Guests conformational equilibria

Fig. 2. Schematic representation of the use of unnatural side chain interactions to alter the conformational equilibrium towards a more ordered structure. The binding of a specific guest displaces the equilibrium towards a conformation that orients the binding side chains complementary to the geometry of the guest...

It has also been reported from circular dichroism (CD) studies [36] that polysaccharide-based CSPs can induce chirality in enantiomeric guests such as (4Z,15Z)-bilirubin-Ixoc (BR) (Fig. 5). Although not optically active, BR has two enantiomeric helical conformations maintained by six intramolecular hydrogen bonds between two carboxylic acid moieties and two pyrromethenone — NH— protons. These (R)- and (5)-helical conformers are in dynamic equilibrium in an achiral solution [37], but some optically active compounds can enantioselectively bind to BR to induce CD spectra in solution [38-40]. A significant induced CD... 

As the receptors have become more complex, forces such as H-bonding and dipole-dipole interactions have augmented the ion-dipole complexation mechanism typical of crown ethers. This has increased the importance of conformational analysis for the receptors and an understanding of structure. Ultimately, however, the binding of a guest by a host molecule is defined by the equilibrium constant for that system in a particular solvent and at a specified temperature. 

Since guest molecules are arranged close together and in one conformer in a host-guest complex, inter- and intramolecular photoreactions of the guest compound would proceed regio- and stereo-selectively and efficiently by irradiation of the complex in the solid state. When an optically active host compound is used, enantio-control of the reaction is expected. When the complexation is applied to an equilibrium mixture of tautomers, one labile tautomer can be isolated as a host-guest complex. By an irradiation of the complex in the solid state, stereo- and enantio-controls of the reaction of the labile tautomer can be achieved. 

The isolation of otherwise higher energy forms (or at least exclusive selection of one equilibrium form) within a CB[n] cavity, as a result of the confinement within the CB[n] nanoreactor and specific host-guest interactions which can result in different stabilities of isomers or conformers within the cavity as opposed to in free solution, well illustrates the potential utility of CB[ ] nanoreactors for controlling guest reactivity. 

The host is not required to adopt chiral forms in the absence of a chiral guest. In a complexed state, (a) the chiral guest biases the equilibrium between enantiomeric conformations of the host and (b) the chiral guest induces the host to change in structure from achiral to chiral upon complexation and biases the newly generated equilibrium through the supramolecular transmission of chirality. 

The mirror image was induced by the addition of (S,S)-5. The observation is considered a result of conformational switching from a non-helical anti form to a helical syn form, followed by biasing the newly generated equilibrium to a particular sense in the syn-formed complex through supramolecular transmission of guest chirality to double-arm-crossing dynamic helicity (case (b) in Scheme 14.1). 

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