Stereoisomerism and Conformations of Pillar[ <i>n</i> ]arenes and Related [1 _n ]Paracyclophanes

Macrocyclic hosts are at the core of artificial molecular recognition and supramolecular assemblies. The key to successful hosts is easy synthesis/functionalization, 3D structures with suitable cavities, and characteristic surfaces inside/outside the macrocycles. While big successes have been limited to a few host families, recent progress in derivatization has enabled a macrocyclic family, pillar[n]arenes, to exhibit drastically modified 3D structures and properties. These results expand the chemistry of pillar[n]arene into that of more general 2,5-disubstituted [1_n]paracyclophanes. This review overviews such unconventional pillar[n]arenes focusing on the 3D structures, which are one of the aforementioned keys to a big success. Various stereoisomerism and conformations of the "broad-sense" pillar[n]arenes are discussed using average torsion angles as description values for continuously inclining π-panels. This makes a sharp contrast to conventional pillar[n]arene chemistry that has limited structural description to an interconvertible pair of discrete (R_p/S_p) conformations.