Transition from enantioselective high performance to ultra-high performance liquid chromatography: A case study of a brush-type chiral stationary phase based on sub-5-micron to sub-2-micron silica particles.

Three brush-type chiral stationary phases (CSPs) differing in the particle size of the starting silica particles have been prepared by covalent grafting of the pi-acidic bis-(3,5-dinitrobenzoyl)-derivative of trans-1,2-diaminocyclohexane (DACH-DNB). Starting silica particles of 4.3, 2.6 and 1.9micron were used to generate the final CSPs using an improved, highly reproducible synthetic methodology, that allowed to assemble and surface-graft the whole chiral selector in only two steps. The different CSPs have been packed in columns of various length and diameters, and fully characterized in terms of flow permeability, kinetic performances and enantioselectivity using a set of test solutes. Very high speed and high resolution applications together with stereodynamic HPLC examples are demonstrated on the columns with reduced particle diameters, on which separations of several enantiomeric pairs are routinely obtained with analysis times in the 15-40s range.

New chiral and restricted-access materials containing glycopeptides as selectors for the high-performance liquid chromatographic determination of chiral drugs in biological matrices.

Two new chiral and restricted-access materials containing glycopeptide antibiotics as chiral selectors (chiro-Glyco-RAM) were designed, suitable for the direct HPLC injection of biological fluids containing chiral drugs without any sample pre-treatment or pre-columns coupling. The external surface of the porous silica support was covered with a bio-compatible hydrophilic polymeric network (polyvinyl alcohol, PVA) while the chiral phase based on either teicoplanin (TE) or teicoplanin aglycone (TAG) was exclusively confined to the internal region. The chiro-Glyco-RAM supports were synthesized by the following steps: (a) introduction of 3-aminopropyl groups on 100 A pore size silica gel; (b) activation of the aminopropylated silica with 1,6-diisocyanatohexane; (c) functionalization of the external region of the porous silica with PVA; (d) covalent linking of TE/TAG to the internal surface. The average pore diameter of the chiro-Glyco-RAM supports, calculated by inverse size-exclusion chromatography (ISEC), was about 80 A and able to exclude macromolecules heavier than about 20,000 Da (such as the most abundant serum proteins) from the pores. The recovery of bovine serum albumin (BSA) was almost quantitative. HPLC analyses of model chiral drugs were performed using hydro-organic mobile phases consisting of an organic solvent (acetonitrile or methanol) and aqueous solutions of ammonium acetate (0.020 M) or ammonium formate (0.0025-0.0050 M).

Synthesis and characterization of novel internal surface reversed-phase silica supports for high-performance liquid chromatography.

An original synthetic method was developed for the preparation of a family of six novel deactivated restricted-access materials (RAMs), belonging to the group of the internal surface reversed-phase (ISRP) supports. The supports (ISRP-RAM phases A-F) have an alkyl-chain (14 methylenes) with two embedded ureido groups bound only to the internal surfaces of the porous silica, and polyvinyl alcoholic groups (PVA, 100,000-->22,000 molecular weight) chemically bound to the external surfaces. The average pore diameters of the prepared ISRP-RAM supports, calculated by inverse size-exclusion chromatography, ranged between 49 A and 88 A, and were able to exclude macromolecules heavier than about 24000 Da (such as serum proteins) from the pores. The novel supports were designed for the determination of a semi-synthetic anticancer drug of the camptothecin family in human plasma, but they represent universal ISRP-RAM supports not limited to such class of compounds.

The contribution of oxazolidinone frame to the biological activity of pharmaceutical drugs and natural products.

The development of resistance by the antibiotics in the Gram-positive pathogenic bacteria over the last twenty years and continuing today has created a need for new antibiotic classes, which may be unaffected by existing bacterial resistance. The oxazolidin-2-ones represent not only a new class with a novel mechanism of action, but also satisfy the requirement for overcoming the resistance mechanisms. Both linezolid and eperozolid, the first chemical candidates, arose from the piperazine subclass, with the first one being chosen further development because of its enhanced pharmacokinetic properties. The main attractive traits of the oxazolidinone series has encouraged further work in the area, and the patent literature reveals that extensive chemical investigation is currently being made. The unexpected early resistance development emphasizes the need for further exploration of features of the oxazolidinone to eliminate these deficiencies. Recently, several changes, involving the C5 side chain as well the N-phenyl heterocyclic ring, give promise for such improvement. Oxazolidinone antibacterial agents comprise also ketolides, derivatives of macrolides, such as erythromycin A, with a newly formed carbamate cycle, with a largely unexplored potential. The oxazolidinone nucleus does not appear only in the structures of antimicrobial drugs, but a number of biological activities are connected with frameworks including the oxazolidinone ring. A partial list of these activities comprises enzyme inhibitors, agonists and antagonists, with a particular citation for a new generation of selective monoamino oxidase inhibitors (befloxatone). The oxazolidinone moiety was found in the structure of few biologically active natural products, such as (-)-cytoxazone and streptazolin. Moreover, in some cases the oxazolidinone ring has been chosen for the preparation of isosteric aza analogues of natural compounds (podophyllotoxin, pilocarpine) that can be more easily synthesised and more hardly inactivated. Finally, the participation of oxazolidinone chiral auxiliaries to several syntheses of natural products must be acknowledged.

Twenty years of research on silica-based chiral stationary phases.

This review provides an overview of twenty years of pioneering work (from 1985 to 2005) of our research group in the preparation and application of enantioselective packing materials for HPLC. After a brief introduction to the rational design of a new chiral stationary phase, a detailed presentation in chronological order of appearance in the literature is given of the currently developed repertoire of chiral stationary phases and their typical applications. Emphasis is placed on the different synthetic strategies exploited to obtain highly efficient, stable, and versatile chiral stationary phases.

Prenylated flavonoids: pharmacology and biotechnology.

Within the flavonoid class of natural products the prenylated sub-class is quite rich in structural variety and pharmacological activity. In the last twenty years a huge number of new structures has been reported, mostly from Leguminosae and Moraceae, with few coming from other genera. The presence, in different forms, of the isoprenoid chain can lead to impressive changes in biological activity, mostly attributed to an increased affinity for biological membranes and to an improved interaction with proteins. Molecules, such as xanthohumol and sophoraflavanone G, while being very structurally simple, show numerous pharmacological applications and are ideal candidates for SAR aimed to the discovery of new drugs. Only recently the biogenesis of these compounds has been more extensively studied and much attention has been focused on the enzymes involved in the modification and transfer of the prenyl unit.

Internal motions in a fulleropyrrolidine tertiary amide with axial chirality.

The kinetic parameters for topomerization around the N-CO bond and enantiomerization around the C-CO bond in N-1-naphthoyl fulleropyrrolidine 1 and N-1-naphthoyl pyrrolidine 2 have been determined by dynamic NMR (line shape simulation and selective inversion transfer). The DeltaS(not =) values are negligible. The DeltaH# value for topomerization of 1 is smaller with respect to that of 2 by 4.3 kcal mol(-1) (explained by the electron-withdrawing effect of fullerene) and the value for enantiomerization is greater by 1.4 kcal mol(-1) (explained by the greater rigidity of the fulleropyrrolidine ring, as confirmed by ab initio analyses).

Enantioselective liquid chromatographic-electrospray mass spectrometric assay of beta-adrenergic blockers: application to a pharmacokinetic study of sotalol in human plasma.

An enantioselective high performance liquid chromatographic-electrospray ionization mass spectrometric (HPLC-ESI-MS) method for the direct determination of several beta-adrenergic blockers was developed and validated. The method is based on the direct separation of the enantiomers of drugs on a laboratory-made chiral stationary phase (CSP) containing covalently bonded teicoplanin (TE) as chiral selector. Detection of the effluent was performed by electrospray ionization mass spectrometry, run in the selected-ion recording (SIR) mode. The method was applied to the pharmacokinetic monitoring of sotalol (STL) in the plasma of five young healthy volunteers, dosed with racemic drug. The limits of quantitation (LOQ) reached 4 ng/ml for both sotalol enantiomers. Such a method, fully validated, offers a novel, fast and very efficient tool for the direct determination of sotalol enantiomers in human plasma, and can be generally applied to the beta-adrenergic blockers stereoselective pharmacokinetics.

A chiral A2 B2 macrocyclic minireceptor with extreme enantioselectivity.

We describe a novel macrocyclic minireceptor that is assembled from a chiral 1,2-diamine and 5-allyloxyisophthalic acid. After immobilization on HPLC silica, the chiral macrocycle preferentially binds the L-enantiomers of simple amino acid derivatives, with enantioselectivities values up to 3.0 kcal/mol. [structure: see text]

Synthesis and interaction with copper(II) cations of cyano- and aminoresorcin[4]arenes.

Following our studies on resorcin[4]arenes, we synthesized new macrocycles containing cyanomethyl and aminomethyl side chains. Three stereoisomers (2a-c) of the former were obtained by BF3*Et2O tetramerization of the corresponding trans-cinnamic acid derivative and were shown to be in the 1,2-alternate, cone, and 1,3-alternate conformations. Conversely, the tetraamino derivative 6a in the cone conformation was prepared from the corresponding tetrabromide 3a. The interactions with Cu(II) cations of the new compounds were analyzed by measurements of 1H NMR and EPR spectra in parallel with molecular modeling calculations.