Geometrical Remote Steric Effects in 4,5-Disubstituted-9,10-dihydrophenanthrenes: Expansion of Prestrained C9–C10 Bond in Di(spiroacridan) Derivatives

By attaching bulky substituents at the bay region of di(spiroacridan)-substituted dihydrophenanthrene 1a, the prestrained C9–C10 bond in the dihydrophenanthrene (DHP) skeleton of 1c–1e is expanded from 1.634(2) to 1.649(3)–1.652(6) Å, which demonstrates the bond expansion through remote steric effects. Twisting deformation of the DHP skeleton is the key to transfer the steric bulkiness from the C4/C5 positions to cause the additional strain in the C9–C10 bond.