Quinine versus carbamoylated quinine-based chiral anion exchangers. A comparison regarding enantioselectivity for N-protected amino acids and other chiral acids.
First, a chiral stationary phase was prepared by immobilization of the naturally occurring alkaloid quinine onto a 3-mercaptopropyl-modified silica gel via a radical addition reaction and it was evaluated for direct HPLC enantioseparation of acidic chiral compounds under buffered hydro-organic mobile phase conditions. Second, its enantioselectivity and retention characteristics for a representative set of N-derivatized alpha-amino acids and other chiral acids were compared with those of a similar weak chiral anion exchanger based on tert.-butyl carbamoylated quinine derivative as chiral selector. The results clearly indicate that the introduction of the carbamoyl functionality at the secondary hydroxyl group at C9 of quinine provides new and additional sites for intermolecular interactions with chiral analytes and this can profoundly change and improve chiral recognition ability, especially for amide, carbamate and sulfonamide derivatives of amino acids including DNB, Bz, Ac, For, Z, Fmoc, Boc and DNS-protected amino acids. The impact of this new and rigid hydrogen donor-acceptor group--directing stereoselective selector-selectand complexation by intermolecular hydrogen bonding--in comparison to the plain quinine selector is further evaluated by alkylation of the nitrogen atom of either the selector carbamate and/or of the amino function of leucine (N-methyl leucine) derivatized either by DNB or DNP groups.