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 natu rally occurring alkaloid quinine onto a 3-mercaptopropyl-modified silica ge l via a radical addition reaction and it was evaluated for direct HPLC enan tioseparation of acidic chiral compounds under buffered hydro-organic mobil e phase conditions. Second, its enantioselectivity and retention characteri stics for a representative set of N-derivatized alpha-amino acids and other chiral acids were compared with those of a similar weak chiral anion excha nger based on tert.-butyl carbamoylated quinine derivative as chiral select or. The results clearly indicate that the introduction of the carbamoyl fun ctionality at the secondary hydroxyl group at C-9 of quinine provides new a nd additional sites for intermolecular interactions with chiral analytes an d this can profoundly change and improve chiral recognition ability, especi ally for amide, carbamate and sulfonamide derivatives of amino acids includ ing DNB, Bz, Ac, For, Z, Fmoc, Boc and DNS-protected amino acids. The impac t of this new and rigid hydrogen donor-acceptor group - directing stereosel ective selector-selectand complexation by intermolecular hydrogen bonding - in comparison to the plain quinine selector is further evaluated by alkyla tion of the nitrogen atom of either the selector carbamate and/or of the am ino function of leucine (N-methyl leucine) derivatized either by DNB or DNP groups. (C) 1999 Elsevier Science B.V. All rights reserved.