Strain-Sequenced Synthesis of a Quadruply Twisted Cycloarene With Alternating [4]/[5]Helicenes

Topologically nontrivial molecular nanocarbons, such as cycloarenes with multiple helical twists, represent attractive targets that anticipate the cavity-defined properties of classical cycloarenes with the topological complexity of strained helicene assemblies, which remain largely unexplored. Herein we report a strain-sequenced synthesis of a quadruply twisted cycloarene consisting of alternating [4]/[5]helicenes along the π-conjugated framework. The synthesis involved a rational macrocyclization of 1,1-diaryl-2,2-diethynylethylene linkers with anthryl spacers, and an InCl₃-catalyzed, eightfold alkyne benzannulation. Mechanistic studies reveal that the high intrinsic energy of the alkyne-containing precursor drives a key C─C σ-bond rearrangement through a spiro-five-membered-ring intermediate to deliver An4 with quadruple helical twists. X-ray crystallography confirms the parallelogram geometry, featuring a well-defined central cavity with distinct 'Cove' and 'Fjord' regions. Bond-length analysis, complemented by theoretical calculations, revealed that both the modeling compound AnM and An4 undergo adaptive π-electron redistribution to maximize the number of aromatic sextets, resulting in stable and locally aromatic ground states consistent with Clar's rule. The electronic structure is dominated by local aromaticity, with negligible global delocalization. This study not only introduces a versatile synthetic strategy for topological nanocarbons with multiple helical twists, but also provides fundamental insights into the interplay between strain, reactivity, and aromaticity in contorted π-systems.