Surveying macrocyclic chemistry: from flexible crown ethers to rigid cyclophanes.

Macrocycles are molecular entities that display a combination of molecular recognition and complexation properties with vital implications for host-guest/supramolecular chemistry. Since the accidental discovery of the crown ethers by Pedersen half a century ago, the chemistry of wholly synthetic macrocycles for structure-specific, highly selective, host-guest complexation has experienced rapid development. While the structural diversity and host-guest chemistry of the original macrocycles are well-known, new derivatives of them are being investigated continuously and reported on today in order to improve their recognition properties as well as to unleash new opportunities in supramolecular chemistry. In this Review, we survey the recent developments of the chemistry of naturally occurring cyclodextrins, along with a variety of synthetic flexible and rigid macrocycles that have drawn their inspiration from Pedersen's ground-breaking discovery of crown ethers in the mid-1960s.