72 | December 2021 www.facs.website What are pillar[n]arenes? Macrocyclic host molecules have an angstrom-level space because of their cyclic structures. Guest molecules that fit the space can be selectively bound by physical interactions such as hydrogen bonding, ionic, and aromatic interactions. This cavity-size-dependent guest binding is a lock-and-key relationship. Such selective guest binding enables their use in medical and material applications. The production of macrocyclic host molecules with new structures and properties has opened up new fields of chemistry with global participation. The history of macrocyclic host molecules is long and started in the 1880s.1 Figure 1 shows some widely used macrocyclic host molecules. Figure 1 Structures of macrocyclic hosts of (a) pillar[n] arene, (b) calix[n]arene, (c) cyclodextrin, (d) crown ether, (e) cucurbit[n]uril and (f) number of reports concerning pillar[n]arenes from 2008 to 2020. During their long history, the development of macrocyclic host molecules, which are now used by many chemists, was limited until recently. To enable their widespread use, macrocyclic host molecules must be easy-to-synthesize or commercially available, have unique host–guest properties, highly symmetric structures, and versatile functionalities, and endow the host–guest products with original properties that come from the structures and chemical compositions of the macrocyclic hosts. Cyclodextrins (Figure 1c), which are composed of sugar units, are typically obtained from natural products and have the longest history among macrocyclic host molecules. Their structures were first discovered by Villiers in 1891.2 Despite their long history, cyclodextrins have been mainly used by chemists until now. Crown ethers were first synthesized by Pedersen in 1967 (Figure 1d).3 They were the first macrocyclic compounds to be synthesized by chemists, therefore synthetic macrocyclic chemistry began with Pedersen’s work. To honor the greatness of his achievement, Pedersen received the Novel Prize for Chemistry at 1987. In the 1980s, calix[n] arenes were popularized by Gutsche (Figure 1b). In these compounds, phenol units are connected by a methylene bridge at the meta-position, therefore they have calixshaped structures, and this is the origin of their name.4 Other calix-shaped meta-bridged phenolic macrocyclic compounds, namely calix[n]resorcinarenes, have also been developed.5 Bowl-shaped macrocyclic hosts, which are formed from ortho-methylene-bridged phenolic units, i.e., cyclotriveratrylenes, have also been produced.6 Cucurbit[n] urils are unique pumpkin-shaped macrocyclic host molecules (Figure 1e).7-9 In 1981, Mock and coworkers identified a hexamer, i.e., cucurbit[6]uril, by X-ray crystallographic analysis.7 However, the chemistry of cucurbit[6]uril was not expanded after this first discovery because of its low solubilities in common solvents. In 2000, 19 years after this first Pillar-Shaped Ma Pillar[n]arenes: From Simple Receptors to Tomoki Ogoshi Prof. Ogoshi is a Professor in the Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering at Kyoto University and Specially Appointed Professor, WPI Nano Life Science Institute, Kanazawa University. He received his Ph.D. degree (2005) under Prof. Yoshiki Chujo at Kyoto University. He worked as a JSPS postdoctoral research fellow (2005–2006) at Osaka University (Prof. Akira Harada). He joined Kanazawa University, where he was promoted to assistant professor in 2006, associate professor in 2010, and full professor in 2015. In 2019, he moved to Kyoto University. He has received the Chemical Society of Japan Award for Young Chemists (2012), the Cram Lehn Pedersen Prize in Supramolecular Chemistry; Royal Society of Chemistry (2013), a Commendation for Science and Technology by the Ministry of Education, Culture, Sports, Science and Technology (2014), the Nozoe Memorial Award for Young Organic Chemists (2016), Lectureship Award MBLA 2016 (2017), Kao Academic Award (2019), and JSPS Prize (2020). He is the inventor of pillar[n] arenes and his current research interests focus on developing novel functional materials and systems based on pillar[n]arenes.
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