Amino acids inclusion complexes

An alternative technique for the resolution of enantiomers which lack polar functional groups is a diastereomeric complex formation via non-ionic mechanisms. These complexes may be of the external or the inclusion type. Chiral amino acids, metal complexes or host compounds may serve as useful complexing agents (Table 2) [49-64]. 

Enantioselective self-assembling of amino acids 209 Host-guest inclusion complexes 213 Reactivity of chiral ion-dipole complexes 233... 

To date, only few very recent gas-phase studies on this subject can be retrieved from the literature, i.e., (i) a gas-phase study on the displacement of several amino acids from the chiral amido esorcinarene 9 (Scheme 9) carried out by Speranza and coworkers using an electrospray-ionization Fourier-transform ion cyclotron resonance (ESl-FT-lCR) mass spectrometer," " and (ii) Lebrilla and coworkers study on the ability of the achiral calix[4]arene 7 and calix[6]arene 8 to form inclusion complexes with natural amino acids under matrix-assisted laser... 

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... 

Chiral crown ethers based on IB-crown-6 I Fig. 4> can form inclusion complexes with ammonium ions and proionated primary amines. Immobilization of these chiral crown ethers on a chromatographic support provides a chiral stationary phase which can resolve most primary amino acids, amines and amino alcohols. However, the stereogenic center must be in fairly close proximity in the primary aininc lor successful chiral separalion. Significantly, ihe chiral crown ether phase is unique in that ii is one of the few liquid chromatographic chiral stationary phases that does not require the presence of an aromatic ring to achieve chiral separations. 

The rates of hydrolysis of the trifhioroacetates (201 X = H, Me) increase in a nonlinear fashion in the presence of jS-CD. Some differences in rate between the two substrates have been explained as being due to different modes of inclusion.173 The novel CDs (202) and (203) have been synthesized in 45% and 66% yields, respectively, and their complexation with various l/d amino acids have been examined. Importantly, (202) and (203) can be detected by fluorescence spectroscopy and they can recognize the size and shape but also the chirality of the amino acids.174 A /j-CD dimer with a linking bipyridyl group (204) has been synthesized and shown to bind both ends of potential substrates into two different cavities of the CD holding the substrate ester carbonyl group directly above a Cu(H) ion bound to die bipyridyl unit. This achieves... 

The chiral resolution on CD-based CSPs depends on the formation of inclusion complexes in the cavities and, therefore, the structures and sizes of analytes are very important for the chiral resolution of racemates on these phases. Amino acids often are considered to be the best class of racemic compounds to use in structural studies. In 1987, Han and Armstrong [55] studied the chiral resolution of amino acids on -CD-based CSPs. It was observed that different retention, separation, and resolution factor values were obtained for different amino acids under identical chromatographic conditions, which indicated that the structures and sizes of amino acids govern their chiral resolution. The same observations may be found in the work of Fujimura et al. [70]. 

Sansone, F., Barboso, S., Casnati, A., Sciotto, D., Ungaro, R., A new chiral rigid cone water soluble peptidocalix[4]arene and its inclusion complexes with alpha-amino acids and aromatic ammonium cations. Tetrahedron Lett. 1999, 40, 4741-4744. 

Kalchenko, O. I., Perret, F., Morel-Desrosiers, N., and Coleman, A. W. (2001) A comparative study of the determination of the stability constants of inclusion complexes of p-sulfonatocalix [4]arene with amino acids by RP-HPLC and H-l NMR, Perkin Trans. 2 258-263. 

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