Substituted podands

The bulk of the work which has been performed on open-chained crown ether and cryptand equivalents, especially for application to general cation binding studies has been accomplished by Vogtle and his coworkers. Vogtle has reviewed both his own and other work in this field . 

Cram has published several studies indicating that complexation of ammonium salts was greatly enhanced by using cyclic polyethers over the corresponding acyclic ones. These molecules were analogs of the binaphthyl systems under study and were generally prepared by phenoxide substitution on the appropriate tosylate. The approach is illustrated below in Eq. (7.9).  

In 1975, Weber and Vogtle showed that open-chained polyethers, 2,6-pyridine-dimethanol, and ortho-xylene derivatives terminated in an 8-quinolinyl group could form stable, 1 1, crystalline complexes with a variety of metal salts. The podands were prepared from 8-hydroxyquinoline and the corresponding dihalides. A typical example is shown in Eq. (7.10). 

In later work, Vogtle and his coworkers prepared analogs of both crown ethers and cryptands. These molecules are designed to have a terminal donor group which is capable of offering a complexed cation additional binding sites. Numerous 

The materials shown and described above were generally prepared from the nucleophilic phenoxide or alkoxide and the appropriate bromide. The syntheses of a variety of such compounds were detailed in a report which appeared in 1977. In the same report, complex stability and complexation kinetics are reported. Other, detailed studies, of a similar nature have recently appeared . Vogtle and his collaborators have also demonstrated that solid complexes can be formed even from simple polyethylene glycol ethers . Crystal structures of such species are also available 

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Salimi A, Eshghi H, Sharghi H, Golabi SM, Shamsipur M. Electrocataljrtic reduction of dioxygen at the surface of glassy carbon electrodes modified by some anthraquinone substituted podands. Electroanalysis 1999 11 114—9. 

Tsukube, H. Shinoda, S. Uenishi, J. Hiraoka, T. Imakoga, T. Yonemitsu, O. Ag -specific pyridine podands Effects of ligand geometry and stereochemically controlled substitution on cation complexation and transport functions. J. Org. Chem. 1998, 63, 3884-3894. 

As an application of amine selective complexation, it was observed that the substitution pattern of amines, i.e. primary, secondary, and tertiary, may be discriminated by combined use of two azophenol hosts, pyridine-04-crown 2 and 04-podand 6 (n = ). 

On the other hand, azophenol 04-podand 6 n = ) shows the complexation-coloration with prim, and sec. amines, included sterically bulky amines such as /-BUNH2, (/-Pr)2NH, and 2,2,6,6-tetramethylpiperidine. Consequently, the combined use of two types of azophenol hosts, 2 and 6, is highly useful to discriminate the substitution pattern of amines by means of coloration. The two end hydroxyl groups in the dyed podands are required for such discrimination of the amine pattern because no complexation-coloration is observed by addition of various amines when the hydroxyl groups are substituted with methoxyl groups. 

For recent examples of the synthesis of peptides and peptoids by the Ugi 4-CR, see (a) O. E. VerciUo, C. K. Z. Andrade, L. A. Wessjohann, Org. I tt. 2008, 10, 205-208. Design and synthesis of cychc RGD pentapeptoids by consecutive Ugi reactions, (b) O. Pando, S. Stark, A. Denkert, A. Porzel, R. Preusentanz, L. A. Wessjohann, J. Am. Chem. Soc. 2011, 133, 7692-7695. The multiple multicomponent approach to natural product mimics tubugis, V-substituted anticancer peptides with picomolar activity, (c) A. Fatima, S. Barreto, O. E. Vercillo, M. A. Birkett, J. C. Caulfield, L. A. Wessjohann, C. K. Z. Andrade, Org. Biomol. Chem. 2011, 9,5024-5027. Fast and efficient microwave-assisted synthesis of functionalized peptoids via Ugi reactions, (d) M. Abbas, L. A. Wessjohann, Org. Biomol. Chem. 2012, 10, 9330-9333. Direct synthesis of sensitive selenocysteine peptides by the Ugi reaction. For an application to peptide-peptoid podands by threefold Ugi-4CRs, see (e) D. G. Rivera, F. Leon, O. Concepcidn,... 

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. 

Polymer-supported podands appear to be more commonly used in solid-liquid systems. A variety of chemical transformations including nucleophilic substitutions at saturated carbon atoms [214,215], esterifications [214], sodium bo-rohydride reduction [214], fluoride-catalyzed Michael addition [214], phenacyl ester synthesis, Darzen s reaction, Wittig alkenylation, and dichlorocarbene reactions can be carried out under these conditions [214],... 

Mac Kenzie, W. M., and Sherrington, D. C., Nucleophilic substitution at determined carbon atom catalysed by supported podands. Polymer, 21, 791, 1980. 

The precursors of cyclophanes 161, i.e., podands 156 were prepared in four steps starting from 3-ethylquinoxalin-2-one 159 (Mamedov et al. 2(X)5a Kalinin and Mamedov 2009), the alkylation of which with dibromooxoalkanes leads to 160 with ethyl moieties. The transformation of ethyl acetyl fragments was accomplished in three stages bromination, the substitution of the bromine atom by the azide group, and conversion of azidoethyl fragments into acetyl function when heated in aqueous acetic acid. 

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