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 la, the prestrained C-9-C-10 bond in the dihydrophenanthrene (DHP) skeleton of le le is expanded from 1.634(2) to 1.649(3)-1.652(6)angstrom, 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 C-4/C-5 positions to cause the additional strain in the C-9-C-10 bond.