Synthetic hydrophobic macrocycle

We also investigated chelate binding by dimers of a synthetic hydrophobic macrocycle, in place ofthecyclodextrins [187]. In the systems examined the chelate effect was weaker than that seen with the cyclodextrin dimers. We also studied the strong binding of cholesterol by some cyclodextrin dimers and a cyclodextrin polymer, and saw that the large sterol could occupy parts of two binding cavities [188]. 

The condensation reactions described above are unique in yet another sense. The conversion of an amine, a basic residue, to a neutral imide occurs with the simultaneous creation of a carboxylic acid nearby. In one synthetic event, an amine acts as the template and is converted into a structure that is the complement of an amine in size, shape and functionality. In this manner the triacid 15 shows high selectivity toward the parent triamine in binding experiments. Complementarity in binding is self-evident. Cyclodextrins for example, provide a hydrophobic inner surface complementary to structures such as benzenes, adamantanes and ferrocenes having appropriate shapes and sizes 12) (cf. 1). Complementary functionality has been harder to arrange in macrocycles the lone pairs of the oxygens of crown ethers and the 7t-surfaces of the cyclo-phanes are relatively inert13). Catalytically useful functionality such as carboxylic acids and their derivatives are available for the first time within these new molecular clefts. 

Recently reported synthetic macrocyclic molecules with hydrophilic cavities containing multiple binding atoms and with hydrophobic exteriors make possible controlled studies of selective cation complexa-tion. Certain of these cation-ligand systems appear to mimic biological systems which have remarkable element specificities. Such cation-ligand systems may be considered as models for the study of this unusual property of living systems. 

We have already reported that synthetic peptide lipids, having ot-amino acid residuefs) interposed between a polar head moiety and a hydrophobic doublechain segment, can be used as models for functional simulation of biomembranes [23]. On this ground, we are to clarify molecular recognition specificity by supramolecular assemblies formed in combination of the macrocyclic receptors with the peptide lipid as artificial cell-surface receptors. 

The earliest recognised examples of synthetic supramolecular structures were the complexes formed from crown ethers and metal cations [19]. Since then numerous macrocycles have been synthesised. Representative examples are the cryptands [20], These differ from crown ethers in that the former contains a tridimensional cavity while the latter are characterised by a hole. Similarly, calix[4]arenes are compounds with a cup -like structure that through lower rim functionalisation gives rise to a hydrophilic and a hydrophobic cavity, thus allowing the reception of ionic species in the former and neutral species in the latter. Most of the above mentioned macrocycles are known for their capability to serve as cation receptors. 

We have also synthesized a catalyst related to 131 in which the cyclodextrin rings were replaced with synthetic macrocyclic binding groups [196], Also, we have examined catalysts related to 131 in which substrate binding involved metal ion coordination, not hydrophobic binding into cyclodextrins or macrocycles [198]. 

Fig. 6 Schematic representation of the synthetic route to obtain constitutional silica mesoporous membranes is (a) filled with mesostructured silica-CTAB, (b) then calcinated, (c) reacted with hydrophobic ODS and finally filled with the hydrophobic carriers. Generation of directional ion-conduction pathways which can be morphologically tuned by alkali salts templating within dynamic hybrid materials by the hydrophobic confinement of ureido-macrocyclic receptors within silica mesopores [130]...

Upper rim substitution is typically more synthetically challenging than the lower rim, but can confer very useful properties on the macrocycle. For example, water soluble / -sulfonatocalixarenes have been synthesized and found to be highly water soluble, unlike the hydrophobic p-t-butyl derivatives. The sulfonated calixarenes form a variety of metal complexes where the metal is usually bound to the sulfonato groups of the calixarene. 

Amino acids may be bound in water by molecules with hydrophobic cavities. Cyclodextrin derivatives were studied extensively. Enantioselectivities are usually modest, with some exceptions. For example, a diphenoxyphos-phoryl-substituted p-cyclodextrin bound serine with 3.6 1 enantioselectivity " and an a-cyclodextrin with a pyr-idinium substituent showed 10 1 enantioselectivity toward the same substrate.Synthetic cyclophanes were also used. The bipyridinium-based macrocycle 36 bound DOPA 37 with 13 R S selectivity in acidic aqueous solution. 

With water as solvent, hydrophobic interactions can be used to drive complex formation. Cyclodextrins were widely used to study hydrophobic binding, and they often show enantioselectivity. However, strong enantiodiscrim-ination is rare, especially for less-functionalized substrates. synthetic water-soluble receptors gave similar results. For example, cyclophane 65 bound the enantiomers of menthol 66 in the ratio 5 4. - while macrocycle 67 bound Naproxen derivatives 68 in D20/Me0H (60 40) with 1.9 ] enantioselectivity. " ... 

The incorporation of hydrophobic subunits such as aryl, alkyl, napthyl, binaphthyl, etc., into a macrocyclic structural framework leads to synthetic host molecules containing large endolipophilic cavities which have the potential of accommodating neutral organic guest species. 

Self-assembly of synthetic facial amphiphile is, however, not restricted to assemblies with low aggregation numbers. For instance, very large and well-defined supramolec-ular assemblies are formed by the rigid-flexible macrocycle constructed from a rigid aromatic oligomer as the hydrophobic face and a flexible oligo (ethylene glycol) as... 

Some synthetic macrocycles can bind hydrophobic groups in water similar to the binding into cyclodextrins. We examined the selective binding of some substrates by dimers of such synthetic macrocycles.We have also examined where catalytic groups should be placed on... 

Another comparison substituted the cyclodextrin-binding group by a synthetic macrocycle that also strongly binds hydrophobic substrates in water solution. We synthesized... 

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