Macropolycyclic receptors

The free macropolycyclic receptor (SS)-208 consisting of two l,l -tetralyl units bridged by four polyether chains between their 2 and 3 positions (211). 

Cryptands (16) and (17) are macropolycyclic receptor molecules which provide a cavity for inclusion of a variety of substrates. Cryptate refers to the complexes. 

Canceill, J., Collet, A., Gabard, J., Kotzybahibert, F., Lehn, J. M., Speleands -macropolycyclic receptor cages based on binding and shaping subunits - synthesis and properties of macrocycle-cyclotriveratrylene combinations. Helv. Chim. Acta... 

Lehn, J. M. (1978), Cryptates Inclusion Complexes of Macropolycyclic Receptor Molecules, Pure Appl. Chem. 50, 871-892. 

Lehn J-M (1978) Cryptates inclusion complexes of macropolycyclic receptor molecules. Pure Appl Chem 50 871-892... 

Undoubtly, cryptands remain the class of macrobicyclic and macropolycyclic receptors looking forward to a great future. The number of possible structures is practically unlimited and depends on a researcher imagination and creativity. Initially, the chemistry of cryptands had focused on the development of new synthetic strategies, either to improve the yields or to built up more complicated molecular architectures, like cylindrical, spherical or lateral macropolycycles, bi- or polynucleating, with hard and soft binding sites, etc. 

Macropolycyclic receptors (cryptands) possess a tridimensional cavity in which guest species can be lodged. By modulating the molecular topology of the host molecule, /. e., size and shape of the cavity, number and type of donors and flexibility of the structure, the coordination of many different substrates (metal cations, ammonium ions, anionic species or neutral molecules) can be achieved. 

The hrst opticaUy active macrobicycles and macropolycycles were repotted by Lehn and co-woricers (187) in 1974. Employing the synthetic methodology developed in Strasbourg for the preparation of cryptands, chiral molecular receptors, such as (5)-191 and (5)-192, have been isolated and characterized. 

Fascinating macrocyclic receptor cages based on a triaza-18-crown-6 derivative (79) and on a rigid cyclotriveratiylene unit (188) can be synthesized (189) by cormecting the two residues via three bridges in a single step. Macropolycycles, such as ( )-193 have been reported (189), and since the cyclotrivera-trylene unit is chiral and has been resolved (188), the preparation of optically active macrocyclic receptor cage molecules should be feasible (see below). 

Larger anionic species such as sulfate, carboxylate and phosphate necessitate the preparation of larger macropolycycles as receptors. 

By combining a receptor unit with a rigid shaping unit, hollow macropolycyclic speleands (Figure 3 20) have been synthesized. NMR experiments indicate that (20) forms both external and internal complexes with methylammonium cation.38... 

Cryptands 7-9 thus function as receptors for spherical cations. Their special com-plexation properties result from their macropolycyclic nature and define a cryptate effect characterized by high stability and selectivity, slow exchange rates, and efficient shielding of the bound ion from the environment [2.17,2.27]. 

Varying the side groups X in 27b affects both the stability and selectivity of the complexes (lateral discrimination), and allows the receptor-substrate interactions in biological systems to be modelled, for instance, the interaction between nicotinamide and tryptophan [2.109b]. One may attach to 27b amino acid residues (leading to parallel peptides [2.109] as in 27c), nucleic acid bases or nucleosides, saccharides, etc. The structural features of 27 and its remarkable binding properties make it an attractive unit for the construction of macropolycyclic multisite receptors, molecular catalysts, and carriers for membrane transport. Such extensions require sepa-... 

Polyammonium macrocycles and macropolycycles have been studied most extensively as anion receptor molecules. They bind a variety of anionic species (inorganic anions, carboxylates, phosphates, etc.) with stabilities and selectivities resulting from both electrostatic and structural effects. 

The complexation of various molecular anions by other types of macrocyclic ligands has been reported [3.1-3.4] in particular with cyclophane-type compounds. Two such receptors are represented by the protonated forms of the macropolycycles 40 [3.29] and 41 [3.30]. Quaternary polybipyridinium compounds also bind anionic substrates [3.31]. Progress is also being made towards the developments of neutral anion receptor molecules [3.32]. The thermodynamic and kinetic data for anion complexation by macrocyclic receptors have been reviewed [2.18c]. 

Multidentate macrocyclic ligands are comprised of molecules with an organic backbone interspersed with heteroatoms able to bind a variety of species. Macropolycyclic ligands are a 3-D extension of macrocyclic ligands, in which more than one macrocycle is incorporated into the same molecule. Multidentate macrocyclic and macropolycyclic ligands attract an increasing interest because they can function as receptors for several kinds of substrates, and because, after coordination, the physical and chemical properties of these substrates can be significantly modified. 

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