Macrocyclic receptors

Enantiomeric recognition of amino compounds by chiral macrocyclic receptors including pyridine or triazole subunits 97CRV3313. 

Macrocyclic receptors made up of two, four or six zinc porphyrins covalently connected have been used as hosts for di- and tetrapyridyl porphyrins, and the association constants are in the range 105-106 M-1, reflecting the cooperative multipoint interactions (84-86). These host-guest complexes have well-defined structures, like Lindsey s wheel and spoke architecture (70, Fig. 27a), and have been used to study energy and electron transfer between the chromophores. A similar host-guest complex (71, Fig. 27b) was reported by Slone and Hupp (87), but in this case the host was itself a supramolecular structure. Four 5,15-dipyridyl zinc porphyrins coordinated to four rhenium complexes form the walls of a macrocyclic molecular square. This host binds meso-tetrapyridyl and 5,15-dipyridyl porphyrins with association constants of 4 x 107 M-1 and 3 x 106 M-1 respectively. 

Polytopic macrocyclic receptors 1, 2 (Figure 10.1) are able to complex the zwitterionic form of the amino acids by a double non-covalent charge interaction [28,29]. The unsymmetrical benzocrown sulfonamide derivative, 2 which contains benzo-18-crown-6 and benzo-15-crown-5 moieties was used as a ditopic receptor for multiple molecular recognition of the amino acids, by combining two non-covalent interactions ammonium-crown hydrogen bonding and carboxylate- complexed Na+-benzo-15-crown-5 charge interactions [28,33]. 

Zhang, X.X., Bradshaw, J.S. and Izatt, R.M. (1997) Enantiomeric recognition of amine compounds by chiral macrocyclic receptors. Chemical Reviews, 97, 3313-3361. 

Barboiu, M. (2004) Supramolecular polymeric macrocyclic receptors-hybrid carrier vs. channel transporters... 

Linking the macrocyclic receptor l,7-diaza-15-crown-5 to the fluorophore 7-nitrobenzofurazane leads to the Hg(ll) selective fluorescence probe NBO-crown <1997JFL231S>. The cryptand was functionalized with an electron-withdrawing fluorophore (NBD) which behaves as a fluorescence on/off signaling system by translocating Cd(ll) inside and outside the cryptand cavity <2004IC4626>. 

Interesting results were reported with dithiocarbamates derived from diamines. Thus, a series of dinuclear zinc(II), cobalt(II), and nickel(II) dithiocarbamate ditopic macrocyclic receptors containing various spacer groups of different sizes has been reported.412,413,414 Spectroscopic investigations and... 

The easily available hexa-O-benzylsucrose (187) was used as a starting material for the preparation of macrocyclic receptors. Several derivatives of type 188 with various cavities and different number of nitrogen and oxygen atoms were prepared (Fig. 60).78 The azacrown macrocycles showed remarkable enantioselectivity towards a-phenylethylammonium cations. For example diazacrown derivative 188 (Y = O X = NBn n = 2) did... 

A certain control of selectivity for simple ligands without altering their fluorescence features is possible when integrating them into more developed, often macrocyclic receptor units [45,46]. 

The amplification of two different macrocyclic receptors for similar diastereomeric compounds has been observed by Sanders and coworkers [13]. A dynamic system of macrocyclic polyhydrazones ranging from dimer to at least undecamer was prepared from the indicated homochiral hydra-zone. Templating with quinine caused the percentage of cyclic tetramer to increase from 63 % to 91 %. The same experiment with quinidine, a diastereomer of quinine, instead amplified the dimer from 9% to 45% (Fig. 5.13). These species were characterized by HPLC and ESI MS. Association... 

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). 

An example of the above mentioned cascade complexation of carboxylates by macrocyclic receptors containing metal ionic centers is the inclusion of oxalate by the dien dicobalt complex 9 (Martell, Mitsokaitis) [12]. Similarly, the -cyclodextrin (jS-CD) 10, modified with a zinc cation bound by a triamine side chain, encapsulates anions like 1-adamantylcarboxylate in its cavity, fixing them by combined electrostatic and hydrophobic interactions [13], Zinc s group achieved the enantioselective transport of the potassium salts of N-protected amino acids and dipeptides by making use of the cation affinity of... 

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. 

Macrocyclic receptors have also been found to complex neutral substrates,21 and complexes of both variable and exact stoichiometries have been prepared. The latter category includes acidic CH— and NH— or polar neutral molecules, such as acetonitrile, nitromethane, benzyl chloride and dimethylmalonitrile. X-Ray data indicate complex formation to be mainly a result of hydrogen bonding and dipole-dipole interactions. Section 21.3.8 contains a more detailed treatment of these complexes. 

The incorporation of chiral units within the macrocyclic skeleton is an important route to the design of macrocyclic receptors capable of enantiomeric or chiral substrate recognition. In order to achieve this goal, the receptor must display a fine structure as a result of chiral barriers, which then allows diastereomeric receptor-substrate interactions, in addition to maintaining the desired cavity properties.18,30,51... 

Complexes are also formed in certain instances between neutral molecules and macrocyclic receptors. Neutral molecules which form such complexes for the most part contain polar O—H, N—H or C—H bonds, and hydrogen bonding interactions are responsible for the solid state structural characteristics of these complexes. For many of these complexes, stoichiometries range considerably, from 1 1 to 1 6 host guest, and include a variety of odd ratios such as 3 2, 2 7, etc. Structural results for these complexes indicate them not to be of the inclusion type in a majority of cases. Thus, discussion in this subsection will be limited to a general overview. A more complete review of neutral molecule complexation can be obtained elsewhere.21... 

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]. 

Chemical transformations may be induced by reaction between a bound substrate and functional groups borne by the macrocyclic receptor unit, as illustrated in structure 74. 

Hydrogen transfer has been induced with macrocyclic receptors bearing 1,4-di-hydropyridyl (DHP) groups. Bound pyridinium substrates are reduced by hydrogen transfer from DHP side chains within the supramolecular species 78 the first-order intracomplex reaction is inhibited and becomes bimolecular on displacement of the bound substrate by complexable cations [5.19]. Reactions with carbonyl or sulphonium substrates have been performed with other DHP containing macrocycles, such as 79 [5.20]. 

Scheme 34. Synthesis of sucrose-based macrocyclic receptors.

S. Jarosz, A. Listkowski, B. Lewandowski, Z. Ciunik, and A. Brzuszkiewicz, Macrocyclic receptors containing sucrose skeleton, Tetrahedron, 61 (2005) 8485-8492. 

Dhaenens, M., Lacombe, L., Lehn, J. M., Vigneron, J. P., Binding of acetylcholine and other molecular cations by a macrocyclic receptor molecule of speleand type. J. Chem. Soc., Chem. Commun. 1984, 1097-1099. 

Scheme 5.1 Macrocyclic receptor 5.4 is able to coordinate two Cu(II) ions that, in turn, bind two azide anions bridging across the two metal centres to give the Cu(II) azido complex [Cu2(5.4) (//-N3) 2(N3) 2].

A neutral macrocyclic receptor of nitrate was designed by a rational approach. [1] A hexagonal grid was constructed around the trigonal nitrate anion and the molecular frame was cut from the chicken wire pattern (Scheme 1). 

Scheme 1. Design of a neutral macrocyclic receptor using a chicken wire pattern...

Macrocyclic Receptor Molecules, J.-M Lehn, Pure andApp. Chem. 1979, 51 979-997. 

Buschmann, H. J., Mutihac, L., and Jansen, K. (2001) Complexation of some amine compounds by macrocyclic receptors, J. Incl. Phenom. Macrocycl. Chem. 39, 1-11. 

Wipff, G., Wurtz, J. M. (1988) Dynamics Views of Macrocyclic Receptors Molecular Dynamics Simulations and Normal Modes Analysis, in Pullman, R. (eds.), Transport through Membranes, Carriers, Channels and Pumps, Reidel, Dorbrecht, pp 1-26. 

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