A Key Fragment in Carbon Schwarzite Unit Cells and Its Triple [6]Helicene Precursor

This study explores two structurally related π-skeletons. The π-skeleton of compounds 1a-e containing three heptagons represents a key fragment in theoretical carbon schwarzites, while that of 2a-b is a triple [6]helicene. Compounds 1a-e were synthesized via Scholl reactions, and using a weaker acid allowed the reaction to stop at an intermediate stage, yielding 2a-b. X-ray crystallography revealed not only distinct stereochemistry of 1b and 2a but also unique supramolecular assemblies in the clathrate of 2a with chloroform. Compound 1b adopts a saddle-like geometry, while 2a exhibits a propeller-like structure with C₃ symmetry, consistent with density functional theory (DFT) calculations. The π-skeleton of 1a-e is flexible, enabling rapid enantiomerization, whereas that of 2a-b is rigid, allowing resolution of 2b into optically pure forms with an absorption dissymmetry factor as high as 0.015. Comparative analysis shows that presence of seven-membered rings in 1a-e does not significantly alter the local aromaticity of the triple [6]helicene framework.