Chiral sensing

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS CHIRALITY PROBES Chirality sense,... 

A polymerization of a bulky methacrylate ester (e.g. trityl methacrylate) using an optically active anionic initiator can give an isotactic polymer, poly 1-methyl-1-[(trityloxy)carbonyl]ethylene of high optical activity owing to the formation of helical polymer molecules with units of predominantly one chirality sense. 

Polymerization of tert-butyl isocyanide using an optically active initiator gives an optically active product comprising helical polymer molecules with units of predominantly one chirality sense. 

There are many procedures to determine the chirality sense of a stereomodcl, the best known being the R and S descriptors of the Cahn-Ingold Prelog (CIP) system. 

There is a fundamental difference between a chirality center and a pseudoasymmetric center and that is that reflection and permutation of ligands have the same effect for the chirality center, but not for the pseudoasymmetric center. This is because, on reflection of the latter, the chirality senses of both the center, seen when the bonds are numbered, and each of the enantiomorphic ligands are reversed. Another way of stating the difference is that Re/Si and rejsi descriptors specify absolute and relative configuration, respectively. Pseudoasymmetric is a most unfortunate term, and in order to avoid it, the classical terms chirality center and pseudoasymmetric center would perhaps best be replaced by more neutral terms, such as stereogenic centers of type 1 and type 2, respectively, in order to emphasize the aspect of stereogenicity. 

Moving two-dimensional enantiomorphs out of the plane into three-dimensional space allows them to become congruent, i.e., identical. However, some two-dimensional aspects remain. When the two faces of a particular figure are examined, one perceives that the faces, and the corresponding half-spaces10, of the three-dimensional space, are enantiomorphic, and their chirality sense can be specified by Re/Si descriptors. Furthermore, for figures with more than one stereogenic center the Ikjul and ZjE descriptions are preserved in three-dimensional space. 

The complete topographic description of a chiral n-point figure in three-dimensional space requires 3n coordinates relative to an external coordinate system. However, these coordinates are impractical because they contain information on the position of the figure, i.e., its translational and rotational state. A more convenient description for the chemist is obtained by incorporating chemical bonds, and the use of 3n — 5 (linear models) or 3n —6 internal ( chemical ) coordinates which are bond lengths bond angles torsion (dihedral) angles chirality sense of tripods. 

A helical unit is inherently chiral. Its chirality sense or helicity can assume two values that correspond to the sign of the torsional angle. Although the definition of the sign is arbitrary (sec Section 1.1.1.). a universal scientific convention2 is followed (Klyne Prelog convention) ... 

In summary, the helieity of a nonplanar arrangement of two trigonal centers connected via a linear array of one or more bonds corresponds to the chirality sense of the core of bonds selected according to the two ligands of highest precedence at each trigonal center. 

In a tour de force in 1956, Cahn, Ingold and Prelog introduced the terms chirality axis (descriptors aR/aS) and chirality plane (descriptors pRjpS) in order to deal with compounds such as allenes, biaryls and cyclophanes. Rules for assigning the chirality sense were devised ad hoc. In 1966 the helieity concept was introduced and it was recognized that its use allows the corresponding models to be treated in an alternative way. The specific proposals, as illustrated in Table 1, however, were only published in 19821. 

An example pertinent lo stereoselective reactions and medicinal chemistry, is a change in the chirality sense in a series of closely related compounds ... 

Crown ethers of the type discussed in this section have been used as sensors, membranes, or materials for chromatography. Shinkai used cholesterol-substituted crown ether 10 as a sensor for chirality in chiral ammonium compounds (Scheme 16). It was found that the pitch of the cholesteric phase exhibited by 10 was changed upon addition of the chiral salt. As the wavelength of reflection for incident light depends on the pitch, a color change was observed that was visible to the naked eye [45, 46]. Such chirality sensing systems were known before but chromophores had to be bound to the crown ether in order to observe color changes [47]. This problem could be overcome by 10, which uses intrinsic properties of the chiral nematic phase. 

Lin Z (2004) Time-resolved fluorescence-based europium-derived probes for peroxidase bioassays, citrate cycle imaging and chirality sensing. PhD thesis, University of Regensburg... 

The asymmetric crystallization of achiral compounds is stimulated by autoseeding with the first crystal formed. Although the chiral sense of the spontaneously formed chiral crystals cannot be predicted, seed crystals of the preferred chirality can be added in a more practical procedure to obtain one enantiomorph of a crystal. 

Chiral refers to the property of chirality. As applied to a molecule, the term has been used differently by different workers. Some apply it exclusively to the whole molecule, whereas others apply it to parts of a molecule. For example, according to the latter view, a meso compound is considered to be composed of two chiral parts of opposite chirality sense this usage is to be discouraged. See enantiomorph... 

In its application to an assembly of molecules, some restrict the term to an assembly in which all of the molecules have the same chirality sense, which is better called enantiopure. Others extend it to a racemic assembly, which is better just called as racemate. The use of the term chiral to describe molecular assemblies should be avoided. 

Chirality is a pervasive property of an object, which means that in theory, a single remote asymmetric center in a macromolecule is enough to make the entire molecule chiral and, in principle, even the more distant residue could sense the asymmetry induced by the stereogenic center. On the contrary, experiences maturated by synthetic chemists in the construction of molecular species for enantioselective recognition speak for the necessity of placing the asymmetric units in close contact to allow chiral sensing and discrimination. The latter, in fact, arises from attractive forces and steric interactions that require close contact between the counterparts. On the contrary, magnetic asymmetry is not a direct consequence of weak interactions, but is more a property of the space which surrounds a chiral object. 

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