Achiral compound, circular dichroism

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... 

An interesting phenomenon whereby achiral compounds occupy chiral cavities has been reported. Steroidal host compounds give rise to the attachment of definite chiral conformations of achiral compounds within cavities, making it possible to observe solid-state circular dichroism spectra. Gdaniec and Polonski reported this type of property for the inclusion compounds of DCA and CA with various aromatic ketones [40a] and benzil [40c], Furthermore, it is possible for the selected conformers to maintain their chiral state temporarily in solution. That is, soon after the inclusion compounds are dissolved, the chirality may be retained for some time. /V-Nitrosopiperidines were found to display this type of dynamic chiral recognition in DCA and CA inclusion compounds [40b], In this case, one can observe the decay of the circular dichroism signal after dissolution of these inclusion compounds in methanol. 

The circular dichroism induced in achiral compounds upon complexation with a chiral material can be used to characterize the nature of the interaction responsible for the association. Studies of this type may be conveniently classified as being the optical activity induced upon dissolution of a solute in a chiral solvent, through association of the species of interest with a co-dissolved chiral solute, or through association of the solute with chiral polymers. 

The power of circular dichroism (CD) spectroscopy is unfortunately limited to the study of chiral molecules having accessible chromophores, and cannot be applied to the study of achiral compounds. However, a great many compounds of interest are not intrinsically dissymmetric, so the induction of chirality into such species is a fundamental requirement for the performance of CD studies. In the present work, investigations into the CD induced in achiral molecules by the action of chiral agents is reviewed. 

There is no special crystallization technique required to prepare chiral crystals from achiral organic compounds. Ordinary crystallization techniques can be employed i.e., a hot saturated solution of a compound in a suitable solvent is slowly cooled, or a saturated solution is slowly evaporated to obtain crystals. Whether the molecule is chiral can be easily differentiated by the measurement of optical rotation in solution using a polarimeter. However, there are great difficulties in measuring the optical rotation of crystalline substances due to the large birefringence [33]. Solid-state circular dichroism (CD) spectral measurements of... 

Examination of A. qfficinarum by Itokawa and his group provided besides the known compound 6 (8) and the ketol 22 (25) two new enones, 28 and 29 (26). Circular dichroism studies revealed the interesting fact that, in contrast to the known components of Alms firma and A. sieboldiana, which contained (S)-6 and (5)-22, in Alpinia officinarum the same alcohols were present as the R enantiomers. On further investigation the same plant also yielded a total of eight new achiral or racemic diarylheptanoids 23, 24, 31 (27), 25, 26, and 30, (28), 33 (29), 27 and 32 (30). Since the highly susceptible P-hydroxyketones can readily undergo dehydration and subsequent addition of methanol, the enones 28, 29, and 30, as well as the P-methoxyketones 23, 25, and 26 may be artefacts. This assumption is also supported by the racemic nature of 23, 25, and 26. 

Strictly, ICD encompasses all kinds of circular dichroism effects except for those of inherently chiral chromophores. This is so because most systems (molecules) may be more or less arbitrarily divided into one achiral chromophore in which the ICD is observed and a remaining component that includes the chirality. Thus, the CD exhibited by a chiral molecule may be considered ICD if the chromophoric entity itself is achiral. With this broader definition of ICD, molecules such as carbonyl compounds (in which the carbonyl chromophore is achiral) display an intramolecular ICD in the first allowed n —> 7t transition around 300 nm due to perturbation from a surrounding chiral (e.g. steroid) skeleton. Distinction between this type of molecule and one which is an inherently chiral chromophore could be difficult to make as the boundaries of the chromophore are often diffuse. For example, when a diene is in immediate proximity of a carbonyl, a composite chromophore is obtained, which, if skewed, is inherently chiral. Therefore, it is practical to restrict the definition of ICD to cases in which the achiral part is... 

It is worth mentioning that the extent of induced circular dichroism on NVC units when inserted in copolymers with intrinsically chiral comonomers is of the same order of magnitude as that extrinsically induced by thermotropic cholesteric mesophases on achiral 9-ethylcarbazole when physically trapped within the mesophase . Higher extrinsic circular dichroism effects are by contrast reported when lyotropic cholesteric mesophases, such as those obtained from polyfy-benzylglutamate) in tetrachloroethane, are used as chiral traps of achiral carbazole derivatives and iso-electronic aromatic compounds ... 

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