Cucurbituril cucurbit uril

Cucurbituril (cucurbit[6]uril, or CB[6]) is a hexameric macrocyclic compoxmd self-assembled from an acid-catalyzed condensation reaction of glycoluril and formaldehyde (Scheme 1). Although its synthesis was first reported by Behrend and coworkers in 1905j its chemical nature and stmcture were unknown until they were revealed by Mock and coworkers in 198iP CB[6] has a cavity of 5.5 A diameter, accessible from the exterior by two carbonyl-laced portals of 4 A diameter. Although the size of the cavity is similar to that of a-cyclodextrin ( < highly symmetrical structure... 

Cucurbituril (cucurbit[6]uril, or CB[6]) is a hexameric macrocyclic compound prepared from an acid-catalyzed... 

Fig. 9. Cucurbituril inclusion chemistry (a) tridimensional stmcture of cucurbit[6]uril (b) conjectured cross-sectional representation of a host—guest...

One of the major applications of glycolurils is in the preparation of the cucurbituril family of molecular receptors. Despite being known for a number of years the popularity of this molecule has grown significantly in the last decade. The most common member of the cucurbituril family is cucurbit[6]uril containing six glycoluril units linked by methylene units 149 (Figure 6). 

Cucurbit [6] uril and increasingly its higher homologues have been used extensively in a variety of host-guest chemistry and nanostructure assemblies. Of particular appeal are rotaxanes and molecular necklaces, mechanically interlocked assemblies of molecules based on CB [6]. Efficient synthesis of ID, 2D and 3D polyrotaxanes and molecular necklaces (cucurbituril beads linked by a macrocyclic molecule string ) has been achieved by a combination of self-assembly and coordination chemistry. We discuss rotaxanes and molecular necklaces in Section 10.7, and cucurbil-based systems are summarised in a recent review.23... 

Looking more at host-guest type systems, the cucurbiturils are often very efficient catalysts in cases where they simultaneously bind two guests, thus increasing effective concentration and changing a bimolecular reaction into a guas/ -unimolecular one. To take one recent example, cucurbit[6]uril (Section 6.2.4) proves to be a very efficient catalyst for the click 1,3-dipolar cycloaddition reaction of an azide with an alkyne, particularly where the substrates are suitable guests for the cavity, as in Scheme 12.20. Cucurbituril binds very effectively to ammonium ions and addition of a small amount... 

Although cucurbit[5]uril (CB[5J) became available only recently, along with other CB homologues. its derivative decamethylcucurbit[5]uril (Me-CB[5]) " was known since 1992, and its chemistry was studied (see Cucurbituril Derivatives). Because CB[5] and Me-CB[5] have essentially identical cavity structures, their chemistry is also expected to be the same. 

Curcubiturils are another class of host molecules that have been assembled into bilayer vesicles in water. Kim and coworkers synthesized an amphiphilic cucurbit[6]uril, which forms vesicles and forms host-guest complexes at the vesicle surface. It is possible to decorate the surface of the host vesicles with guest molecules. Exposure of cucurbituril vesicles to a fluorescent spermidine derivative led to fluorescent vesicles. Exposure of the cucurbituril vesicles to a-mannose-substituted spermidine led to vesicles coated with a-mannose, which binds specifically to the lectin concanavalin A. Concanavalin A does not bind when the vesicles are coated with a galactose spermidine... 

Cucurbit[n]urils (n = 1 and 8) (38) have also been used for the molecular recognition of zwitterionic species in aqueous medium (Figure 16). These systems show a hydrophobic cavity with the urea-type carbonyls pointing to the corresponding entrances. This peculiarity implies binding properties with some resemblance to other cavitands (CDs, calixarenes, or resorcinarenes). The size of the cucurbituril receptor can be controlled by the number of monomers present in the structure, and they are usually named as CBn (with n being the number of repeating units). 

The mode of binding in cucurbiturils and their ability to act as pseudorotaxanes is exemplified by the structure shown in Figure 5, showing the host-guest complex between cucurbit[6]uril and apara-xylenediammonium ion. 

Jeon WS, Moon K, Park SH, Chun H, Ko YH, Lee JY, Lee ES, Samai S, Selvapalam N, Rekharsky MK, Sindelar V, Sobrasingh S, Inoue Y, Kaifer AE, Kim K. Complexation of ferrocene derivatives by cucurbit[7]uril host a comparative study of the cucurbituril and cyclodextrin host famiUes. J Am Chem Soc 2005 127 12984-9. 

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