In 1981, the macrocyclic methylene-bridged glycoluril hexamer (CB[6]) was dubbed "cucurbituril" by Mock and co-workers because of its resemblance to the most prominent member of the cucurbitaceae family of plants--the pumpkin. In the intervening years, the fundamental binding properties of CB[6]-high affinity, highly selective, and constrictive binding interactions--have been delineated by the pioneering work of the research groups of Mock, Kim, and Buschmann, and has led to their applications in waste-water remediation, as artificial enzymes, and as molecular switches. More recently, the cucurbit[n]uril family has grown to include homologues (CB[5]-CB[10]), derivatives, congeners, and analogues whose sizes span and exceed the range available with the alpha-, beta-, and gamma-cyclodextrins. Their shapes, solubility, and chemical functionality may now be tailored by synthetic chemistry to play a central role in molecular recognition, self-assembly, and nanotechnology. This Review focuses on the synthesis, recognition properties, and applications of these unique macrocycles.

We determined the values of Ka for a wide range of host-guest complexes of cucurbit[n]uril (CB[n]), where n = 6-8, using 1H NMR competition experiments referenced to absolute binding constants measured by UV/vis titration. We find that the larger homologues--CB[7] and CB[8]--individually maintain the size, shape, and functional group selectivity that typifies the recognition behavior of CB[6]. The cavity of CB[7] is found to effectively host trimethylsilyl groups. Remarkably, the values of Ka for the interaction of CB[7] with adamantane derivatives 22-24 exceeds 10(12) M(-1)! The high levels of selectivity observed for each CB[n] individually is also observed for the CB[n] family collectively. That is, the selectivities of CB[6], CB[7], and CB[8] toward a common guest can be remarkably large. For example, guests 1, 3, and 11 prefer CB[8] relative to CB[7] by factors greater than 10(7), 10(6), and 3000, respectively. Conversely, guests 23 and 24 prefer CB[7] relative to CB[8] by factors greater than 5100 and 990, respectively. The high levels of selectivity observed individually and collectively for the CB[n] family renders them prime components for the preparation of functional biomimetic self-sorting systems.

Im Jahre 1981 tauften Mock und Mitarbeiter das makrocyclische methylenverbrückte Glycoluril‐Hexamer (CB[6]) auf den Namen “Cucurbituril” – da ihre Form einem Kürbis ähnelt, dem bekanntesten Mitglied der Cucurbitaceae‐Familie in der Pflanzenwelt. In den folgenden Jahren wurden die grundlegenden Bindungseigenschaften von CB[6] – die hohe Affinität und Selektivität sowie der Einschluss von Gästen – in bahnbrechenden Arbeiten der Gruppen von Mock, Kim und Buschmann aufgezeigt; es folgten Anwendungen in der Wasseraufarbeitung, künstlichen Enzymen und molekularen Schaltern. Erst vor kurzem wuchs die Cucurbit[n]uril‐Familie um Homologe (CB[5]–CB[10]), Derivate, Verwandte und Analoga, sodass sie nun die Molekülgrößen von α‐, β‐ und γ‐Cyclodextrinen umfasst und übersteigt. Auch die Formen, Löslichkeiten und chemischen Funktionalitäten können inzwischen durch spezielle Synthesen den Erfordernissen angepasst werden. Durch diese Eigenschaften sind die Cucurbiturile prädestiniert für eine zentrale Rolle in molekularer Erkennung, Selbstorganisation und Nanotechnologie. Dieser Aufsatz stellt Synthesen, Erkennungseigenschaften und Anwendungen dieser einzigartigen Makrocyclen vor.

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The ability of cucurbit [8]uril (CB[8]) to control the folding of diaryl ureas 1 and 2 in water was investigated. Compounds 1 and 2 contain two ureidyl C-N bonds which can each populate two conformational states resulting in a conformational ensemble comprising at least three states. We find that the presence of CB[8] results in the selective population of the (E,E)-2 conformer by the formation of CB[8]·(E,E)-2 complex at CB[8]:2 stoichiometries of 1: < 1; at higher stoichiometries, an unfolding process takes place during the formation of CB[8]·(Z,Z)-2 2 . In contrast, compound 1 forms the 2:2 complex CB[8] 2 ·(Z,Z)-1 2 over a broad range of CB[8]:2 stoichiometries. The absolute stoichiometries of these complexes were established by diffusion ordered spectroscopic methods (DOSY). The folding of 2 into the (E,E)-2 conformer under the formation of CB[8]·(E,E)-2 is responsive to the presence of guests in its environment. For example, the addition of 8 results in the expulsion of (E,E)-2 from the cavity of CB[8] followed by its unfolding to the thermally preferred mixture of (Z,Z)-and (Z,E)-2 conformers. These results suggest that complexation within synthetic molecular containers-just like their natural counterparts the chaperones-may be an efficient route to control the folding behaviour of non-natural oligomers in aqueous solution.

The four-component ensemble comprising CB[6], beta-CD, and guests 1 and 2 forms complexes CB[6].1H+ and beta-CD.2 at pH < 7 but swaps partners forming complexes CB[6].2 and beta-CD.1 at pH > 13. The intermolecular nature of the switching process suggests application as a basis for stimuli-responsive reconfigurable systems.

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