Amino alcohol-derived chiral stationary phases

An updated minireview of chiral stationary phases (CSPs) based on amino alcohols is presented. In this minireview, we focused on amino alcohols as starting materials in preparation of chiral catalysts for asymmetric organic synthesis and CSPs for chiral separations. Among the various CSPs, we summarized the important developments and applications of the amino alcohol-based Pirkle-type CSPs, ligand exchange CSPs, α-amino acid-derived amino alcohol CSPs, and symmetric CSPs from their first appearance to the present day to propose ideas for the development of new CSPs with improved performance.

An Adjustable Cleft Based on an 8-sulfonamide-2-naphthoic Acid with Oxyanion Hole Geometry

Cleft type receptors showing the oxyanion hole motif have been prepared in a straightforward synthesis starting from the commercial 3,7-dihidroxy-2-naphthoic acid. The double H-bond donor pattern is achieved by the introduction of a sulfonamide group in the C-8 position of naphthalene and a carboxamide at the C-2 position. This cleft, whose geometry resembles that of an oxyanion hole, is able to adjust to different guests, as shown by the analysis of the X- ray crystal structures of associates with methanol or acetic acid. Combination of hydrogen bonds and charge-transfer interactions led to further stabilization of the complexes, in which the electron-rich aromatic ring of the receptor was close in space to the electron-deficient dinitroaromatic guests. Modelling studies and bidimensional NMR experiments have been carried out to provide additional information.

Solvent-sensitive circularly polarized luminescent compounds bearing a 9,9′-spirobi[fluorene] skeleton

Chiral 9,9′-spirobi[fluorene] derivatives with a donor–π–acceptor system were prepared and found to exhibit solvent-sensitive circularly polarized luminescence.

Enantioselective carbohydrate recognition by synthetic lectins in water

Carbohydrate receptors with a chiral framework have been generated by combining a tetra-aminopyrene and a C3-symmetrical triamine via isophthalamide spacers bearing water-solubilising groups. These "synthetic lectins" are the first to show enantiodiscrimination in aqueous solution, binding N-acetylglucosamine (GlcNAc) with 16 : 1 enantioselectivity. They also show exceptional affinities. GlcNAc is bound with Ka up to 1280 M-1, more than twice that measured for previous synthetic lectins, and three times the value for wheat germ agglutinin, the lectin traditionally employed to bind GlcNAc in glycobiological research. Glucose is bound with Ka = 250 M-1, again higher than previous synthetic lectins. The results suggest that chirality can improve complementarity to carbohydrate substrates and may thus be advantageous in synthetic lectin design.

Molecular recognition of isomeric protonated amino acid esters monitored by ESI-mass spectrometry

Two new 9,9'-spirobifluorene-derived crown ethers were prepared and used to recognise constitutionally isomeric amino acid derivatives. The performance of the receptors was evaluated by ESI-mass spectrometry using the isomer labelled guest method (ILGM). This method revealed the preferred binding of L-norleucine and L-leucine compared to L-isoleucine for both receptors. Furthermore, non-covalent isotope effects demonstrate the relevance of dispersive interactions for the overall binding event. These effects also provide hints for the relative spatial orientation of the guest molecules within the host-guest complex, and thereby prove the importance of the spirobifluorene moiety for the observed binding of the protonated amino acid esters.

High-performance liquid chromatography chiral stationary phases based on low-molecular-mass selectors

A review of HPLC chiral stationary phases (CSPs) based on low molecular mass selectors is given. The review is focused on brush- and monomeric-type CSPs obtained by covalent linkage of chiral selectors, with emphasis on those obtained by total synthesis. Emphasis is given to new, emerging aspects like enantioseparations on receptor-like chiral stationary phases and dynamic enantioselective chromatography of stereolabile compounds.

Atomistic modeling of enantioselection in chromatography

A review of atomistic molecular modeling studies related to chromatographic separations of enantiomers is presented. Only those types of calculations where direct interactions between a selector and a selectand are involved are described in this review; omitted are regression models. An emphasis is placed on comparing methods used for sampling potential energy surfaces implementing different methodologies like quantum and molecular mechanics for energy calculations, and molecular dynamics and Monte Carlo sampling strategies for simulations. Type I-V chiral stationary phases and additives for capillary electrophoresis and ion-pair chromatography are covered in this review.

New chiral stationary phases based on (R)-1-naphthylethylamine bound to 2,4,5,6-tetrachloro-1,3-dicyanobenzene

Chiral functionalization of 2,4,5,6-tetrachloro-1,3-dicyanobenzene (1) by regioselective nucleophilic substitution of one or two chlorine atoms by optically pure (R)-(+)-1-naphthylethylamine (NEA), or by a glycine unit as a spacer to (R)-NEA, enables the preparation of brush-type chiral selectors (2, 3, 9, 13). By the introduction of the 3-aminopropyltriethoxysilyl (APTES) group, reactive intermediates 4a/b, 5, 10a/b, and 14a/b are obtained (a/b indicate a mixture of regioisomers with APTES in 6- and 2-position). Binding of these to silica gel afforded four novel chiral stationary phases (CSPs) 6, 7, 15, and 16. HPLC columns containing CSPs with (R)-NEA directly linked to polysubstituted aromatic ring (6, 7) are not very effective in resolution of most of the 23 racemic analytes, whereas the columns with distant pi-basic subunits (15, 16) exhibited higher resolving efficacy, in particular towards the isopropyl esters of racemic N-3,5-dinitrobenzoyl-alpha-amino acids. Effective resolution of test racemates reveals the importance of the presence of the hydrogen bond donor amido group and the distance between the persubstituted benzene ring in 1 and the pi-basic naphthalene ring of (R)-NEA. Copyright 1999 Wiley-Liss, Inc.

Recognition of neutral species with synthetic receptors

The design of synthetic receptors for neutral molecules is an area of intense current interest. The area has grown from early work on cyclodextrin or single crown ether complexation to encompass a wide array of receptor shapes and structures. Furthermore, the range of substrate selectivities has increased from simple aromatic or metal ion substrates to include key biological components such as peptides and carbohydrates. Recent advances have included the application of split bead combinatorial methods for the identification of receptors for oligopeptides, the design of self-assembling spherical receptors, the use of multiple hydrogen bonding groups for carboxylic acid and carbohydrate recognition and the achievement of impressive catalytic effects with synthetic receptors.