Recognition, enantiomers

Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound has two chiral centers but both centers have the same four substituents attached, the total number of isomers is three rather than four. One isomer of such a compound is not chiral because it is identical with its mirror image it has an internal mirror plane. This is an example of a diaster-eomer. The achiral structure is denoted as a meso compound. Diastereomers have different physical and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound. 

Chiral active pharmaceutical ingredients, 18 725-726. See also Enantio- entries Chiral additives, 6 75—79 Chiral alcohols, synthesis of, 13 667-668 P-Chiral alcohols, synthesis of, 13 669 Chiral alkanes, synthesis of, 13 668-669 Chiral alkenes, synthesis of, 13 668—669 Chiral alkoxides, 26 929 Chiral alkynes, synthesis of, 13 668-669 Chiral ammonium ions, enantiomer recognition properties for, 16 790 Chiral ansa-metallocenes, 16 90 Chiral auxiliaries, in oxazolidinone formation, 17 738—739... 

While the distinct amino acid residues have mostly only a modulating effect (see Table 1.9) (e.g., FMOC-protected amino acids), the type of protection group or derivative formed decides on the molecular and chiral recognition mechanism and hence on the obtained elution order as well as the level of enantiomer recognition (i.e., magnitudes of a-values) that can be afforded. From a practical point of view, we may distinguish between two groups of IV-derivatives ... 

Enantiomer recognition is a general principle in chemistry. Molecular recognition is achieved by numerous electronic and steric factors including chirality. This is also the case among molecules with the same atomic composition and connectivity. As illustrated in Scheme 27, chiral (/ ,/ )- or (5,5)-tartaric acid may be seen as a homochiral dimer of the / - or 5-pyramidal radicals, respectively meso tartaric acid is a result... 

Reactivity. Enantiomer recognition in solution results in various intriguing stereochemical outcomes for organic reactions. The molecular interaction may be direct or may occur by way of some other atoms or molecules. 

Vegvari A, Fdldesi A, Heten)d C, Kochegarova O, Schmid MG, and Kudirkaite V. A new easy-to-prepare homogeneous continuous electrochromatographic bed for enantiomer recognition. Electrophoresis 2000 21 3116. 

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... 

Chankvetadze, B.,Yamamoto, C., Okamoto, Y. Extremely high enantiomer recognition in HPLC separation of racemic... 

Appropriate substrate-catalyst matching can achieve interesting enantiomer recognition. Chemical kinetic resolution is observed during hydrogenation of 3-substituted itaconate esters with Rh-dipamp catalysts , and of various prochiral secondary alcohols with binap . 

The simplicity of use, the relatively low cost and the broad range of possible guest molecules (small organic molecules, ions but also biological macromolecules) have since led to the important development of this technique, as illustrated by the increasing numbers of publications over recent years [17-25]. The fields of application of these imprinted polymer networks are very diverse. We can mention chromatographic supports (particularly for the separation of enantiomers) recognition elements in the preparation of specific sensors, catalysts, systems for stereospecific synthesis, and selective adsorbents. 

SCHEME 24. Enantiomer recognition of zinc alkoxides homochiral versus hetero-chiral dimerization. 

Naemura, K. Ogasahara. K. Hirose, K. Tobe. Y. Preparation of homochiral azophenolic crown ethers containing l-phenylethane-1.2-diol and 2,4-dimethyl-3-oxapentane-l,5-diol as a chiral subunit Enantiomer recognition behaviour towards chiral 2-aminoetlianol derivatives. Tetrahedron Asymmetry 1997. 8 (1). 19-22. 

---