Indirect HPLC Enantioseparations

As mentioned above, the different solubility of diastereomers offers a challenge for their separation by diastereomeric crystallization. The indirect HPLC separation of enantiomers relies on their different interaction with a (achiral) stationary phase. The interaction of the enantiomeric pair (R, S) with one (let us assume it to have S configuration) enantiomer of a chiral derivatizing reagent may be expressed as follows  

The diastereomeric compounds RS and SS obtained by this way may be resolved using achiral HPLC columns, which are less expensive. In addition, the chiral derivatizing reagents available in both configurations allow us to revert the migration order of enantiomers. These are advantages of indirect enantioseparation techniques. 

The useful chiral derivatizing reagents (CDR) for indirect enantioseparations are summarized in a review article by Lindner [104]. 

The number of reported indirect enantioseparations is currently decreasing compared with direct enantioseparations on CSP. The reason for this is on the one hand the development of more universal, stable and effective CSPs for direct enantioseparations, and on the other hand the inherent limitations of indirect enantioseparation techniques. 

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Direct HPLC enantioseparation techniques, which are free of many disadvantages of GC, indirect and chiral mobile phase HPLC methods, have gained unequivocal prevalence in bio-analytical studies. Several methods have been advanced so much that they allow enantiose-lective determination not only of the parent chiral drugs but also of their pharmacologically relevant metabolites [121]. As already mentioned above, a direct injection of biofluids offers several advantages in terms of analysis time and sample recovery. Precolumns packed with achiral or chiral packings, or with the recently developed so-called restricted-access packing materials, may be useful in this case. 

Chromatographic separation of two antimers, most often referred to as enantiosep-aration, can be carried out following either a direct or an indirect strategy. It is perhaps noteworthy that this duality of alternative options is characteristic not only of enantioseparations by means of thin layer chromatography (TLC), which is the primary subject of this book, but also of high-performance liquid chromatography (HPLC) and gas chromatography (GC). 

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