Several monolithic chiral stationary phases for reversed-phase electrochromatography have been prepared within the confines of untreated fused silica capillaries by the direct copolymerization of the chiral monomer 2-hydroxyethyl methacrylate (N-L-valine-3,5dimethylanilide) carbamate with ethylene dimethacrylate, 2-acrylamido-2-methyl-1-propanesulfonic acid and butyl or glycidyl methacrylate in the presence of a porogenic solvent. The hydrophilicity of the stationary phase, which may be enhanced further by the hydrolysis of the epoxide functionalities of the glycidyl methacrylate moieties within the monolith, was found to have a pronounced effect on the enantioseparation. Using the most hydrophilic monolithic capillary column and optimized elution conditions, a separation of N-(3,5-dinitrobenzoyl)leucine diallylamide enantiomers with an efficiency of 61 000 plates m 21 and a resolution of 2.0 was achieved.
Porous size monodisperse methacrylate beads containing amino and hydroxyl groups, which can be used as a platform for the production of chiral separation media, have been prepared using the staged templated suspension polymerization process. The monomers 2-hydroxyethyl methacrylate and several tert-butoxycarbonyl-protected aminoalkyl methacrylates were used for the preparation of both hydroxyl- and amino-functionalized beads. Attachment of a chiral selector based on L-valine-3,5-dimethylanilide through a carbamate and a urea linkage, respectively, provides chiral stationary phases with excellent enantioselectivities in the separation of the enantiomers of 3,5-dinitrobenzamido derivatives of alpha-amino acids. For comparison purposes, a separation medium was also prepared by the direct polymerization of a chiral monomer analogous to the hydroxyethyl methacrylate-based stationary phase. The chiral stationary phases prepared with the N-methyl-2-aminoethyl methacrylate platform exhibit the best selectivity, and separation factors as high as 15 were achieved under normal-phase HPLC conditions.
A parallel library of 108 4-aryl-1,4-dihydropyrimidine (DHPM) enantiomers, which are potential selectors for chiral HPLC separations, was synthesized using the single-step Biginelli multicomponent condensation. The individual compounds were screened by observing the enantioselectivity for resolution on a "brush-type" L-(3,5-dinitrobenzoyl)leucine-based chiral stationary phase, and separation factors alpha up to 12 were achieved. The best candidates from the library contained an ortho-substituted aromatic group at C4 carbon atom of the pyrimidine ring and an alkyl substituent at N1 nitrogen atom. Resolution of the enantiomers of the lead compound, 4-(9-phenanthryl)-DHPM 8, using semipreparative chiral HPLC followed by attachment to monodisperse macroporous aminomethacrylate beads, provided the novel polymer based chiral stationary phase with good enantioselectivities in the resolution of several pi-acidic aryl-dihydropyrimidines and derivatized profens. In addition, 3,5-dinitrobenzamido derivatives of alpha-amino acids could be resolved under normal phase HPLC conditions with separation factors up to 8.
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