Polysaccharide-type packing

Mourier s report was quickly followed by successful enantiomeric resolutions on stationary phases bearing other types of chiral selectors, including native and deriva-tized cyclodextrins and derivatized polysaccharides. Many chiral compounds of pharmaceutical interest have now been resolved by packed column SFC, including antimalarials, (3-blockers, and antivirals. A summary is provided in Table 12-2. Most of the applications have utilized modified CO, as the eluent. 

Chiral separations result from the formation of transient diastereomeric complexes between stationary phases, analytes, and mobile phases. Therefore, a column is the heart of chiral chromatography as in other forms of chromatography. Most chiral stationary phases designed for normal phase HPLC are also suitable for packed column SFC with the exception of protein-based chiral stationary phases. It was estimated that over 200 chiral stationary phases are commercially available [72]. Typical chiral stationary phases used in SFC include Pirkle-type, polysaccharide-based, inclusion-type, and cross-linked polymer-based phases. 

In contrast, CSPs have achieved great repute in the chiral separation of enantiomers by chromatography and, today, are the tools of the choice of almost all analytical, biochemical, pharmaceutical, and pharmacological institutions and industries. The most important and useful CSPs are available in the form of open and tubular columns. However, some chiral capillaries and thin layer plates are also available for use in capillary electrophoresis and thin-layer chromatography. The chiral columns and capillaries are packed with several chiral selectors such as polysaccharides, cyclodextrins, antibiotics, Pirkle type, ligand exchangers, and crown ethers. 

Chiral columns are packed with stereo-specific sorbents for the separation of stereoisomers in the sample.27 Many chiral columns operate in hydrophilic interaction mode (e.g., Pirkle-type) whereas others are used in reversed-phase mode (proteins, macrocyclic antibiotics, polysaccharides, etc.). The use of these columns is critical in the development of chiral drugs. Most chiral columns are quite expensive and many older chiral columns have low efficiencies and limited lifetimes. Examples of chiral separations are shown in Chapter 6. Alternately, convention columns can also be used for chiral separations using a chiral selector in the mobile phase.27... 

One important type of chiral packing material is derivatized polysaccharides, which provide a chiral lattice, but separation is improved by the addition of structural features that enhance selectivity. One group of compounds includes aroyl esters and carbamates, which are called Chiralcels (also spelled Chiracel) two of the most important examples are the 4-methylbenzoyl ester, called Chiralcel OJ, and the 3,5-dimethylphenyl carbamate, called Chiralcel OD. There is a related series of materials derived from amylose rather than cellulose, which have the trade name Chiralpak. 

They can be employed in medical, packing, paper, food, mechanical engineering, textile, electronic and other applications. The composites were evaluated for crystallinity, rheology absorptivity, biodegradabiUty and mechanical, gelling, pasting, filmforming, adhesive, antimicrobial properties, optical, separation, as well as water repellency, dye uptake, water vapor permeability, and flre-retardancy. In addition to food applications, composites based on more than two types of polysaccharides have rarely been used and many possible combinations remain unexplored (Table 6.3). 

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