Enantioselective High-performance Liquid Chromatography Research Articles

Polydopamine-based surface modification of copolymeric nanoparticles as a targeted drug delivery system for cancer therapy

An enantioselective high performance liquid chromatography method has been developed and validated by evaluating the suitability of newly introduced immobilized polysaccharide chiral stationary phases, the effect of different organic modifiers and temperature including the entropy and enthalpy on resolution of the (R,S)-(−) & (S,R)-(+) emtricitabine enantiomers on rat dried blood spots. Both the enantiomers were extracted from dried blood spots using ethanol: methanol (80:20 v/v) mixture and separated on an immobilized amylose tris-(3,5-dimethyl phenyl carbamate) chiral stationary phase using n-hexane:ethanol (65:35 v/v) as a mobile phase at a flow rate of 0.8 mL/min. The detection was carried out at 280 nm using photo diode array detector connected to a polarimeter in series to determine their order of eluton. The method was validated with respect to limits of detection and quantification, linearity, accuracy and precision. The calibration curves were linear over the concentration range of 0.5–500 μg/mL for both enantiomers and the correlation coefficient (r2) was >0.998. The overall recovery of (R,S)- & (S,R)-enantiomers of emtricitabin from DBS were 90.4 and 90.6%, respectively. The limits of detection and quantification of enantiomers were 0.26, 0.30 and 0.85, 0.92 μg/mL for (R,S)- and (S,R)-emtricitabin enantiomers, respectively. The assay was specific and precise (RSD <10%). The stability of emtricitabin was also performed and the results were found to be well within the limits. The effect of hematocrit on extraction of emtricitabin enantiomers from dried blood spots was evaluated and no interference from endogenous substances was observed.

Human Serum Albumin as Chiral Selector in Enantioselective High-Performance Liquid Chromatography.

Enantioselective high-performance liquid chromatography (eHPLC) using chiral stationary phases (CSPs) is surely the most used technique for the determination of the enantiomeric excess (e.e.) of chiral drugs, a fundamental parameter for reliable studies on the relationship between stereochemistry and pharmacological activity. A key aspect of this enantioseparation technique is the efficiency of the chiral selector, which can be optimized to obtain higher selectivity and a wider applicability. Thus, the determination of the mechanisms behind chiral recognition is very important to predict and improve the enantioselectivity of CSPs. The present review deals with the preparation and use of CSPs for eHPLC with human serum albumin (HSA) as chiral selector, with particular emphasis on the modulation of the chromatographic performance. HSA-based CSPs allow a relatively easy prediction of the binding sites involved in the retention of analytes and the possibility to improve the selectivity of enantioresolution by modulating the binding process, using either reversible or covalent modifications of the protein. Significant improvements of the chromatographic parameters, such as reduction of analysis time and increase of enantioselectivity, have been obtained for selected analytes by using competitors for a particular binding site of HSA dissolved in the mobile phase or by selectively modifying the protein structure at single amino acid residues.

The role of chromatographic and chiroptical spectroscopic techniques and methodologies in support of drug discovery for atropisomeric drug inhibitors of Bruton’s tyrosine kinase

Atropisomers are stereoisomers resulting from hindered bond rotation. From synthesis of pure atropisomers, characterization of their interconversion thermodynamics to investigation of biological stereoselectivity, the evaluation of drug candidates subject to atropisomerism creates special challenges and can be complicated in both early drug discovery and later drug development. In this paper, we demonstrate an array of analytical techniques and systematic approaches to study the atropisomerism of drug molecules to meet these challenges. Using a case study of Bruton’s tyrosine kinase (BTK) inhibitor drug candidates at Bristol-Myers Squibb, we present the analytical strategies and methodologies used during drug discovery including the detection of atropisomers, the determination of their relative composition, the identification of relative chirality, the isolation of individual atropisomers, the evaluation of interconversion kinetics, and the characterization of chiral stability in the solid state and in solution. In vivo and in vitro stereo-stability and stereo-selectivity were investigated as well as the pharmacological significance of any changes in atropisomer ratios. Techniques applied in these studies include analytical and preparative enantioselective supercritical fluid chromatography (SFC), enantioselective high performance liquid chromatography (HPLC), circular dichroism (CD), and mass spectrometry (MS). Our experience illustrates how atropisomerism can be a very complicated issue in drug discovery and why a thorough understanding of this phenomenon is necessary to provide guidance for pharmaceutical development. Analytical techniques and methodologies facilitate key decisions during the discovery of atropisomeric drug candidates by characterizing time-dependent physicochemical properties that can have significant biological implications and relevance to pharmaceutical development plans.

A study of ofloxacin and levofloxacin photostability in aqueous solutions

Introduction. The photostability is one of the most important properties of drugs. A comprehensive study of ofloxacin (OFX) and levofloxacin (LVX) photostability in aqueous solutions was performed. Ofloxacin is a chemotherapeutic agent belonging to the second generation fluoroquinolones and is a racemate of (R)-(+)-ofloxacin and (S)-(-)-ofloxacin (LVX).Material and Methods. Samples of OFX and LVX were subjected to stress conditions of UV irradiation using a mercury-vapor lamp. The study involved development of enantioselective high-performance liquid chromatography (HPLC) and high-performance capillary electrophoresis (HPCE) methods for separation of OFX enantiomers and their degradation products. These methods were used to monitor the degradation process of OFX and LVX under irradiation and to determine the kinetics of degradation of these antibacterial agents. Moreover, the identification of photoproducts was also attempted. The structure of the main photoproducts was examined by mass spectrometry (MS).Results and Conclusions. Using HPLC method it was possible to observe two products of OFX degradation and only one for LVX, while using HPCE method eight products of OFX degradation and six of LVX were observed. Some of the photoproducts retain character of optically active compounds. The trend of the photodegradation of both tested compounds was described by autocatalytic reaction proceeding according to the Prout-Tompkins model. Some of the products of the decomposition catalyze this reaction. The rate of degradation was similar for both enantiomers but t0.5 was slightly longer for LVX than OFX. Based on MS experiments the photodegradation products of the studied fluoroquinolones and possible pathways of UV induced decay were identified.

3-(Phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole: A fascinating molecular framework to study the enantioseparation ability of the amylose (3,5-dimethylphenylcarbamate) chiral stationary phase. Part I. Structure-enantioselectivity relationships

Chiral stationary phases (CSPs) based on amylose (3,5-dimethylphenylcarbamate) (ADMPC) exhibit a wide-range of enantioselectivity in high-performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC). Although this class of CSPs has been extensively used, chiral discriminations at receptorial level, which are useful to develop predictive molecular models, have been rarely reported in the literature.Herein, we describe the results obtained in the enantioselective HPLC of a set of six C5-chiral 4,5-dihydro-(1H)-pyrazole derivatives on the ADMPC-based Chiralpak AD-3 CSP (CSP) under normal-phase and polar organic conditions. Using pure methanol as a mobile phase the exceptional enantioseparation factor value of 50 at 25°C was found for one of the investigated analytes. To the best of our knowledge, the enantiomeric bias represents the most outstanding enantioseparation ever recorded on ADMPC-based CSPs.Systematic variations in chemical groups in specific positions of the 3-(phenyl-4-oxy)-5-phenyl-4,5-dihydro-(1H)-pyrazole molecular framework resulted in peculiar changes in retention and enantioselectivity. A careful analysis of the chromatographic data permitted to advance some hypotheses concerning the role played by the individual chemical groups in determining the exceptional enantioseparation.In particular, under methanol-rich mode, the prenyl moiety of the second eluted enantiomer of the better resolved analyte was recognized as a critical structural element to establish direct and favorable solvophobic interactions with apolar portions of selector.

Enantiomerization of Allylic Trifluoromethyl Sulfoxides Studied by HPLC Analysis and DFT Calculations.

Enantiomerization of allylic trifluoromethyl sulfoxides occurs spontaneously at room temperature through the corresponding allylic trifluoromethanesulfenates via a [2,3]-sigmatropic rearrangement. Dynamic enantioselective high-performance liquid chromatography (HPLC) analysis revealed the stereodynamics of these sulfoxides ranging from chromatographic resolution to peak coalescence at temperatures between 5 and 53 °C. The rate constant of enantiomerization and activation parameters were determined and compared with Density Functional Theory (DFT) calculations.

Comparison of reversed-phase enantioselective HPLC methods for determining the enantiomeric purity of (S)-omeprazole in the presence of its related substances

A simple analytical high-performance liquid chromatography (HPLC) method was applied for the enantiomeric excess determination of esomeprazole ((S)-OME), the enantiopure active ingredient contained in drug products, in the presence of its potential organic impurities A-E. The enantioselective separation was accomplished on the immobilized-type Chiralpak ID-3 chiral stationary phase (CSP) under reversed-phase conditions. The results were evaluated and compared with those obtained by the official enantioselective method of European Pharmacopoeia used as the reference for checking the enantiomeric excess of (S)-OME. It has been established that the use of the Chiralpak ID-3 CSP allows the determination of the enantiomeric purity of (S)-OME without any interference coming from its chiral and achiral related substances. The analytical procedure of the drug regulatory agencies based on the AGP CSP suffered instead from poor specificity due to overlap of the peaks pertinent to the achiral impurity A and the chiral impurity (R)-OME (impurity F).

“Fit-for-purpose” development of analytical and (semi)preparative enantioselective high performance liquid and supercritical fluid chromatography for the access to a novel σ1 receptor agonist

A rapid and straightforward screening protocol of chiral stationary phases (CSPs) in HPLC and SFC resulted in three different methods “fit-for-purpose”, i.e. analysis and scale-up to semi-preparative enantioselective chromatography. The efficient use of these three methods allowed expedited preparation of an important drug discovery target, (R/S)-1, a potent new sigma 1 (σ1) receptor agonist. The approach taken resulted in significant savings of both time and labor for the isolation of enantiomers compared to the development of a stereo-selective synthesis.The enantiomers of 1 have been isolated allowing studies of their chirooptical properties and an in-deep comparative examination of the pharmacological profile for the individual enantiomers.

Stopped-Flow Enantioselective HPLC-CD Analysis and TD-DFT Stereochemical Characterization of Methyl Trans-3-(3,4-Dimethoxyphenyl)Glycidate.

Caffeic acid-derived polyethers are a class of natural products isolated from the root extracts of comfrey and bugloss, which are endowed with intriguing pharmacological properties as anticancer agents. The synthesis of new polyether derivatives is achieved through ring-opening polymerization of chiral 2,3-disubstituted oxiranes, whose absolute configurations define the overall stereochemistry of the produced polymer. The absolute stereochemistry of one of these building blocks, methyl trans-3-(3,4-dimethoxy-phenyl)glycidate (3), was therefore characterized by the combination of enantioselective high-performance liquid chromatography (HPLC), electronic circular dichroism (ECD) spectroscopy, and time-dependent density functional theory (TD-DFT) calculations. Initial efforts aiming at the isolation of enantiomers by means of a standard preparative HPLC protocol followed by offline ECD analysis failed due to unexpected degradation of the samples after collection. The stopped-flow HPLC-CD approach, by which the ECD spectra of enantiomers are measured online with the HPLC system, was applied to overcome this issue and allowed a fast, reliable, and chemical-saving analysis, while avoiding the risks of sample degradation during the collection and processing of enantiomeric fractions. Subsequent TD-DFT calculations identified ( as the first eluted enantiomeric fraction on the Lux Cellulose-2 column, therefore achieving a full stereochemical characterization of the chiral oxirane under investigation.

Determination of the Enantiomerization Barrier of the Residual Enantiomers of C3 -Symmetric Tris[3-(1-Methyl-2-Alkyl)Indolyl]Phosphane Oxides: Case Study of a Multitasking HPLC Investigation Based on an Immobilized Polysaccharide Stationary Phase.

The residual enantiomers of three tris-(3-indolyl)-phosphane oxides bearing different alkyl groups (methyl, ethyl or i-propyl) in position 2 of the indole rings constituting the blades were separated on the immobilized type Chiralpak IC column in polar organic and reversed-phase modes. The good enantioselectivity and versatility of the IC CSP allowed easy isolation of the enantiomerically highly enriched samples suitable for configurational stability studies. The enantiomerization barriers of residual phosphane oxides were evaluated both by off-column techniques (CD signal and enantiomeric purity decay kinetics) and by dynamic enantioselective high-performance liquid chromatography (HPLC).

Rat dried blood spot analysis of (R,S)-(-)- and (S,R)-(+)- enantiomers of emtricitabin on immobilized tris-(3,5-dimethylphenyl carbamate) amylose silica as a chiral stationary phase.

An enantioselective high performance liquid chromatography method has been developed and validated by evaluating the suitability of newly introduced immobilized polysaccharide chiral stationary phases, the effect of different organic modifiers and temperature including the entropy and enthalpy on resolution of the (R,S)-(-) & (S,R)-(+) emtricitabine enantiomers on rat dried blood spots. Both the enantiomers were extracted from dried blood spots using ethanol: methanol (80:20 v/v) mixture and separated on an immobilized amylose tris-(3,5-dimethyl phenyl carbamate) chiral stationary phase using n-hexane:ethanol (65:35 v/v) as a mobile phase at a flow rate of 0.8mL/min. The detection was carried out at 280nm using photo diode array detector connected to a polarimeter in series to determine their order of eluton. The method was validated with respect to limits of detection and quantification, linearity, accuracy and precision. The calibration curves were linear over the concentration range of 0.5-500μg/mL for both enantiomers and the correlation coefficient (r(2)) was >0.998. The overall recovery of (R,S)- & (S,R)-enantiomers of emtricitabin from DBS were 90.4 and 90.6%, respectively. The limits of detection and quantification of enantiomers were 0.26, 0.30 and 0.85, 0.92μg/mL for (R,S)- and (S,R)-emtricitabin enantiomers, respectively. The assay was specific and precise (RSD <10%). The stability of emtricitabin was also performed and the results were found to be well within the limits. The effect of hematocrit on extraction of emtricitabin enantiomers from dried blood spots was evaluated and no interference from endogenous substances was observed.

Comparative pharmacokinetic studies of racemic oxiracetam and its pure enantiomers after oral administration in rats by a stereoselective HPLC method

Oxiracetam (ORC), a nootropic drug used for improving the cognition and memory, has an asymmetric carbon in its structure and exists as (S)- and (R)-ORC. The pharmacokinetic profiles of racemic oxiracetam and its pure enantiomers in rats were evaluated and compared by enantioselective high-performance liquid chromatography, which was performed on a Chiralpak ID column with a mobile phase of hexane–ethanol–trifluoroacetic acid (78:22:0.1, v/v/v). The method was validated with respect to selectivity, linearity, accuracy and precision, stability and the limit of quantification. The validation acceptance criteria were met in all cases. A saturating phenomenon of (S)-ORC was observed when the dosage ranged from 200mg/kg to 800mg/kg. The two enantiomers showed similar profiles in the absorb phase, and reached the maximum concentration at 2h after oral administration. However, compared with the racemate group, the AUC/dose and Cmax/dose ratios of (S)-ORC were higher and Cl/f was lower in enanpure (S)-ORC group. The Cmax of (S)-ORC decreased from 21.3±5.0μg/ml to 13.2±4.2 when (R)-ORC was co-administrated at the dose of 200mg/kg. AUC0–t values of (S)-ORC were different after oral administration of 200mg/kg (S)-ORC and 400mg/kg racemic ORC (96.7±15.5 and 50.1±16.3μgh/ml). The higher absorption and slower elimination suggest that enantiopure (S)-ORC could be a promising drug that efficiently reduces clinical dosage, improves therapeutic indices, decreases toxicology risks, and results in increased therapeutic ration.

Dispersive liquid–liquid microextraction and HPLC to analyse fluoxetine and metoprolol enantiomers in wastewaters

Sample extraction is a major step in environmental analyses due both to the high complexity of matrices and to the low concentration of the target analytes. Sample extraction is usually expensive, laborious, time-consuming and requires a high amount of organic solvents. Actually, there is a lack of miniaturized methodologies for sample extraction and chiral analyses. Here, we developed a dispersive liquid–liquid microextraction (DLLME) to extract the pharmaceuticals fluoxetine and metoprolol, as models of basic chiral compounds, from wastewater samples. Compounds were then analysed by enantioselective high-performance liquid chromatography. We monitored the influence of sample pH, extracting and dispersive solvent and respective volumes, salt addition, extracting and vortexing time. The DLLME method was validated within the range of 1–10 µg L−1 for fluoxetine enantiomers and 0.5–10 µg L−1 for metoprolol enantiomers. Accuracy ranged from 90.6 to 106 % and recovery rates from 54.5 to 81.5 %. Relative standard deviation values lower than 7.84 and 9.00 % were obtained for intra- and inter-batch precision, respectively.

A new lavandulol-related monoterpene in the sex pheromone of the grey pineapple mealybug, Dysmicoccus neobrevipes.

The grey pineapple mealybug, Dysmicoccus neobrevipes, is a serious pest that attacks a variety of crops in tropical regions. Recently, it was recorded on an island in southwestern Japan, suggesting that its distribution is expanding. As a measure against this expansion, a monitoring tool is urgently needed. In this study we determined the structure of the sex pheromone of D. neobrevipes in order to develop an efficient lure for monitoring traps. Volatiles collected from virgin adult females were fractionated by high performance liquid chromatography (HPLC) and gas chromatography, and fractions were tested for attractiveness to males in a laboratory bioassay. A single compound was isolated which was as attractive to males as the crude collections, and this was proposed to be the main, if not the only, component of the female-produced sex pheromone. The structure of this was determined to be (E)-2-isopropyl-5-methylhexa-3,5-dienyl acetate by gas chromatography/mass spectrometry and nuclear magnetic resonance analyses. This compound was synthesized through four steps, and the synthetic chemical was as attractive as the natural product in a greenhouse bioassay. The enantiomers of the synthetic acetate were obtained by enantioselective HPLC fractionation of the corresponding alcohols, and the natural pheromone was shown to be the (+)-isomer. The carbon skeleton of this novel compound is related to lavandulol, a monoterpene with an unusual non-head-to-tail connection of isoprene units that is often found in mealybug pheromones.

Proline functionalization of the mesoporous metal-organic framework DUT-32.

The linker functionalization strategy was applied to incorporate proline moieties into a metal-organic framework (MOF). When 4,4'-biphenyldicarboxylic acid was replaced with a Boc-protected proline-functionalized linker (H(2)L) in the synthesis of DUT-32 (DUT = Dresden University of Technology), a highly porous enantiomerically pure MOF (DUT-32-NHProBoc) was obtained, as could be confirmed by enantioselective high-performance liquid chromatography (HPLC) measurements and solid-state NMR experiments. Isotope labeling of the chiral side group proline enabled highly sensitive one- and two-dimensional solid-state (13)C NMR experiments. For samples loaded with (S)-1-phenyl-2,2,2-trifluoroethanol [(S)-TFPE], the proline groups are shown to exhibit a lower mobility than that for (R)-TFPE-loaded samples. This indicates a preferred interaction of the shift agent (S)-TFPE with the chiral moieties. The high porosity of the compound is reflected by an exceptionally high ethyl cinnamate adsorption capacity. However, postsynthetic thermal deprotection of Boc-proline in the MOF leads to racemization of the chiral center, which was verified by stereoselective HPLC experiments and asymmetric catalysis of aldol addition.

Conventional Chiralpak ID vs. capillary Chiralpak ID-3 amylose tris-(3-chlorophenylcarbamate)-based chiral stationary phase columns for the enantioselective HPLC separation of pharmaceutical racemates.

A comparative enantioselective analysis using immobilized amylose tris-(3-chlorophenylcarbamate) as chiral stationary phase in conventional high-performance liquid chromatography (HPLC) with Chiralpak ID (4.6 mm ID × 250 mm, 5 µm silica gel) and micro-HPLC with Chiralpak ID-3 (0.30 mm ID × 150 mm, 3 µm silica gel) was conducted. Pharmaceutical racemates of 12 pharmacological classes, namely, α- and β-blockers, anti-inflammatory drugs, antifungal drugs, dopamine antagonists, norepinephrine-dopamine reuptake inhibitors, catecholamines, sedative hypnotics, diuretics, antihistaminics, anticancer drugs, and antiarrhythmic drugs were screened under normal phase conditions. The effect of an organic modifier on the analyte retentions and enantiomer recognition was investigated. Baseline separation was achieved for 1-acenaphthenol, carprofen, celiprolol, cizolirtine carbinol, miconazole, tebuconazole, 4-hydroxy-3-methoxymandelic acid, 1-indanol, 1-(2-chlorophenyl)ethanol, 1-phenyl-2-propanol, flavanone, 6-hydroxyflavanone, 4-bromogluthethimide, and pentobarbital on the 4.6 mm ID packed with a 5 µm silica column using conventional HPLC. Nonetheless, baseline separation was achieved for aminoglutethimide, naftopidil, and thalidomide on the 0.3 mm ID packed with a 3 µm silica capillary column.

Determination of levamisole and tetramisole in seized cocaine samples by enantioselective high-performance liquid chromatography and circular dichroism detection

Levamisole, an anthelmintic drug, has been increasingly employed as an adulterant of illicit street cocaine over the last decade; recently, the use of tetramisole, the racemic mixture of levamisole and its enantiomer dexamisole, was also occasionally observed. A new enantioselective high-performance liquid chromatography (HPLC) method, performed on cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phases in normal-phase mode, was validated to determine the enantiomeric composition of tetramisole enantiomers in seized cocaine samples. Furthermore, the hyphenation of the validated HPLC method with a circular dichroism (CD) detection system allowed the direct determination of elution order and a selective monitoring of levamisole and dexamisole in the presence of possible interferences. The method was applied to the identification and quantitation of the two enantiomers of tetramisole in seized street cocaine samples.

Enantioselective high performance liquid chromatography and supercritical fluid chromatography separation of spirocyclic terpenoid flavor compounds

Chiral spirocyclic terpenoids are abundant natural flavors with significant impact particularly on the food industry. Chromatographic methods for analytical and preparative separation of these compounds are therefore of high interest to natural product chemists in academia and industry. Gas chromatography on chiral stationary phases is currently the standard method for the separation of volatile terpenoids, limiting the scale to analytical quantities. We report herein high performance liquid chromatography (HPLC) and supercritical fluid chromatography (SFC) protocols for the chiral separation of several racemic spirocyclic terpenoids such as the important flavors theaspirane and vitispirane. A screening of mobile phases and 16 commercially available chiral stationary phases (CSPs) largely based on polysaccharides led to identification of protocols for the separation of all terpenoids tested. SFC methods were found to be particularly useful for the separation of these spirocyclic flavors due to the volatility and low polarity of the compounds. The reported chiral HPLC and SFC protocols are scalable alternatives to gas chromatographic separations of volatile terpenoid flavors.

Enantioselective HPLC determination and pharmacokinetic study of secnidazole enantiomers in rats.

Secnidazole is a long-lasting nitroimidazole antimicrobial agent that is used as racemic mixture in clinical settings. We developed and validated an enantioselective high-performance liquid chromatography method to determine secnidazole enantiomers in rat plasma. Secnidazole enantiomers and S-(-)-ornidazole (internal standard) were extracted from 50 μL of rat plasma using diethyl ether-dichloromethane (3:2, v/v). Baseline resolution (Rs=2.45) was achieved within 7.0 min on a Chiral-AGP column (150 mm × 4.0mm, 5 μm) at 20°C. The mobile phase consisted of 10mM ammonium acetate-methanol (96:4, v/v) and was delivered at a flow rate of 0.5 mL/min with ultraviolet detection at 318 nm. The method was linear over the concentration range 0.500-100 μg/mL for both enantiomers. The lower limit of quantification was 0.500 μg/mL for both enantiomers. The relative standard deviation values for intra- and inter-day precision were 0.8-8.6 and 1.8-8.2% for S-(+)-secnidazole and R-(-)-secnidazole, respectively. The relative error values of accuracy ranged from -7.8 to 1.1% for S-(+)-secnidazole and from -7.3 to -0.1% for R-(-)-secnidazole. The method was successfully used to determine the pharmacokinetic properties of secnidazole enantiomers in rats after administration of the racemate and individual enantiomers. The pharmacokinetic results indicate that the disposition of secnidazole enantiomers is not stereoselective and chiral inversion and enantiomer-enantiomer interaction do not occur in rats.

Chirality in the Absence of Rigid Stereogenic Elements: Steric and Electronic Effects on the Configurational Stability of C3 Symmetric Residual Tris‐Aryl Phosphanes

Residual stereoisomers result whenever closed subsets of appropriately substituted interconverting isomers (the residual stereoisomers) are generated from a full set of stereoisomers under the operation of a favored stereomerization mechanism. In the case of the three-bladed propellers, differentiation of the edges of the blades and strict correlation in the motion of the rings are the prerequisites for the existence of residual stereoisomers. In these systems, the two-ring flip mechanism is the lowest energy process. It does not interconvert all possible conformational stereoisomers generated by helicity and the three-blade-hub rotors. In the case of C3 symmetric systems, two noninterconverting subgroups (the residual stereoisomers) are generated, each one constituted of quickly interconverting diastereoisomers. A series of tris-aryl phosphanes, structurally designed for existing as residual enantiomers or diastereoisomers, bearing substituents differing in size and electronic properties on the aryl rings, were synthesized and characterized. The configurational stability of residual phosphanes, evaluated by dynamic (1) H- and (31) P-NMR analysis and by dynamic enantioselective high-performance liquid chromatography (HPLC), was found 10 kcal mol(-1) lower than that shown by the corresponding phosphane-oxides. In accordance with the calculations, an unexpectedly low barrier for phosphorus pyramidal inversion was invoked as responsible for the scarce configurational stability of the residual tris-arylphosphanes.