Chirality/Chiral

Holzspanplatte chiral chiral chiral chromatography enantioselektive Chromatographie chirality Chiralitht chisel... 

The remaining three isomers are chiral CHIRAL DICHLOROCYCLOHEXANES... 

A propargyl substrate having a substituent at the propargyl position is centrally chiral and an allenic product from the SN2 substitution reaction will be axially chiral. Chirality transfer in the SN2 reaction, accordingly, may be achieved starting from an enantiomerically enriched propargyl electrophile [29]. The reactions in Scheme 3.11 are some recent examples of the center to axis chirality transfer by Pd-catalyzed SN2 reactions [41, 42]. 

Most drugs and biological molecules are chiral. Chirality means handedness, that is, left-hand and right-hand mirror images. This is because of the existence within the molecules of chiral centers. For example, a carbon atom attached to four different groups can be oriented in such a way that two different molecules that are mirror images are obtained (Fig. 3.11). 

A stereochemical property of compounds arising from the ability of an enzyme s active site to distinguish between two chemically identical substituents covalently bound to a tetrahedral center (usually carbon and, in some cases, phosphorus). Prochirality is also termed prostereoisomerism. The classical example is citrate with its two carboxymethyl group substituents. Likewise, the Cl carbon atom of ethanol has two prochiral hydrogens. See Chirality Chirality Probes... 

PROCESSIVITY OF KINESIN ATPase PROCHIRALITY CHIRALITY CHIRALITY PROBES PROCOLLAGEN N-PEPTIDASE PRODUCT... 

CHIRALITY CHIRALITY PROBES PROCHIRALITY PROSTHETIC GROUP... 

Many other biological structures exhibit LC behavior. For instance, the concentrated protein solution that is extruded by a spider to generate silk is actually an LC phase. The precise ordering of molecules in silk is critical to its renowned strength. DNA and many polypeptides can also form LC phases. Since biological mesogens are usually chiral, chirality often plays a role in these phases. 

The achiral things are (c) and (h). with planes of symmetry (e). with a center of symmetry and two planes of symmetry and (/), with a center of symmetry. This leaves (a), (b), (d). (g). and (/) as chiral. [Chiral derives from the Greek for hand.)... 

Restek Corporation. 1997. A Guide to the Analysis of Chiral Compounds by GC. www.restekcorp.com/chiral/chiral.htm. 

Molecules that are not superimposable on their mirror images are chiral Chiral molecules exists as enantiomers but achiral molecules cannot exist as / enantiomers. A carbon atom to which four different groups are attached is a chiral center. Not all molecules that contain a chiral center are chira/L Nbt all chiral molecules contain a chiral center. 

The results surveyed in the preceding two sections provide a first clue to the origin of chirality chiral patterns can emerge spontaneously in an initially uniform and isotropic medium, through a mechanism of bifurcations far from thermodynamic equilibrium (see Figs. 4 and 5). On the other hand, because of the invariance properties of the reaction-diffusion equations (1) in such a medium, chiral solutions will always appear by pairs of opposite handedness. As explained in Sections III.B and III.C this implies that in a macroscopic system symmetry will be restored in the statistical sense. We are left therefore with an open question, namely, the selection of forms of preferred chirality, encompassing a macroscopic space region and maintained over a macroscopic time interval. 

Keywords Axial chirality Bent-core molecule Chirality Chirality control Deracemization... 

Chirality (handedness, from Greek cheir = hand) is the term used for objects, including molecules, which are not superposable with their mirror images. Molecules which display chirality, such as (S)-(+)-lactic acid (/, Fig. 1) are called chiral. Chirality is often associated with a chiral center (formerly called an asymmetric atom ), such as the starred carbon atom in lactic acid (Fig. 1) but there are other elements that give rise to chirality the chiral axis as in allenes (see below) or the chiral plane, as in certain substituted paracyclophanes.1,2)... 

An important approach to stereochemical problems is to make use of the concept of chirality. Chirality (7), namely, the phenomenon that a chiral object and its mirror image cannot be superimposed, has been classified according to different elements of chirality. Chiral molecules may contain chiral centers, axes, and/or planes (2, 3). 

This concept of the chiral amplification has been applied to supramole-cular systems that involve self-assemblies composed of chiral-achiral and chiral-chiral (R/S) components, resulting in the formation of chiral supra-molecules [20] including helical aggregates. 

The main topics are adsorption-induced chirality, chiral recognition, transfer of chirality from single molecules into supramolecular structures, and cooperatively driven chiral amplification phenomena. 

To this point, various physicochemical properties of drugs such as lipophilicity, ionization, and partition coefficient have been discussed. While these are certainly major factors, there is an additional factor that can influence drug distribution, namely chirality. Chirality is a relatively unique structural characteristic of certain molecules that can exist in two asymmetric, nonsuperimposable isomers (enantiomers) due to the presence of a chiral center (a carbon atom that is attached to four different functional groups (see Chapters 5 and 13). 

In the study of mixtures, differentiation between enantiomers is a two level problem which is somewhat independent of whether the LC system is chiral or conventional. The problems common to both systems are the effects of overlapping bands on the performance of the detectorfs). Overlap can be between chiral-achiral species on the one hand and co-eluted chiral-chiral with achiral on the other. On first thought the chiral-achiral distinction should be relatively easy if a chiroptical detector is used because the achiral compounds will not interfere with the detection measurement. In addition the ability of the chiroptical detector to measure both positive and negative signals makes the confirmation of the enantiomeric structure elementary [3,4], As pointed out earlier, enantiomers co-elute from conventional columns and two detectors in sequence will provide the information to measure the enantiomeric ratio provided the mixture is not racemic. Partial or total overlap of the band for a non-chiral species with the chiral eluate band increases significantly the difficulty in measuring an enantiomeric ratio. In this instance the total absorbance that is measured may include a contribution from the non-chiral species which without correction will lead to an overestimation of the amount of chiral material and an erroneous value for the enantiomeric ratio. Under these circumstances there is no other LC option but to develop a separation that is based upon a chiral system. 

Molecules such as 2-chlorobutane are termed chiral. Chiral molecules exist as either of two stereoisomeric structures. These stereoisomers, a pair of nonsuperimposable mirror images, are called enantiomers. 

Carbons I, 4, 5, and 6 are all bonded to two or more hydrogens, so they are not chirality centers. Carbon 2 is bonded to two methyl groups, so it is not a chirality center either. However, carbon 3 is bonded to four different groups (hydrogen, methyl, propyl, isopropyl) and is a chirality center. Because the molecule has one chirality center, it is chiral. Chirality centers are sometimes marked with an asterisk ( ). 

Molecules without reflection symmetry (no a plane) are called dissymmetric or chiral. Chirality (from the Greek cheir , hand) is the property displayed by any object (e.g., a hand) which is nonsuperimposable on its mirror image. If aCB( > 1) is also absent the structure lacks all elements of symmetry and is called asymmetric (point group C,). A carbon atom bearing four different substituents (asymmetric carbon atom) is a classical example of this point group. 

Beginning with Pasteur s work in 1860 [4] the fields of stereochemistry and biology were dominated for almost nine decades by the phenomenon now called chirality. Chiral molecules are those for which a three-dimensional model of the molecule is not superimposable on the mirror image of the model. Since the operation determining the existence of chirality is reflection in a plane mirror, this... 

Two of the above products are chiral (chirality centers are starred). None of the products are optically active each chiral product is a racemic mixture. 

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