in the chirality of the polyene perimeter helix for one given configuration of the 4a,4b-ethane unit. C and T can be distinguished by the position of atom pair H4a, X4, or of the equivalent atom pair H4b, X5 with respect to the the mean molecular plane. In C, atoms H4a and X4 are on one side of the mean molecular plane, while in T atoms H4a and X4 are on opposite sides of this plane. The same holds for the atom pair H4b, X5. The C conformation is assigned to the primary photocyclization product (L modification) in which the intersubstituent distance X4−X5 is short and ...