Arene Synthesis by Extrusion Reaction. Part 16. Coplanar and Stable Derivatives of 13,14-Didehydro-tribenzo(a,c,e)cyclooctene: Synthesis of 5,6-Didehydro-1,1,14,14-tetramethyl-10,11-methano-1H-benzo(5,6) cycloocta(1,2,3,4-def)fluorene