Asymmetry, center

Initially, only the nitrile of L-mannonic acid was found on addition of hydrocyanic acid to L-arabinose this acid retained the original arabinose in the asymmetry centers 3, 4, and 5. The new center of asymmetry created in this way at C-2 was first considered by Fischer to be racemic. This would have meant that the L-mannonic acid should be a partial racemate however, attempts to separate it into two stereoisomers failed. The idea of a partial racemate led to the question as to whether L-mannonic acid and D-gluconic acid (which are enantiomorphous on carbon atoms 3, 4, and 5) could form such a partial racemate (which would still be optically active). Such a compound could not be isolated, but negative results have only limited values as proof. ... 

In spite of the fact that asymmetric carbon atoms1 are formed in the synthesis of vinyl polymers, head-to-tail linear vinyl polymers showing remarkable optical activity in solution cannot be prepared by the usual polymerization processes unless asymmetry centers are already present in the monomers (6, 13). In fact, by considering the principal types of vinyl polymers prepared up to now, the ideal isotactic macromolecules, having such terminal groups as to render both terminal segments of the macromolecule chemically identical, possess a symmetry plane and therefore the polymers cannot show optical activity they might be considered as a peculiar type of meso compounds. 

An additional and frequently dominating contribution to optical activity can result from the conformation in helix-forming polymers. A helix is either left- or right-handed. In the immediate vicinity of an asymmetry center, therefore, an additional asymmetry, and consequently an additional optical activity, is produced. For this reason, a helix should always be optically active. However, since a solution consists of many molecules, it is only possible for the helices to make a contribution to the optical activity if the helical rotation is the same in all the molecules, or if there is a larger proportion of one helix present. 

Structures [VIII] and [IX] are not equivalent they would not superimpose if the extended chains were overlaid. The difference has to do with the stereochemical configuration at the asymmetric carbon atom. Note that the asymmetry is more accurately described as pseudoasymmetry, since two sections of chain are bonded to these centers. Except near chain ends, which we ignore for high polymers, these chains provide local symmetry in the neighborhood of the carbon under consideration. The designations D and L or R and S are used to distinguish these structures, even though true asymmetry is absent. 

Chiral separations are concerned with separating molecules that can exist as nonsupetimposable mirror images. Examples of these types of molecules, called enantiomers or optical isomers are illustrated in Figure 1. Although chirahty is often associated with compounds containing a tetrahedral carbon with four different substituents, other atoms, such as phosphoms or sulfur, may also be chiral. In addition, molecules containing a center of asymmetry, such as hexahehcene, tetrasubstituted adamantanes, and substituted aHenes or molecules with hindered rotation, such as some 2,2 disubstituted binaphthyls, may also be chiral. Compounds exhibiting a center of asymmetry are called atropisomers. An extensive review of stereochemistry may be found under Pharmaceuticals, Chiral. 

The asterisk signifies an asymmetric carbon. AH of the amino acids, except glycine, have two optically active isomers designated D- or L-. Isoleucine and threonine also have centers of asymmetry at their P-carbon atoms (1,10). Protein amino acids are of the L-a-form (1,10) as illustrated in Table 1. 

The functional reaction center contains two quinone molecules. One of these, Qb (Figure 12.15), is loosely bound and can be lost during purification. The reason for the difference in the strength of binding between Qa and Qb is unknown, but as we will see later, it probably reflects a functional asymmetry in the molecule as a whole. Qa is positioned between the Fe atom and one of the pheophytin molecules (Figure 12.15). The polar-head group is outside the membrane, bound to a loop region, whereas the hydrophobic tail is... 

Reductive alkylation with chiral substrates may afford new chiral centers. The reaction has been of interest for the preparation of optically active amino acids where the chirality of the amine function is induced in the prochiral carbonyl moiety 34,35). The degree of induced asymmetry is influenced by substrate, solvent, and temperature 26,27,28,29,48,51,65). Asymmetry also has been obtained by reduction of prochiral imines, using a chiral catalyst 44). Prediction of the major configurational isomer arising from a reductive alkylation can be made usually by the assumption that amine formation comes via an imine, not the hydroxyamino addition compound, and that the catalyst approaches the least hindered side (57). 

This reviews contends that, throughout the known examples of facial selections, from classical to recently discovered ones, a key role is played by the unsymmetri-zation of the orbital phase environments of n reaction centers arising from first-order perturbation, that is, the unsymmetrization of the orbital phase environment of the relevant n orbitals. This asymmetry of the n orbitals, if it occurs along the trajectory of addition, is proposed to be generally involved in facial selection in sterically unbiased systems. Experimentally, carbonyl and related olefin compounds, which bear a similar structural motif, exhibit the same facial preference in most cases, particularly in the cases of adamantanes. This feature seems to be compatible with the Cieplak model. However, this is not always the case for other types of molecules, or in reactions such as Diels-Alder cycloaddition. In contrast, unsymmetrization of orbital phase environment, including SOI in Diels-Alder reactions, is a general concept as a contributor to facial selectivity. Other interpretations of facial selectivities have also been reviewed [174-180]. 

In normal carbonium-ion chemistry, reaction proceeds from a precursor with a tetrahedral carbon capable of asymmetry hence, the stereochemistry of displacement in an aliphatic system can be ascertained by observation of the fate of the chiral center from reactant to product. An ethylenic system, of course, has no such chiral center, and hence there can be no change in optical configuration as the reaction proceeds. However, the stereochemistry of vinylic displacement and hence the symmetry and geometry of the intermediate can be... 

Figure 4.51. Distribution of experimental data. Six experimental formulations (strengths 1, 2, resp. 3 for formulations A, respectively B) were tested for cumulative release at five sampling times (10, 20, 30, 45, respectively 60 min.). Twelve tablets of each formulation were tested, for a total of 347 measurements (13 data points were lost to equipment malfunction and handling errors). The group means were normalized to 100% and the distribution of all points was calculated (bin width 0.5%, her depicted as a trace). The central portion is well represented by a combination of two Gaussian distributions centered on = 100, one that represents the majority of points, see Fig. 4.52, and another that is essentially due to the 10-minute data for formulation B. The data point marked with an arrow and the asymmetry must be ignored if a reasonable model is to be fit. There is room for some variation of the coefficients, as is demonstrated by the two representative curves (gray coefficients in parentheses, h = peak height, s = SD), that all yield very similar GOF-figures. (See Table 3.4.)...

Both the alkyl and the acyl have two asymmetric centers the iron and the )3-carbon. Accordingly, each composition exists as a pair of racemic mixtures. When the two diastereomeric racemic mixtures of the acyl are separately subjected to the decarbonylation in Eq. (54), only partial (<50%) epimerization is observed by NMR spectroscopy. This indicates that in the reactive intermediate, presumably three-coordinate CpFe(PPh3)COCH2-CH(Me)Ph, the iron substantially retains its asymmetry, and is therefore not planar. 

The views perpendicular to b, on the other hand, show considerably more asymmetry. The lack of a two-fold axis in the crystal is most evident in the view along c. In all three of the PbTX-1 structures studied, a short intermolecular distance is calculated between the 012 hydroxyl and the 05 ether oxygens. The two molecules involved in the short contact are related by a C-centerihg (or pseudo C-centering) operation. In the PbTX-1 dimethyl acetal crystal, the 012-05 distance is 2.87 A, in PbTX-1 it is 2.86 A, and in dihydro PbTX-1 it is 2.84 A. Dihydro... 

The ability of cobalt(II), nickel(II), and copper(II) to exhibit a greater tendency than Zn(II) towards bidentate coordination is further illustrated by structural comparisons within a series of bridging carbonate complexes (188). For example, of the complexes [TpPr 2]M 2(/x-C03) (M = Mn, Fe, Co, Ni, Cu, Zn), only the zinc derivative does not exhibit bidentate coordination at both metal centers (151,153). Furthermore, the carbonate ligand in the complexes [TpPr 2]M 2(/x-C03) (M = Mn, Fe, Co, Ni, Cu) also exhibits varying degrees of asymmetry that closely parallel the series of nitrate complexes described earlier (Fig. 47 and Table IX). 

In organic stereochemistry the terms center of chirality or center of asymmetry are often used usually they refer to an asymmetrically substituted C atom. These terms should be avoided since they are contradictions in themselves a chiral object by definition has no center (the only kind of center existing in symmetry is the inversion center). 

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