Chiral separations Subject

As in tic, another method to vaUdate a chiral separation is to collect the individual peaks and subject them to some type of optical spectroscopy, such as, circular dichroism or optical rotary dispersion. Enantiomers have mirror image spectra (eg, the negative maxima for one enantiomer corresponds to the positive maxima for the other enantiomer). One problem with this approach is that the analytes are diluted in the mobile phase. Thus, the sample must be injected several times. The individual peaks must be collected and subsequently concentrated to obtain adequate concentrations for spectral analysis. 

Chiral separation validation in lie may be accomplished by recovering the individual analyte spots from the plate and subjecting them to some type of chiruptical spectroscopy such as circular dichroism or optical roiary dispersion, Alternatively, the plates may be analyzed using a scanning densiloineier. 

As in ilc. another method to validate a chiral separation is to collect the individual peaks and subject them to some type of optical spectroscopy. 

Much progress has also been made in the development of methods for authenticity control of flavor compounds, such as chiral separations or isotope ratio analysis. However, a comprehensive discussion of this subject would have gone beyond the limits of this chapter. The interested reader is, therefore, referred to some recent articles published in this field [109-111],... 

Since Pasteur separated crystalline sodium ammonium tartrate manually in 1848, many researchers have worked on the subject of enantiomeric separation. In 1939 Henderson and Rule fully separated derivatives of camphor by column chromatography using lactose as a stationary phase material [1]. Gil-Av et al. [2] were able to separate amino acid derivatives on a polysiloxane-based stationary phase by gas chromatography (GC) in 1966. Since then many approaches for a successful distinction between enantiomers have been developed for capillary GC and liquid chromatography [3]. It is still a current topic for researchers searching for chiral separation with SciFinder [4] results in 812 hits and searching for chiral recognition leads to 285 hits for the year 2003 only. 

The bifunctional catalysts NAP-Mg-PdOs and NAP-Mg-OsW were separated subjected to tandem Heck-AD and AD-N-oxidation reactions in a single pot to obtain chiral diols, as shown in Scheme 5.14. 

Developments in chiral separations are also rapidly occurring and were the subject of a recent review by Ward. Whilst many applications utilize capillary electrophoresis, applications are also found using GC, LC, and supercritical fluid chromatography. 

Transition metals complexes of tartaric acid have been the subject of research in all the main areas of practical applications of chirality biochanistry and medicinal chan-istry [1], asymmetric catalysis [2-4], and chiral separations [5-9]. It is not surprising that the rapid development of the chemistry of vanadium in recent decades smoothly covered all of these areas by itself. The main attention has been devoted to the insulin-mimetic activity of vanadium complexes of tartaric acid [10,11]. Chirality of some insu-linomimetic dinuclear vanadyl(IV)-tartrate complexes has been considered as well, resulting in an interesting finding that both the complexes of naturally occurring L-tartaric acid and racemic tartaric acid are highly active [12]. 

A compromise among all the properties mentioned herein should be established, depending on the technique used and on the particular application. Preparative separation of enantiomers is still an open subject which requires further investigation in the search of new chiral selectors and techniques well adapted to large scale processes. 

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