title: Novel cinchona carbamate selectors with complementary enantioseparation characteristics for N-acylated amino acids.
abstract: The synthesis and chromatographic evaluation of the enantiomer separation capabilities of covalently immobilized calix[4]arene-cinchona carbamate hybrid type receptors derived from quinine (QN) and its corresponding C9-epimer (eQN) in different solvents are reported. The receptors display complementary enantiomer separation profiles in terms of elution order, chiral substrate specificity, and mobile phase characteristics, indicating the existence of two distinct chiral recognition mechanisms. The QN-derived receptor binds the (S)-enantiomers of N-acylated amino acids more strongly, shows preferential recognition of open-chained amino acids, and superior enantioselectivity in polar media such as methanol/acetic acid. In contrast, the eQN congener preferentially recognizes the corresponding (R)-enantiomers, displays good enantioselectivity (alpha up to 1.74) for cyclic amino acids, and enhanced stereodiscriminating properties in apolar mobile phases, e.g., chloroform/acetic acid. A comparison of the enantiomer separation profiles with those of the corresponding QN and eQN tert-butyl carbamate congeners indicates no significant level of cooperativity between the calix[4]arene module and the cinchona units in terms of overall chiral recognition, most probably as a consequence of residual conformational flexibility of the calixarene module and the carbamate linkage.
authors.affiliation: Institute of Analytical Chemistry, University of Vienna, Vienna, Austria.
authors.forename: Karl Heinz
authors.initials: KH
authors.lastname: Krawinkler
authors.forename: Norbert M
authors.initials: NM
authors.lastname: Maier
authors.forename: Rocco
authors.initials: R
authors.lastname: Ungaro
authors.forename: Francesco
authors.initials: F
authors.lastname: Sansone
authors.forename: Alessandro
authors.initials: A
authors.lastname: Casnati
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Lindner
mesh_terms: D000596:Amino Acids / Q000737:chemistry* / Q000302:isolation & purification; D047250:Calixarenes; D002219:Carbamates / Q000737:chemistry*; D002725:Chloroform / Q000737:chemistry; D002845:Chromatography; D002853:Chromatography, Liquid; D002929:Cinchona / Q000737:chemistry*; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008968:Molecular Conformation; D011083:Polycyclic Compounds; D013237:Stereoisomerism
chemical_list: D000596:Amino Acids; D002219:Carbamates; D011083:Polycyclic Compounds; D047250:Calixarenes; D002725:Chloroform