Novel Cinchona Carbamate Selectors With Complementary Enantioseparation Characteristics for N-Acylated Amino Acids KARL HEINZ KRAWINKLER, 1 NORBERT M. MAIER, 1 * ROCCO UNGARO, 2 FRANCESCO SANSONE, 2 ALESSANDRO CASNATI, 2 AND WOLFGANG LINDNER 1 1 Institute of Analytical Chemistry, University of Vienna, Vienna, Austria 2 Dipartimento di Chimica Organica e Industriale, Universita ´ di Parma, Parma, Italy ABSTRACT The synthesis and chromatographic evaluation of the enantiomer sepa- ration 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 sepa- ration 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 en- antioselectivity in polar media such as methanol/acetic acid. In contrast, the eQN con- gener preferentially recognizes the corresponding (R)-enantiomers, displays good en- antioselectivity ( 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. Chirality 15:S17–S29, 2003. © 2003 Wiley-Liss, Inc. KEY WORDS: chiral recognition; enantiomer separation; liquid chromatography; sol- vent influence; cinchona-calixarene carbamate hybrid type selectors; C9- epiquinine; Boc-amino acids Carbamates of the cinchona alkaloids quinine (QN) and quinidine (QD) are well known for their broad enantiomer separation capabilities for chiral acidic analytes. 1 Over the recent years, this class of anion exchangers has been suc- cessfully deployed as chiral selectors in different enantio- mer separation techniques, including high-performance liquid chromatography (HPLC), 1 capillary electrophoresis (CE), 2 and capillary electrochromatography (CEC). 3–5 Combined experimental liquid evidence from chromato- graphic, 1 calorimetric, 6 NMR spectroscopic, and molecular modeling studies 7 allowed us to advance a detailed picture of the molecular mechanism governing enantioselective as- sociation of open-chained N-acylated amino acids to these selectors. For this class of analytes, cinchona carbamates selectively bind one of the enantiomers at a well-defined domain of the receptor due to the formation of an array of concerted noncovalent interactions, involving intermolecu- lar ion-pairing, – stacking, hydrogen-bonding, and steric increments (see Fig. 1). This recognition mechanism is of a general nature as it rationalizes the often outstanding levels of enantioselectivity (up to  > 30) for various classes of open-chained N-acylated amino acids and permits highly reliable predictions of elution orders. Unfortunately, the recognition model provides insufficient information on the subtle structural factors that control the substrate- compatibility of cinchona carbamate type selectors. Specifi- cally, the fact that cinchona carbamate selectors perform quite poorly for pharmaceutically important cyclic amino acids is beyond the explanatory potential of the advanced mechanistic model. Prompted by the ever-increasing importance of cyclic amino acids in various fields of life sciences, 8,9 we have initiated a research program aimed at improving the enan- tioselective binding properties of cinchona carbamate type selectors for these poorly recognized analytes (mismatch analytes). To facilitate the dedicated redesign of preexist- ing receptor systems for this and other classes of analytes, we are currently assembling a structural database of novel Contract grant sponsor: Austrian Research Foundation (FWF); Contract grant number: P14179-CHE; Contract grant sponsor: European Commu- nity; Contract grant number: COST D 11/0008/98, STSM, March 2001. *Correspondence to: Norbert M. Maier, Institute of Analytical Chemistry, University of Vienna, Wa¨hringer Strae 38, A-1090, Vienna, Austria. E-mail: norbert.maier@univie.ac.at Received 16 December 2002; Accepted 31 March 2003 CHIRALITY 15:S17–S29 (2003) © 2003 Wiley-Liss, Inc.