Polysaccharide carbamate derivatives

Preweighed 1.0 mg samples of each polysaccharide carbamate derivative were submerged in aqueous solutions at three pH values 3.1 7.0 and 11.3. Three milliliter aliquots were withdrawn at periodic intervals and analyzed by ultraviolet spectroscopy. Typical results are shown in Figures 3 and 4 for pendant hydrolysis rates of carbamate derivatives of cellulose and chitin respectively as a function of pH. 

Cellulose, Amylose and Other Polysaccharide Carbamate Derivatives... 

Polysaccharide-coated CSP are prepared by coating of their benzoate or carbamate derivatives on substrate from a solution with the following evaporation of the solvent. These phases can be used under RP conditions with aqueous eluents or in normal-phase conditions with limited number of solvents to avoid dissolution of the coated layer [80, 81]. 

The nature and the position of the substituents introduced into the benzene ring of the carbamate derivatives essentially define the chiral recognition ability of these CSPs [82]. Higher long-term stability of polysaccharide phases was achieved with covalent bonding of polysaccharide to the surface of the support [83]. 

Okamoto and Kaida 99j as well as other research groups also reported the enan-tiodiscrimination ability of CSPs derived from other polysaccharides than cellulose or amylose, including chitosan derivatives 1109.155,1-56], heparin [174], and some others. In some cases chitosan, 3,5-dimethylphenyl carbamate derivatives have shown higher discrimination capability than the cellulose or amylose analogues. 

In a few cases, the option of preparing tailor-made CSPs for a particular racemic structure has been applied. For example, we prepared on an empirical basis a particular polysaccharide-based CSP for the separation of the enantiomers of the enantiomers of the LTD4 antagonist iralukast and of the antimalaria agent benflumethol [87]. These two racemic drugs were only poorly resolved on the commercially available polysacharide-based phases whereas an excellent separation was obtained on the carbamate derivative of cellulose obtained from cellulose and 3-chloro-4-methylphe-nylisocyanate. The prepared CSP was also used to perform pharmacokinetic studies. 

Phenylurethane (carbamate) derivatives of insoluble polysaccharides, produced by reaction with PhNCO in neutral organic solvents, are used in determination of molecular weight distributions (see Section 4.5.1) carbamates are acid stable and base labile and have better alternatives as protecting groups in synthesis. 

Aromatic carbamate derivatives of polysaccharides are today the most used CSP in chiral HPLC, but unfortunately, they are not ideal selectors for TLC because of the detection difficulty by UV light. However, this problem may be overcome in the future by the use of cycloalkylcarbamates [54]. 

Chemical stabilizers have been used to reduce the rate of oxygen-promoted degradation of polysaccharides at T>225°F. Methanol and sodium thiosulfate are the most commonly used (86). Sodium dithio-carbamate, alkanolamines, and thiol derivatives of imidazolines, thiazolines, and other heterocyclic compounds have also been tested for this application. Calcined dolomite (B7) and Cu(l) and Cu(ll) salts (88) have been reported to increase the thermal stability of HEC. 

Among optically active polymers, polysaccharide derivatives are particularly valuable. Polysaccharides such as cellulose and amylose are the most readily available optically active polymers and have stereoregular sequences. Although the chiral recognition abilities of native polysaccharides are not remarkable, they can be readily converted to the esters and carbamates with high chiral recognition abilities. The chiral recognition mechanism of these derivatives has been clarified to some extent. 

Lithium salts in organic solvents with high solubility parameters have been previously used to dissolve strongly hydrogen-bonded polyamidesfciy and polysaccharides /for viscosity studies and for preparation of films or fibers. We are not aware of any previous attempts to utilize these solvent systems to prepare ester or carbamate polysaccharide derivatives... 

Table I lists physical data for a number of the carbamate and ester derivatives of cellulose, chitin, amylose, amylopectin, and dextran synthesized as described in the Experimental Section. The solubility of the polysaccharide starting materials as well as that of the produced derivatives allows for macromolecular characterization through techniques including UV, NMR, IR, high pressure liquid chromatography, etc.

Derivatives (Table I) formed by the reaction of the chosen polysaccharide with an isocyanate or acid chloride are carbamates and esters respectively. However, chitin or poly[(1+4)(N-acetyl-2-amino-2-deoxy-f3-glucopyranose)] actually used in this experiment contains approximately 16% free amine groups which can form urea and amide derivatives with the above reagents. 

More recent developments in the field of the Pirkle-type CSPs are the mixed r-donor/ r-acceptor phases such as the Whelk-Of and the Whelk-02 phases.The Whelk-Of is useful for the separation of underiva-tized enantiomers from a number of families, including amides, epoxides, esters, ureas, carbamates, ethers, aziridines, phosphonates, aldehydes, ketones, carboxylic acids, alcohols and non-steroidal anti-inflammatory drugs.It has been used for the separation of warfarin, aryl-amides,aryl-epoxides and aryl-sulphoxides. The phase has broader applicability than the original Pirkle phases. The broad versatility observed on this phase compares with the polysaccharide-derived CSPs... 

More often, carbamates of dextran are prepared by one-step conversion of the polysaccharide with isocyanates or isothiocyanates. This path is especially useful for the conjugation of fluorescent dyes. Fluorescent-marked dextrans are commercial products today. Fluorescent dextran derivatives with different molecular weights and substituents are available as invaluable tracers for studies on microcirculation and vascular permeability in health and disease [8]. Fluorescein isothiocyanate (FITC) dextran (actually the thiocarba-mate) has been well established in this area of research since the 1970s [358]. The isothiocyanate of fluorescein is covalently bound to dextran leading to... 

Additional carbamate-modified polysaccharides, (I), were prepared by Ohnishi [1] and used in separating enantiomeric racemates of both non-aromatic heterocyclics and aromatic derivatives. 

In a broad survey of various analytes about 80 of the compounds tested could be separated on the following four CSPs amylose tris(3,5-dimethylphenyl carbamate) (commercialized as Chiralpak AD), amylose tris[(S)-a-methylbenzyl carbamate] (Chi-ralpak AS), cellulose tris(3,5-dimethylphenyl carbamate) (Chiralcel OD), and cellulose tris(4-methyIbenzoate) (Chiralcel OJ), all coated onto the surface of macroporous y-aminopropyl silica. These columns behave excellently in non-aqueous HPLC mode, but they seem to be also well suitable for SFC separations. Other less widely used, but also commercially available polysaccharide-derived CSPs, e.g. cellulose triacetate, cellulose tribenzoate, and cellulose tricinnamate, or cellulose tris(phenylcarbamate), cellulose tris(4-chlorophenylcarbamate), cellulose tris(4-methylphenylcarbamate) selectors... 

Expansion of these investigations to other types of polysaccharide derivatives led to the development of silica-coated versions of cellulose and amylose carbamates and benzoates [100-102]. These CSPs showed particularly versatile chiral recognition profiles, resolving an extremely broad assortment of chiral analytes. Several of these CSPs have been commercialized by Daicel Chemical Industries, Ltd., the most important ofwhich are depicted in Fig. 7.5. 

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. 

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