Stationary phases cellulose derivatives

The Diacel columns can be used for the separation of a wide variety of compounds, including aromatic hydrocarbons having hydroxyl groups, carbonyls and sulfoxides, barbiturates, and P-blockers (35,36). There are presendy nine different cellulose derivative-based columns produced by Diacel Chemical Industries. The different columns each demonstrate unique selectivities so that a choice of stationary phases is available to accomplish a separation. 

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. 

Okamoto et al [85] performed the optical resolution of primaquine and other racemic drugs by high performance liquid chromatography using cellulose and amylose tris-(phenylcarbamate) derivatives as chiral stationary phases. Primaquine and other compounds were effectively resolved by cellulose and/or amylose derivatives having substituents such as methyl, tertiary butyl, or halogen, on the phenyl groups. 

These polysaccharide-based stationary phases appear to be the most useful in organic, bio-organic and pharmaceutical analysis. Of the above-mentioned derivatives three of them, namely cellulose tris-(3,5-dimethylphenylcarbamate), amylose tris-(3,5-dimethylphenylcarbamate) and cellulose tris-(4-methylbenzoate), have very complementary properties and numerous publications have demonstrated that they have been able to achieve the chiral resolution of more than 80% of the drugs currently available on the market. " These CSPs are known under the commercial names, Chiralcel OD-H , Chiralpak AD and Chiralcel OJ , respectively (Figure 4). Their very broad enantiorecognition range is also the... 

For example, cyclodextrins form chiral cavities which adsorb the corresponding enantiomers with different affinity while cellulose triacetate crystallizes in the form of helical substructures in which the enantiomers may be incorporated with different rates. For amino acid derived stationary phases there are two types of enantiomer differentiating interactions a brush-like hydrogen bond and dipole interaction plus a /[-complex donor or acceptor interaction with the aromatic residues in the amino acid. 

Okamoto, Y., Aburatani, R., and Hatada, K. (1987) Cellulose tribenzoate derivatives as chiral stationary phases for high-performance liquid chromatography, J. Chromatogr. 389, 95-102. 

Francotte, E.R. and Huynh, D. (2002) Immobilized halogenophenylcarbamate derivatives of cellulose as novel stationary phases for enantioselective drug analysis, J. Pharm. Biomed. Anal. 27, 421-429. 

Kubota, T., Yamamoto, C., and Okamoto, Y. (2003) Preparation of chiral stationary phase for HPLC based on immobilization of cellulose 3,5-dimethylphenylcarbamate derivatives on silica gel, Chirality 15, 77-82. 

The majority of preparative separations undertaken at Ultrafine have used chiral stationary phases based on either cellulose or amylose derivatives. In one project 500 mg of salts of both the (R,R)- and (S,S)-Formoterol (6) enantiomers were prepared using an OJ column with a resulting ee of >97%.45 A loading of 200 mg/mL on a semipreparative column was achievable without loss of resolution or purity relative to the racemate. 

Cellulose, starch, and their derivatives are commonly used as chromatographic stationary phases. They are, in principle, potential hosts for inducing CD activity in small molecules and could be used with effect for analysis in homogeneous media with chiroptical detection. An example might be the starch (amylose)-iodide complex [86]. Low aqueous solubility however is an obstacle to their general use in homogeneous solutions. Linear oligomers of maltose are more soluble than starch and could theoretically be used as alternatives to Cy, however they do not really compete in terms of the stability of the association complexes. 

HPLC was used to evaluate the enantiomeric resolution of dihydrope-pidine enantiomers (including nimodipine), using phenylcarbamates of polysaccharides as a chiral stationary phase [35]. A column (25 cm x 4.6 mm) packed with the arylcarbamate derivatives of amylase, cellulose, and xylem was used. Detection was effected using polarimetry at 435 nm. Using xylem bis-(3,5-dichlorophenylcarbamate) and a mobile phase (flow rate of 0.5 mL/min) of 0.1%, diethylamine in hexane-propan-2-ol (17 3) yielded separation of nimodipine. 

Figure 35 CEC enantioseparations of 2-(benzylsulfinyl)benzamide in capillaries packed with derivitized with differing amounts of the polysaccharide derivative cellulose tris(3,5-dichlorophenylcarbamate). The stationary phase contained (a) 4.8%, (b) 1.0%, and (c) 0.5% (w/w) of the chiral selector. (Reprinted from Ref. 1 56, with permission.)...

The polysaccharide stationary phases derived from cellulose and amylose are today among the most popular CSPs due to their versatility. 

A CSP consists of a chiral selector, which either alone constitutes the stationary phase or which has been immobilised to a solid phase. The chiral selector is a low molecular weight compound or a polymer, either synthetic or natural. A broad range of CSPs has been developed. Examples of CSPs that have been used successfully include polysaccharides, such as cellulose and its derivatives [6] and cyclodex-trins [7], and proteins, e.g. bovine serum albumin, aj-acid glycoprotein, cellulase, trypsin and a-chymotrypsin [8]. Several different synthetic polymers have also proven to be useful CSPs, for example the Blaschke-type CSPs (polyacrylamides and polymethacrylamides) [9] and the Pirkle-type CSPs [10]. 

Benzoic acid derivatives often contain amino, hydroxy, carboxy, and nitro groups. Analysis of substimted benzoic acids by thin layer chromatography was performed on silica gel, polyamide, and cellulose containing UF254 fluorescent indicator. For the mobile phase, different mixtures were used hexane-acetic acid hexane-ethyl acetate-formic acid chloroform-methanol-phosphoric acid cyclohexane-acetic acid benzene-ethanol etc. Because benzoic acid derivatives have similar retention parameters, their separation requires a thorough optimization of conditions (the nature of the stationary phase, the composition of the mobile phase, and the pH of the solutions). 

The enantiomeric purity of 2-arylpropionic acids used as pharmaceuticals was determined after derivatizing to the corresponding amides with various PAA and HAA. End analysis was by HPLC-UVD, with simultaneous measurement at 230 and 254 nm, using a cellulose-based stationary phase. The best derivative for routine determinations was chosen in each case after establishing the chromatographic parameters of the enantiomeric pair of derivatives197. 

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