Amylose dimethylphenyl carbamate

Y. Bereznitski, R. Lobrutto, N. Variancaval, R. Thompson, K. Thopmpson, R Sajonz, L. S. Crocker, J. Kowal, D. Cai, M. Journet, T. Wang, J. Wyvratt, and N. Grinberg, Mechanistic aspects of chiral discrimination on an Amylose tris(3,5-dimethylphenyl)carbamate. Enantiomer 1 (2002), 305. 

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... 

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. 

J. M. Characterization of a thermally induced irreversible conformational transition of amylose tris(3,5-dimethylphenyl-carbamate) chiral stationary phase in enantioseparation of dihydropyrimidinone acid by quasi-equilibrated liquid chromatography and solid-state NMR, Anal. Chem., 2003, 75, 5877-5885. 

Yashima, E., Fukaya, H., Okamoto,Y. (3,5-Dimethylphenyl)carbamates of cellulose and amylose regioselectively bonded to silica gel as chiral stationary phases for high-performance liquid chromato-... 

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. 

Guard column 25 x 4.6 10 pm Chiralpak AD amylose tris-3,5-dimethylphenyl carbamate... 

SMB parameters Cellulose tris(p-methyl- benzoate) beads Poly(N-acryloyl-amino acid ester) silica CSP Cellulose tris(p-methylbenzoate) coated on silica (Chiralcel OJ) Amylose tris(3,5-dimethylphenyl-carbamate) coated on silica (Chiralpak AD)... 

The popularity of the poly(saccharide) derivatives as chiral stationary phases is explained by the high success rate in resolving low molecular mass enantiomers. It has been estimated that more than 85% of all diversely structured enantiomers can be separated on poly(saccharide) chiral stationary phases, and of these, about 80% can be separated on just four stationary phases. These are cellulose tris(3,5-dimethylphenyl carbamate), cellulose tris(4-methylbenzoate), amylose tris(3,5-dimethylphenyl carbamate), and amylose tris(l-phenylethyl carbamate). Typically, n-hexane and propan-2-ol or ethanol mixtures are used as the mobile phase [111]. Both the type and concentration of aliphatic alcohols can affect enantioselectivity. Further mobile phase optimization is restricted to solvents compatible with the stationary phase, such as ethers and acetonitrile, as binary or ternary solvent mixtures, but generally not chloroform, dichloromethane, ethyl acetate, or tetrahydrofuran. Small volumes of acidic (e.g. tri-fluoroacetic acid) or basic (n-butylamine, diethylamine) additives may be added to the mobile phase to minimize band broadening and peak tailing [112]. These additives, however, may be difficult to remove from the column by solvent rinsing to restore it to its original condition. 

Zhang, T, Kientzy, Ch., Franco, P Ohnishi, A., Kagamihara, Y, Kurosawa, H, (2005b) Solvent versatility of immobilized 3,5-dimethylphenyl-carbamate of amylose in enantiomeric separations by HPLC. /. Chromatogr., 1075, 65-75... 

In practice, it has been found that certain derivatives e.g. tris (3,5-dimethylphenyl carbamate) render the coating less culnerable to solvent dissolution. As a consequence this stationary phase can be used with buffered methanol/water or acetonitrile/water with certain care being taken. Therefore, the tris(3,5-dimethylphenyl carbamate) derivatives of both cellulose and amylose can, with caution, be used in the reversed phase mode. As example of a chiral separation using the different modes is shown in figure 8.6. 

The most effective cellulose and amylose based chiral stationary phases are those derivatized with the various substituted tris(3,5-dimethylphenyl carbamates). Okamoto et al. [10], carried out an extensive study on the effect of the different substituent groups on the chiral selectivity of the stationary phase to a range of protected amino acid derivatives. An example of one of their separations is given in figure 11.12. 

Okamoto Y, Aburatani R, Eukumoto T, Hatada K (1987) Chromatographic resolution. XVII. Useful chiral stationary phases for HPLC. Amylose tiis(3,5-dimethylphenyl carbamate) and tiis(3,5-dichlorophenylcarbamate) supported on silica gel. Chem Lett 9 1857-1860... 

FIGURE 4.4 Chromatograms showing chiral resolution of racemic compounds on (a) Microcrystalline cellulose frw(3,5-dimethylphenyl)carbamate of cellulose in the 1 1 (w/w) ratio as stationary phase. Eluent n-hexane/propan-2-ol (80 20, v/v). Visualization anisaldehyde reagent. Migration distance = 6 cm at room temperature. Enantiomers and racemic propranolol [36]. (b) Microcrystalline cellulose tris(2,3-dimethylphenyl)carbamate of cellulose in the 1 1 (w/w) ratio as stationary phase. Eluent n-hexane/propan-2-ol (90 10, w/w). Enantiomers and racemic bupranolol [36]. (c) Amino-propyl sUanized sihca gel (3 g) coated with rnj(cyclohexyl)carbamates of amylose (0.75 g) as stationary phase. Eluent n-hexane/propan-2-ol (90 10, v/v). Racemates (a) 2,2,2-trifluoro-(9-antryl)ethanol, (b) Troger s base, and (c) benzoin ethyl ether. Visualization UV (254 nm). Migration distance = 6.2 cm at room temperature [37]. 

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