Racemates imprinted chiral phases

Figure 6.27 illustrates a sample chromatogram for racemic resolution on a MIP stationary phase, whilst Table 6.6 summarises the developments in the field of molecularly imprinted chiral separation technology. 

Despite this, there are examples of molecularly imprinted chiral stationary phases that are capable of resolving more than the raceme corresponding to the template. In these cases, minor structural differences are possible without compromising the separation. For example, a polymer imprinted with L-phenylalanine anilide efficiently separated the protected amino acids with different side chains or amide substituents [36]. Anilides of all aromatic amino acids were resolved as well as p-naphthylamides and -nitroanilides of leucine and alanine [40]. 

Figure 7 Chiral HPLC separation of 2-arylpropionic acid derivatives on nonimprinted (a) and (5)-naproxen-imprinted stationary phase (b). (1) Racemic ketoprofen, (2) racemic ibu-profen, (3) (R)-naproxen, (4) (5)-naproxen. Mobile phase, 20 mM phosphate buffer pH 3.2 -acetonitrile 1 + lv/v. Columns 100 x 4.6 mm. Flow rate, ImL/min. Detection, UV 254 nm. Reproduced from Ref. 45, with permission.

The most popular and commonly used chiral stationary phases (CSPs) are polysaccharides, cyclodextrins, macrocyclic glycopeptide antibiotics, Pirkle types, proteins, ligand exchangers, and crown ether based. The art of the chiral resolution on these CSPs has been discussed in detail in Chapters 2-8, respectively. Apart from these CSPs, the chiral resolutions of some racemic compounds have also been reported on other CSPs containing different chiral molecules and polymers. These other types of CSP are based on the use of chiral molecules such as alkaloids, amides, amines, acids, and synthetic polymers. These CSPs have proved to be very useful for the chiral resolutions due to some specific requirements. Moreover, the chiral resolution can be predicted on the CSPs obtained by the molecular imprinted techniques. The chiral resolution on these miscellaneous CSPs using liquid chromatography is discussed in this chapter. 

In spite of the development of more successful and reliable CSPs (Chaps. 2-8), these miscellaneous types of CSP have their role in the field of the chiral resolution also. The importance of these CSPs ties in the fact that they are readily available, inexpensive, and economic. Moreover, these CSPs can be used for some specific chiral resolution purpose. For example, the CSP based on the poly(triphenylmethyl methacrylate) polymer can be used for the chiral resolution of the racemic compounds which do not have any functional group. The CSPs based on the synthetic polymers are, generally, inert and, therefore, can be used with a variety of mobile phases. The development of CSPs based on the molecularly imprinted technique has resulted in various successful chiral resolutions. The importance and application of these imprinted CSPs lies in the fact that the chiral resolution can be predicted on these CSPs and, hence, the experimental conditions can be designed easily without greater efforts. Because of the ease of preparation and the inexpensive nature of these CSPs, they may be useful and effective CSPs for chiral resolution. Briefly, the future of these types of CSP, especially synthetic polymers and polymers prepared by the molecularly imprinted technique, is very bright and will increase in importance in the near future. 

The high selectivity of M IPs is demonstrated when an optically active compound is imprinted the resulting MIP will normally resolve the racemate. Numerous reports on MIP chiral stationary phases have appeared [184—188]. Chiral templates studied include amino acids [26, 29, 120, 139, 189-192], peptides [139, 192, 193], carbohydrates [58, 194, 195] and dmgs [127, 196]. 

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