Chiral ligand exchange chromatography on polygalactomannan (Guaran)

Summary The Chiral discriminating properties of the legume seed (Cyamopsis tetragonalobus) polysaccharide, guaran, has been investigated by this laboratory. Due to the presence of cis-hydroxyl groups in its molecule, guaran a galactomannan, forms tetracoordinated polymeric complexes with boron, resulting in cross-linking and gelling. Ligand exchange between the polymeric boron complex and a low molecular weight chiral diol can take place with stereoselectivity. The principle has been applied for the resolution of (±) mandelic acid by using ligand exchange chromatography.     

Access to the enantiopure pyrrolobenzodiazepine (PBD) dilactam nucleus via self-disproportionation of enantiomers

A short synthesis of an enantiopure pyrrolobenzodiazepine (PBD) dilactam featuring early installation of the C2-C3 unsaturation is reported. An enantioselective cooperative catalytic cascade followed by self-disproportionation of enantiomers via sublimation afforded the enantiopure 2,3-dihydro-1H-pyrrole key intermediate, 1. N-Acylation followed by reduction and lactam formation furnished the PBD dilactam.     

Chromatographic resolution, chiroptical characterization and urinary excretion of the enantiomers of sulindac

Summary The chromatographic resolution of the enantiomers of sulindac has been achieved using a Chiralpak AD CSP (10 μm, 250×4.6 mm) with a mobile phase of hexane: ethanol (85∶15 v/v) containing trifluoroacetic acid (0.05% v/v) at a flow rate of 1.0 mL min⁻¹. Under these conditions the enantiomers eluted with separation and resolution factors of 1.43 and 2.46 respectively. Semipreparative isolation of the enantiomers and their characterization by circular dichroism spectroscopy and NMR, in the presence of a chiral shift reagent, indicated that the elution order was (−)-(S)- before (+)-(R)-sulindac. The enantiomeric composition of sulindac in urine following administration of the racemic drug to man was determined by sequential achiral-chiral chromatography. Achiral analysis was carried out using a Spherisorb S5 ODS2 stationary phase (5 μm, 250×4.6 mm) and a mobile phase of aqueous acetic acid (2% v/v; pH 3.5): acetonitrile: THF (50∶48∶2 by volume) at a flow rate of 1.0 mL min⁻¹. The HPLC eluate containing sulindac (retention time 4.9 min) was collected and following workup, the enantiomeric composition of the drug was determined using the CSP. Over the 24 h collection period sulindac was excreted predominantly as theR-enantiomer, but the enantiomeric composition was found to vary markedly with time which is presumably associated with the complex metabolism of the drug.     

Direct determination of enantiomeric purity of FMOC amino acids with high performance liquid chromatography

Summary Analytical methods are described which allow a direct determination of enantiomeric purity of seventeen FMOC amino acids commonly used in peptide synthesis. The corresponding ester derivatives can be separated directly on cellulose tris(3,5-dimethylphenylcarbamate) (Chiralcel-OD). The methods are suitable for primary as well as secondary FMOC amino acids. The presence of a highly sensitive fluorescence moiety within the molecule, in combination with large separation factors (-values between 1.5–2.2) allowed for a general detection limit below 0.05%. In several cases the antipode has been determined in the ppm-range. An interesting result has been observed with respect to the elution order of the FMOC amino acid esters. The elution order of the Trp enantiomers is opposite to that obtained with the other amino acids. This is contrary to the generally held belief that elution order is identical within a homologous series of racemates when chromatographed under identical conditions on the same chiral stationary phase. In addition, the inversion of elution of the Pro enantiomers depending on the estertype indicates a competition of different separation mechanisms.     

Models for thyroxine: Aromatic iodine-assisted self-assemblies

Trying to understand the different recognition factors between thyroxine and its binding proteins, the chemistry of the ternary complexes between copper and o-iodobenzoylglycine (I-hip) [or o-iodobenzoylglycilglycine (B^IGG)] as simple thyroxine models were studied with 2,2′-bipyridyl as secondary ligand.     

Preparation and Evaluation of Mono (6A-N-Ethylenediamine-6A-Deoxy)-Functionalized β-Cyclodextrin Bonded Stationary Phases for Normal and Reverse-Phase High-Performance Liquid Chromatography

The preparation, characterization and potential liquid chromatographic applications of two totally and partly functionalized -cyclodextrin-bonded stationary phases were presented. Separation of a series of organophosphorus racemic compounds was achieved under normal and reverse-phase conditions. Separation factor was up to 1.812 and resolution up to 4.072. The influence of chromatographic conditions was investigated and the possible chiral recognition mechanism was proposed.     

Bile acids as stationary phase in liquid chromatography

Summary Bile acids chemically bonded onto aminopropylsilica have been employed as stationary phases in liquid chromatography. Bile acid aggregates were dynamically formed around molecules chemically anchored on the supports when the eluent contained bile salts. The bile salt aggregates achieved the separation of 1,1′-binaphthyl-2,2′-diyl-hydrogenphosphate enantiomers and dansyl amino acids.     

Diastereoselective synthesis of substituted 1,3,6-triazabicyclo[3.1.0]hexanes

A general method was developed for the synthesis of substituted 1,3,6-triazabicyclo[3.1.0]hexanes via intramolecular aminomethylation of the NH group of the diaziridine ring by the reactions of 3-aminomethyl-1,3-dimethyldiaziridine with aliphatic, aromatic, and heteroaromatic carbonyl compounds. These reactions with aldehydes proceeded diastereoselectively to form mixtures of two racemates, viz., 1R*,2R*,5R*,6R* and 1R*,2S*,5R*,6R*, in a ratio of (3—20) : 1, the predominant diastereomer being isolated in all cases. The reactions with symmetrical ketones gave rise exclusively to the (1R*,5R*,6R*) racemate. The predominant diastereomer 1R*,2R*,5R*,6R*-2-(2-bromothien-5-yl)-1,3,6-triazabicyclo[3.1.0]hexane crystallized as a conglomerate. The structure of one of its enantiomers was established by X-ray diffraction analysis.