Stereoisomers chiral recognition

Dissolving metal reduction of camphor produces a mixture of bor-neol, isobomeol, and pinacol coupling products (Scheme 39). The ratios of the stereoisomers are affected profoundly by whether the starting ketone is enantiomerically pure or racemic, implying the chirality recognition at the stage of ketyl radical (65). 

Chiral recognition The ability of a chiral host to discriminate between the stereoisomers of chiral guests. Can refer to thermodynamic factors (e.g. stability constants for com-plexation of enantiomeric guests) or kinetic factors (e.g. rates of reaction or transport of enantiomeric guests). 

Table 8.3 lists some examples of optodes with simple receptor layers. Among them are calixarenes, which are interesting for ISEs also, since they are highly selective ligands for cations (Chap. 7, Sect. 7.1.2). Above all, calixarenes are highly efficient receptors for certain organic analytes. Optical properties of such systems have been used successfully for chiral recognition of stereoisomers. As an example, when a special calixarene was combined with the... 

It is because the chiral selector was a relatively simple molecule having naturally its stereoisomer that the chiral recognition mechanism could be fully established in the case of DNB-derivatized amino acid enantiomer separation. Most chiral selectors are very complicated molecules making extremely difficult to predict a priori a chiral recognition mechanism. 

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. 

The four different groups attached to a chiral carbon can be different elements, isotopes, or functional groups, and chiral centers can be present in bodi open-chain molecules or cyclic compounds. The recognition of chirality and chiral centers in molecules is an important step in determining the numbers of stereoisomers that are possible for a given compound. 

Chirality of derivatized cyclodextrin was used for recognition of stereoisomers. Phenylazobenzoyl modified y-cyclodextrin was anchored onto silica gel used as stationary phase in HPLC and photoresponsive chromatographic behavior of dansyl amino acid enantiomers was studied [64],... 

Despite the long recognition of chirality in POPs, it has only been fairly recently that studies of chiral POPs have been possible, after the introduction of analytical technology for stereoisomer separation by enantioselective gas chromatography (GC), high performance liquid... 

In 1980, K. B. Sharpless (then at the Massachusetts Institute of Technology, presently at The Scripps Research Institute) and co-workers reported a method that has since become one of the most valuable tools for chiral synthesis. The Sharpless asymmetric epoxidation is a method for converting allylic alcohols (Section 11.1) to chiral epoxy alcohols with very high enan-tioselectivity (i.e., with preference for one enantiomer rather than formation of a racemic mixture). In recognition of this and other work in asymmetric oxidation methods (see Section 8.16A), Sharpless received half of the 2001 Nobel Prize in Chemistry (the other half was awarded to W. S. Knowles and R. Noyori see Section 7.14). The Sharpless asymmetric epoxidation involves treating the allylic alcohol with tert-butyl hydroperoxide, titanium(IV) tetraisopropoxide [Ti(0—/-POJ, and a specific stereoisomer of a tartrate ester. (The tartrate stereoisomer that is chosen depends on the specific enantiomer of the epoxide desired). The following is an example ... 

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