Cyclic tris-[5]helicenes with single and triple twisted Möbius topologies and Möbius aromaticity.

A number of singly (180 degrees) twisted, largely single- stranded and thus conformationally rather fragile, Mobius molecules have been synthesized within the last 15 years, which are aromatic with 4n electrons, thus violating the Huckel rule. Annulenes with significantly higher twist (e.g. 540 degrees) that retain a full cyclic conjugation path have been elusive, mainly because of the high strain and loss of orbital overlap. Recently, a topological strategy was devised to project the "twist" into "writhe", thus reducing the strain. However, orbital overlap was still severely reduced within the flexible building blocks. We now present a single and a triple twisted annulene with fully conjugated peripheries. They are unique in their pronounced band shape and conformational robustness as they are made up of three fully katacondensed [5] helicene fragments. The triple twisted molecule exhibits a strong diatropic ring current in the outer periphery, even though the pi system includes 4n electrons. The diatropic current is counterbalanced by a paratropic current in the sigma system, resulting in no net manifestation of macrocyclic aromaticity. The key step of the synthesis of both Mobius compounds is a Perkin condensation of complementary bifunctional bismaleates leading to a flexible macrocycle containing alternating benzene and biphenyl fragments. Subsequent photocyclization yields a separable mixture of rigid diastereomeric tris- helicene macrocycles of the above topologies.