Centrohexacyclic or “ K 5 ” Molecules: Development of a Growing Family of Topologically Nonplanar Organic Compounds

This article gives a comprehensive review of the synthetic development of centrohexacyclic organic compounds (“centrohexacyclanes”) during the past decade. With their unique feature of having a molecular constitution that corresponds to the complete graph K 5 , centrohexacyclanes represent the majority of the rare class of topologically nonplanar compounds. Starting from the few previously known organic, element–organic and inorganic K 5 compounds, the syntheses of various organic centrohexacyclanes are discussed, based mainly on three different strategies that have emanated from the chemistry of centropolyindanes: the propellane, fenestrane, and “broken fenestrane” routes. In addition, the first results of some targeted transformations of centrohexaindane, the T d ‐symmetrical hexabenzoannelated centrohexaquinane, are presented.