Chiral podands

The synthesis of a typical compound containing two units of dihydrobenzoin is shown as an example in Scheme 7 [26]. Reaction of 2,6-bis(bromomethyl)l,4-dimethoxybenzene 41 with the dibutyltin derivative of optically active dihydrobenzoin 59 yields a chiral podand 60 in 63%. Cyclization of 60 with the ditosylates of oligoethylene glycols, followed by oxidation with CAN and treatment with dinitrophenylhydrazine, affords the desired chiral dyed acerand 53. Some chiral azophenol acerands 51-58 are synthesized in a similar manner. 

Martinez-Diaz et al. have reported the synthesis of chiral podands and macrocycles based on 1,2,4-triazole (Figure 64) [89]. They have observed enantioselective transportation of chiral ammonium cations through supported liquid membranes. 

It has been argued above that a threefold symmetrical chiral podand may simplify the stereochemistry of key catalytic intermediates for cases in which it only acts as bidentate ligand in the stereoselectivity-determining step in other words, for metal complexes with a stereoelectronic preference for non-deltahedral coordination geometries. Palladium(ll)-catalyzed allylic substitutions provide appropriate test reactions alongthese lines [27], and it was possible to study the dynamic exchange in model systems for both the Pd" and Pd intermediates of this catalytic reaction. 

Chiral calix[4]arene podands were made using A-benzyl histidine methyl ester.33 These histidyl calixarenes 12a,b were studied in complexation experiments with Co(H), but no use was made of their chirality. The same is true for a chiral calix[4]arene capped tetraphenyl porphyrin which is C4-symmetrical due to the four L-alanine derived linkers.34... 

Kobuke et al. [40] demonstrated that the podand 28 only forms weak complexes with alkali metal ions. However, when a chiral macrocyclic crown ether structure is formed by complexation with boric acid, alkali metal ions are bound much more strongly (Scheme 10). Stiffening, provided by covalent B-O bonds, improves the preorganization of the ligands that is required for the complexation of metal ions. 

The most extensively examined method of stereoselective SLM separation is carrier-facilitated transport with chiral carriers. Different macrocychc compounds, transition metal complexes, phosphates, lariat ethers, podands. 

Biological Ligands, p. 88 Chiral Guest Recognition, p. 236 Complexation of Fullerenes, p. 302 Concave Reagents, p. 5 i 1 Cyclophanes Definition aiTd Scope, p. 414 Drug Delivery, p. 484 Glycoluril-Based Hosts, p. 597 Podands, p. 1106... 

Chiral Guest Recognition, p. 236 Crown Ethers, p. 326 Cryptands, p. 334 Enzyme Mimics, p. 546 Ion-Selective Electrodes, p. 747 lonophores, p. 760 Kinetics of Complexation, p. 116 Lariat Ethers, p. 782 Macrocycle Synthesis, p. 830 Podands, p. 1106... 

The kind and grouping of amino acid side chains are known to determine protein secondary structures. The X-ray crystal structure of the ferrocene 7 bearing the podand dipeptide chains (-Gly-L-Phe-OEt) shows the same chirality-organized structure based on two C2-symmetrical intramolecular interchain hydrogen bonds between CO (Gly) and NH (Gly of another strand) of each podand dipeptide chain. However, the ferrocene 7 exhibits... 

Enantiomer selective coloration of optically active amines, our important project, was realized by chiral azophenol crown 4 incorporated with two units of optically active hydrobenzoin. The synthetic route is shown in Scheme 2. Reaction of 2,6-bis(bromomethyl)-l,4-dimethoxybenzene 22, which is derived from hy-droquinone monomethylether 19 by a three-step procedure, with the dibutyltin derivative 26 of optically active dihydrobenzoin gives optically active podand 23 in 63% yield. Cyclization of 23 with the ditosylate of polyethylene glycol, followed by oxidation with ceric ammonium nitrate (CAN) and treatment with dinitrophenylhy-drazine, affords the desired chiral azophenol crowns 4n. 

One such nonnatural function of natural ionophores is the complexation of organic ammonium cations for example, protonated amino acid esters. Since natural ionophores are chiral compounds, the process can be enantioselec-tive. Binding of ammonium cations by such receptors as monensin or lasalocid (Fignre 1) is efficient bnt lack the expected enantioselectivity. However, cyclic or podand-type derivatives of monensin like 1 or 2 (Figure 1) show significant enantioselectivity the ratios of binding constants of the R and S enantiomers of protonated methyl esters of phenylglycine, phenylalanine, and leucine to 2 are A r/ZCs = 5.1, 6.2, and 7.6, respectively. Crown-ether type derivatives like 1 show lower enantioselectivities. 

Steroidal podands similar to 3 are, in fact, quite well-adapted for enantioselective recognition. The three functionalised sites on the steroid nucleus may be modified to give asymmetrical derivatives of general form 10, capable in principle of the three-point contact required for the classical model of enantioselectivity. Whether or not this idealised situation is ever achieved in practice, the chiral, functionalised steroidal a-face appears to provide an excellent environment for enantioselection. 

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