Racemization of enantiomers

As well as selectivity changes at low temperatures, such as those reported by Sander and Wise [78,79] for the separation of PAHs (Figure 18-13), subambient column temperatures can also alter chromatographic separations, by reducing the rate of the racemization of enantiomers and structural isomeri-... 

Racemic compounds, or dl pairs, arise from (a) deliberate racemization of enantiomers by interconversion (the mechanism is not usually significant), (b) mixing the enantiomers in a 1 1 molar ratio, and (c) synthesis in the absence of a biassing influence that would cause one enantiomer to predominate. 

Extended version of Payne-type inversion involving aUylic alcohol was appUed to the synthesis of the polar pharmacophoric subunit of (+)-scyphostatin (Scheme 5.45) [122]. Although this is not DKR of enantiomers, the stereoinversion process is conceptually unique and can be applied to racemization of enantiomer. 

Figure 9.2 Racemization of enantiomers as an example of a chemical reaction. The associated entropy production and the time variation of A are shown in (a) and (b). State functions A and G as functions of are shown in (c) and (d)...

As discussed in the previous section, isomers or impurities can be successfully removed from a host material, if solubilities are sufficiently different. Therefore, a worst-case scenario for any impurity removal or isomeric separation is the resolution of a racemate of enantiomers, since enantiomers have equivalent physical and chemical properties in an achiral medium (ordinary solvent) and hence the same solubility (39,55). Furthermore, 50% of the sample can be regarded as impurity. 

Steinreiber J, Faber K, Griengl H. De-racemization of enantiomers versus de-epimerization of diastereomers-classification... 

From 1 Steinreiber, K. Faber, H. Griengl, De-racemization of enantiomers versus de-epimerization of diastereomers-classification of dynamic kinetic asymmetric transformations (DYKAT), Chemistry 14 (2008) 8060. Copyright 2008 Wiley). 

Fig. 8.33 DYKAT of 1,3-diols via lipase-catalyzed acyl-transfer in combination with Ru-catalyzed epimerization of hydroxyl groups. G=chiral carbon, convertible for equilibration and acyl migration, but not for the irreversible step H=chiral carbon, convertible for equilibration, acyl migration and the irreversible step l=chiral carbon, convertible for acyl migration, stable chirality. (From J. Steinreiber, K. Faber, H. Griengl, De-racemization of enantiomers versus de-epimerization of diastereomers-chssification of dynamic kinetic asymmetric transformations (DYKAT), Chemistry 14 (2(X)8), 8060. Copyright 2008 Wiley).

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