Chromatography using chiral media

TA2909). The two anfi-isomers were separated using chiral medium pressure liquid chromatography (MPLC). 

Another means of resolution is to use a chiral material in a physical separation. Currently, many resolutions are done using medium- or high-pressure chromatography with chiral column-packing materials. Resolution by chromatography depends upon differential adsorption of the enantiomers by the chiral stationary phase. Differential adsorption occurs because of the different fit of the two enantiomers to the chiral adsorbent. Figure 2.7 shows such a separation. Topic 2.1 provides additional detail on several types of chiral stationary phases. 

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. 

The retention of the enantiomers in the column arises mainly from the equilibrium between the chiral selector selectand. A large excess of chiral additive causes the equilibrium to shift to the association side. An increase in the polarity of the medium decreases the strength of the hydrogen bonding between the selectand and the selector and shifts the equilibrium towards the dissociation side. Subsequently, the same selector was bound to a silica support and packed into an HPLC column it was also incorporated into a polysiloxane backbone and used as a chiral phase in gas chromatography in a similar manner previously used for Chirasil-Val [40,41]. 

Chromatography can also be used to separate chiral forms of molecules. Given the necessity for a chiral environment to differentiate enantiomeric forms, resolution can only be achieved where the chromatographic medium is chiral and/or the eluent is chiral. As... 

---