Two strained hexahelicenophanes

The crystal structures of the [6]helicenes 4,13-(1,10-decamethylenedioxy)he xahelicene, C36H34O2, (I), and 4,13-(1,8-octamethylenedioxy)hexahelicene, C 34H30O2, (II), show strong steric interactions between the terminal benzene rings and the polymethylenedioxy chains. The shortest ring A and F distanc es amount to 2.941 (3) and 2.902 (3) Angstrom, respectively. The increased steric energy of the ground state is responsible for a significantly lower racemization barrier of (I) and (II) in comparison to the unsubstituted [6] helicene.