Chiral stationary phases cellulose esters

The potential for use of chiral natural materials such as cellulose for separation of enantiomers has long been recognized, but development of efficient materials occurred relatively recently. Several acylated derivatives of cellulose are effective chiral stationary phases. Benzoate esters and aryl carbamates are particularly useful. These materials are commercially available on a silica support and imder the trademark Chiralcel. Figure 2.4 shows the resolution of y-phenyl-y-butyrolactone with the use of acetylated cellulose as the adsorbent material. 

The ester and carbamate derivatives of cellulose and amylose are among the most successful and versatile chiral stationary phases for liquid and supercritical fluid chromatography [1,4,28,107-109]. These phases are prepared by reaction of the poly(saccharide) with an acid chloride (ester derivative) or phenylisocyanate (carbamate... 

Before synthetic chiral stationary phases were developed, attempts were made to use naturally occurring chiral materials for the stationary phase. Quartz, wool, lactose and starch were inadequate but triacetylated cellulose has met with some success. The synthetic stationary phases introduced by Pirkle are able to interact with solute enantiomers in three ways, one of which is stereochemically dependent. Typically these interactions are based on hydrogen bonding, charge transfer (rc-donoi -acceptor based) and steric repulsive types. An independent chiral stationary phase therefore consists of chiral molecules each with three sites of interaction bound to a silica (or other) support. Early work in this area demonstrated that 5-arginine bound to Sephadex would resolve 3,4-dihydroxy-phenylalanine, and that direct resolution of chiral helicenes could be accomplished with columns packed with 2-(2,4,5,7-tetranitro-9-fluorenylideneaminoxy)-propionamide or tri-P-naphthol-diphosphate amide. Amino acid esters have also been resolved with a silica bound chiral binaphthyl crown ether, but better separations are achieved with A-acylated amino acid derivatives with amino-acid derived chiral stationary phases. 

A side effect of a lack of temperature control is that changes can alter the refractive index of the mobile phase, causing basehne disturbances and reducing sensitivity The problem is principally with refractive index detection [39], but it can also influence spectroscopic detectors and their light path can be distorted. Temperature has also been reported to alter the nature of some stationary phases. For example, it caused a change in the chiral selectivity of the resolution of dihydropyrimidone acid and its methyl ester on amylose and cellulose stationary phases [40],... 

The derivatised glucose can act as a chiral site and result in diastereomeric interactions with enantiomers which together with the steric fit requirements within the cavity and the different interactions with the cellulose strands provides the basis for enantioselective interaction and subsequent resolution. The acetate, benzoate and phenylcarbamate glucose ester give superior resolution and selectivity compared with the parent material. Hydrophobic mobile phases are most commonly encountered though aqueous based eluants can be used with many versions of these materials. These stationary phase packings have been used to separate a wide range of pharmaceutical compounds [80]. 

Both cellulose and amylose contain 5 chiral centers per unit and thus the polymeric material offers a large number of chirally interactive centers and a high probability of interaction. There are basically two common types of cellulose and amylose derivatives that are used as stationary phases. The first type are simple esters as shown below... 

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