Chirality/Chiral enantiomers

The three water ligands located at meridional positions of the J ,J -DBFOX/Ph aqua complexes may be replaced by another molecule of DBFOX/Ph ligand if steric hindrance is negligible. Based on molecular model inspection, the hetero-chiral enantiomer S,S-DBFOX/Ph looks like a candidate to replace the water ligands to form the heterochiral meso-2 l complex J ,J -DBFOX/Ph-S,S-DBFOX/... 

Let s return for a last look at Pasteur s pioneering work. Pasteur took an optically inactive tartaric acid salt and found that he could crystallize from it two optically active forms having what we would now call the 2R,3R and 2S,3S configurations. But what was the optically inactive form he started with It couldn t have been meso-tartaric acid, because meso-tartaric acid is a different chemical compound and can t interconvert with the two chiral enantiomers without breaking and re-forming chemical bonds. 

Recall from Section 1.9 that some molecules can exist as chiral enantiomers that are mirror images of each other. Although enantiomers may appear to be superficially identical, they may differ markedly in their metabolism and toxic effects. Much of what is known about this aspect of xenobiotics has been learned from studies of the metabolism and effects of pharmaceuticals. For example, one of the two enantiomers that comprise antiepileptic Mesantoin is much more rapidly hydroxylated in the body and eliminated than is the other enantiomer. The human cytochrome P-450 enzyme denoted CYP2D6 is strongly inhibited by quinidine, but is little affected by quinine, an optical isomer of quinidine. Cases are known in which a chiral secondary alcohol is oxidized to an achiral ketone, and then reduced back to the secondary alcohol in the opposite configuration of the initial alcohol. 

We consider a production of chiral enantiomers R and S from an achiral substrate A in a closed system. Actually, in the Soai reaction, chiral molecules are produced by the reaction of two achiral reactants A and B as A + R or A + B -> S. But in a closed system a substrate of smaller amount controls... 

When the initial system is sufficiently far from 0, the system approaches Si or S2 Both correspond to homochiral states with 0i OO = 1. When the initial configuration is close to the racemic state or the diagonal line r = s, the system approaches a racemic fixed point U4 at first. But, while the recycling process returns the chiral enantiomers back to achiral substrate, the majority enantiomer increases its population at the cost of the minority one along the flow curve diverging from U4 to Si or to S2, and eventually the whole system becomes homochiral. 

There is a theoretical study on the asymptotic shape of probability distribution for nonautocatalytic and linearly autocatalytic systems with a specific initial condition of no chiral enantiomers [35,36]. Even though no ee amplification is expected in these cases, the probability distribution with a linear autocatalysis has symmetric double peaks at 0 = 1 when ko is far smaller than k -,kototal number of all reactive chemical species, A, R, and S. This can be explained by the single-mother scenario for the realization of homo chirality, as follows From a completely achiral state, one of the chiral molecules, say R, is produced spontaneously and randomly after an average time l/2koN. Then, the second R is produced by the autocatalytic process, whereas for the production of the first S molecule the... 

A further shortcoming of the most commonly used topological indices is the inability to take into account stereo-specific properties of molecules, such as atomic chiralities, enantiomers (R-and S-isomers), and - and I J-diastereomers (Z- and E-isomers). Topological descriptors proposed to fill the gap to account for chirality and ZE-isomerism can be found in Schultz et al. (1995), de Julian-Ortiz et al. (1998), and Golbraikh et al. (2001) as well as in Lekishvili (1997 2001), and Golbraikh et al., (2002), respectively. It is probably a question of availability and time until the power of these descriptors is demonstrated and they begin to be used routinely in QSPR analysis. 

Which of the following is/are true regarding chirality a all drugs have chiral enantiomers b a symmetrical carbon atom is required c the presence of a humanized section is required d very few drugs are sold as single chiral enantiomers e none of the above. 

The procedure that we propose to enhance the concentration of a particulap enantiomer when starting with a racemic mixture, that is, to purify the mixture) is as follows [259], The mixture of statistical (racemic) mixture of L and irradiated with a specific sequence of three coherent laser pulses, as described below. These pulses excite a coherent superposition of symmetric and antisymmetric vibrational states of G. After each pulse the excited system is allowed to relax bg t to the ground electronic state by spontaneous emission or by any other nonradiativ process. By allowing the system to go through many irradiation and relaxatio cycles, we show below that the concentration of the selected enantiomer L or can be enhanced, depending on tire laser characteristics. We call this scenario lat distillation of chiral enantiomers. 

The chirality of (d) depends on the configuration at both of the chirality centers. The (R.R) and (5,5) isomers are chiral enantiomers the (R,S) isomer is an achiral meso compound. 

The hfac ligand forms very stable and volatile complexes, therefore there is an opportunity for their use in separation and metal vapor deposition reactions. An analogous reaction with PbO will lead to the formation of dimer complexes, which exist in the solid state as a chiral enantiomer connected with the third diglyme oxygen of one unit oriented toward the Pb center of the other unit. Despite the dimer strucmre, this complex is... 

Because one stereoisomer of 1,3-dibromocyclopentane is superimposable on its mirror image, there are only three stereoisomers, not four. A is an achiral meso compound and B and C are a pair of chiral enantiomers. A and B are diastereomers, as are A and C. 

Problem 9.36 The ds and trans isomers of 2,3-dimethyloxirane both react with OH to give 2,3-butanediol. One stereoisomer gives a single achiral product, and one gives two chiral enantiomers. Which epoxide gives one product and which gives two ... 

A dynamic H and NMR study revealed the atropisomerism of compound 130 (R = Pr ) which exists at room temperature as a pair of two, nonplanar, axially chiral enantiomers due to the steric interaction between the isopropyl and 1,2-dithiole groups. 

The electrochemical behavior of C76 has been easier to study than that of the other members of the higher fullerenes because it exists as a pair of chiral enantiomers with Do symmetry, and these are isolated as a 1 1 racemic mixture [23]. As shown in Table 4, up to six cathodic waves have been observed in a toluene/acetonitrile mixture at — 15°C, although the sixth reduction is irreversible [24]. In dichloromethane (DCM) and benzoni-... 

Several chiral substituted a-phenyl-alkyl alcohols, which were partially enantio-merically enriched, were successfully purified by this method after esterification with 9-anthroic acid optical purities higher than 95% were obtained.Fluorescence studies of racemic compounds show a characteristic a-type excimer emission, whereas the chiral enantiomer exhibits a typical y-type monomer emission. Figure 7 shows the spectra of l-(2-chlorophenyl)ethyl 9-anthroate for a crystalline sample of enantiomeric (1) and racemic (2) at 77 K. It has been demonstrated that in the absence of configurational disorder chiral impurities in the range of 1 % can be detected by this method. ... 

In some instances the carbonyls in pinacolic coupling reactions exhibit chirality (enantiomer) recognition, that is the ability of an enantiomer to recognize a molecule of like chirali and react exclusively... 

Three reviews of chiral molecules have appeared.52,53,26 The first of these discusses using 7r contacts from supported metal complexes to separate enantiomers by GC and HPLC.52 The second is useful to those reading Portugese.53 The third addresses changing the transport phase affinity of chiral enantiomers by adding chiral additives such as cyclodextrin or a chiral metal complex that form second-sphere contacts in the mobile phase.26... 

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