Cyclodextrins host selectivities

Host-guest inclusion complexes, 262—263 antibiotic hosts, 231—233 cahxarene hosts, 228—231 chiral crown ether hosts, 213—218 cyclic oligosaccharide hosts, 218—222 cyclodextrin host selectivities, 223/ host molecular size, 221 hnear ohgosaccharide hosts, 222—228 ir- TT stacking interactions, 217 proteic hosts, 231 Human 15-hpoxygenase, 52/... 

The geometric (size and shape) complementarity between the cyclodextrin host and the organometallic guest determines a well manifested selectivity in the formation of inclusion complexes and can be used for the separation of ferrocene from dime-thylferrocene (only the latter forms a complex with y5-CD) [471], and of (benzene)-chromium tricarbonyl, (7 -C6H6)Cr(CO)3 from (hexamethylbenzene)-chromium tricarbonyl, ( y -C6Me6)Cr(CO)3 (only the latter forms a complex with y-CD) [471], Cyclodextrin host-guest complexation also affords the resolution of hydroxyethylferrocene enantiomers [489]. 

Amphiphilic macrocyclic host molecules have been investigated for many years. Among others, it is known that amphiphilic crown ethers, " cryptands, calixarenes, cyclodextrins, and curcubitnrils can form bilayer vesicles in aqueous solution. However, the host-guest chemistry of such host vesicles remained largely unexplored for many years. Darcy and Ravoo prepared bilayer vesicles composed entirely of amphiphilic cyclodextrin host molecules. These vesicles have a membrane that displays a high density of embedded host molecules that bind hydrophobic guest molecules such as f-butylbenzyl and adamantane derivatives. The characteristic size-selective inclusion behavior of the cyclodextrins is maintained, even when the host molecules are embedded in a hydrophobic... 

Structurally developed cyclodextrins. Effective procedures for the selective functionalization of peripheral hydroxyl groups on the cyclodextrins have been developed. A motivation for these studies has been to produce suitably functionalized hosts which will induce enhanced reaction rates... 

Metalated container molecules can be viewed as a class of compounds that have one or more active metal coordination sites anchored within or next to a molecular cavity (Fig. 2). A range of host systems is capable of forming such structures. The majority of these compounds represent macrocyclic molecules and steri-cally demanding tripod ligands, as for instance calixarenes (42), cyclodextrins (43,44), and trispyrazolylborates (45-48), respectively. In the following, selected types of metalated container molecules and their properties are briefly discussed and where appropriate the foundation papers from relevant earlier work are included. Porphyrin-based hosts and coordination cages with encapsulated metal complexes have been reviewed previously (49-53) and, therefore, only the most recent examples will be described. Thereafter, our work in this field is reported. 

Quantum yields determinations lead to analogous conclusions, although differences in the ortho-to-para ratio are found probably because of experimental reasons. However, the ortho selectivity seems to be well established. Phenyl acetate irradiated in water gives the following quantum yields of product formation 0.16 (ortho), 0.067 (para), and 0.048 (phenol) in the presence of an excess of (3-cyclodextrin, they change to 0.23 (ortho), 0.053 (para), and 0.27 (phenol) [260]. As can be seen, the ortho product is favored in the hydrophobic microenvironment of the cyclodextrin. Phenol quantum yield is enhanced with respect to the irradiation without cyclodextrin, which has been interpreted in terms of H abstraction from the inner walls of the host oligosaccharide. 

However, for the positional isomers of phthalate (56-58), the response selectivity was different for the two types of membranes. Whereas membranes 1 and 2 showed responses in the order of 56 (ortho) > 57 (meta) > 58 (para), membrane 53 interestingly showed a different response order, i.e., 57 (meta) > 58 (para) > 56 (ortho), a selectivity which is quite different from that expected on the basis of simple electrostatic effects. Such a difference in the selectivity is possibly due to host-guest complexation involving not only electrostatic interactions but also inclusion into the P-cyclodextrin cavity, which is capable of recognizing differences in the steric structures of the guests. 

It should be stressed that there is not alwaysjustice in reseach evaluation. The selective formation of inclusion complexes by cyclodextrins (such as 11) was established by Cramer [6] at least 15 years earlier than that by crown ethers. However, cyclodextrin studies forming an independent branch of host-guest chemistry seem underestimated in spite of their considerably greater practical importance at present than that of other host macrocycles (crown ethers 17, calixarenes 18, etc.). Sometimes they are even totally neglected by discussing inclusion phenomena [7]. 

Some authors based their approach to selective binding of the more lipophilic a-amino acids in water on hydrophobic effects using water-soluble, cavity-containing cyclophanes for the inclusion of only the apolar tail under renouncement of any attractive interaction of the hosts with the zwitterionic head . Kaifer and coworkers made use of the strong affinity of Stoddart s cyclobis(paraquat-p-phenylene) tetracation 33 for electron-rich aromatic substrates to achieve exclusive binding of some aromatic a-amino acids (Trp, Tyr) in acidic aqueous solution [48]. Aoyama et al. reported on selectivities of the calix[4]pyrogallolarene 34 with respect to chain length and t-basicity of aliphatic and aromatic amino acids, respectively [49]. Cyclodextrins are likewise water-soluble and provide a lipophilic interior. Tabushi modified )S-cyclodextrin with a 1-pyrrolidinyl and a carboxyphenyl substituent to counterbalance the... 

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