Guests chiral

This chapter provides (i) a brief review of the chemistry involved in chiral host-chiral guest recognition involving primary amines (ii) a description of a nonchromatographic (equilibrium or bind-release based) separation process devel-... 

FIGURE 3.6 Chiral helical hexagonal structure of urea and thiourea adducts. A chiral guest generates one helical form. (Straight chain alkyl guests fit in the urea adduct channel larger molecules such as trans-decalin fit in thiourea adduct channel.)... 

These results allowed the proposal, at the beginning of the 1980s, of a different molecular model for cholesteric induction 65,66 This model is sketched in Figure 7.15 in the case when both nematic host and chiral guest have a biaryl structure. Nematic molecules exist in chiral enantiomorphic conformations of opposite helicity in fast interconversion. The chiral dopant has a well-defined helicity (M in Figure 7.15) and stabilizes the homochiral conformation of the solvent In this way, the M chirality is transferred from the dopant to the near molecule of the solvent and from this to the next near one and so on. This... 

Figure 5. Top Tetraurea calixarene monomers 37 and 38 bearing chiral amino acid ester residues (isoleucine and valine methyl esters, respectively) attached to the urea functions. Norcamphor 39 was the chiral guest used to detect the chirality transfer from the outside to the inner cavity.

Figure 6. Top The two enantiomeric forms of the "american football" monomer 40 and views of the tetrameric capsule formed from four identical monomers along the mutually perpendicular C2-axes. Bottom Chiral guests 41 -46 and the corresponding de values obtained from 1H-NMR integrations.

A convenient method to demonstrate guest binding in mPE systems is circular dichroism spectroscopy (CD). CD is a method which allows the determination of chiral excess in a system [56]. In the absence of a chiral influence, the mPE helix exists as a racemic mixture of both M and P helices and displays no CD signal. Upon the addition of a chiral guest, diastereomeric complexes are... 

When the analyte is chiral, however, the stereochemical issues evolve to how a chiral host interacts with a chiral guest. The nature and magnitude of these diastereomeric interactions ultimately control the DCL evolution and what types of hosts are amplified. 

Mesophase with a helicoidal superstructure of the director, formed by chiral, calamitic or discotic molecules or by doping a uniaxial nematic host with chiral guest molecules in which the local director n precesses around a single axis. 

In the early 1990s, several studies were published on the computation of selector-selectand interactions in chiral CE. This relates basically to the interactions between CDs and their chiral guests, which seem to be caused by the fact that CDs are rather rigid molecules of medium size and therefore calculations for these molecules are easier and faster and may be rather precise. In addition, many CDs are well studied by alternative techniques for structure elucidation. Among these, X-ray crystallographic data are of the highest interest. 

The CD band intensity originated from the host (8) is enhanced in the bilayer vesicle ([0] -i-1.5 x 10 deg cm dmol at 247 nm) relative to the corresponding value in the HEPES buffer at 30.0 °C. In addition, no CD spectral change was observed for at least one day at 30.0 °C. This result indicates that the four pyridinium moieties bound to the chiral L-valine residues in 8 assume highly restricted conformations in the bilayer membrane. Thus, the hybrid assembly seems to furnish a chiral guest-binding site different from that provided by 8 alone in aqueous media without the vesicle. 

Such chiral luminescent dendrimers, whose fluorescence is quenched significantly more strongly by one enantiomer of a chiral guest molecule than by its mirror-image counterpart, would be of interest for rapid determination of the... 

Measurement of circular dichroism can even permit elucidation of relatively small structural changes. CD spectroscopy is also suitable for the solution of specific application-relevant questions. Studies of the sensor properties of chiral dendrimers make use of the fact that complexation of chiral guest molecules induces changes in the CD bands of the host dendrimers. Thus guest-selective chiroptical effects observed in titration experiments with enantiomeric guest molecules give an indication of the potential of the chiral dendrimer to act as an enantioselective sensor [87]. 

SPR spectroscopy and RIfS were used as optical transduction principles for the detection of halogenated diethers as chiral guests with modified cyclodextrin as the stationary phase [7]. 

Figure 6.29 (a) molecular model of the chiral tweezers 6.53, (b) the chiral guest that forms part of... 

Among the different types of compounds whose complexation properties have been studied are various amides linear oxoamide 9 [22], fumaramide 10 [23,24] and methanetricarboxamide 11 [25], biphenyl derivatives 12 [26], and derivatives of tartaric acid 13-16, that can also be prepared in an optically active form [27], The above-mentioned chiral hosts have been found to form inclusion complexes with chiral guests 17 and 18. Molecular recognition between chiral hosts and... 

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