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The capacity of nonsteroidal antiinflammatory drugs (NSAIDs) to prevent prostanoids biosynthesis through the inhibition of COX-2 enzyme is related to their structural backbone, based on the fusion of a cis-stilbene unit with a variety of heterocyclic and carbocyclic rings. By this route, a series of new selective COX-2 inhibitors was developed, by maintaining the 4-methylsulfone or 4-methylsulfonamide substituent on the phenyl moiety, essential for their activity. In this frame, two novel propyl sulfoxide derivatives were synthesized, which proved selective and sufficiently potent COX-2 inhibition activity when tested as racemates. In the present study, the use of a cellulose tris(3,5-dichlorophenylcarbamate)-based chiral stationary phase, in a polar-organic mode of elution, enabled the successful enantioseparation of the investigated compounds. The developed chromatography method reveals a useful tool of monitoring in view of a proper forthcoming enantioselective synthetic protocol. Moreover, the optimized chromatographic conditions allowed the isolation of appropriate amounts of single enantiomers for the electronic circular dichroism studies that, coupled with in silico simulations, allowed assessing the absolute configuration of each species. [ABSTRACT FROM AUTHOR]

Peptide stereoisomer analysis is of importance for quality control of therapeutic peptides, the analysis of stereochemical integrity of bioactive peptides in food, and the elucidation of the stereochemistry of peptides from a natural chiral pool which often contains one or more D-amino acid residues. In this work, a series of model peptide stereoisomers (enantiomers and diastereomers) were analyzed on a zwitterionic ion-exchanger chiral stationary phase (Chiralpak ZWIX(+) 5 µm), in order to investigate the retention and separation performance for such compounds on this chiral stationary phase and elucidate its utility for this purpose. The goal of the study focused on 1) investigations of the effects of the sample matrix used to dissolve the peptide samples; 2) optimization of the mobile phase (enabling deriving information on factors of relevance for retention and separation); and 3) derivation of structure-selectivity relationships. It turned out that small di- and tripeptides can be well resolved under optimized conditions, typically with resolutions larger than 1.5. The optimized mobile phase often consisted of methanol-tetrahydrofuran-water (49:49:2; v/v/v) with 25mM formic acid and 12.5mM diethylamine. This work proposes some guidance on which mobile phases can be most efficiently used for peptide stereoisomer separations on Chiralpak ZWIX. [ABSTRACT FROM AUTHOR]

A chiral chromatography method enabling the simultaneous diastereo- and enantioseparation of N α -Boc- N 4 -(hydroorotyl)-4-aminophenylalanine [Boc-Aph(Hor)-OH, 1 ] was optimized with a quinine-based zwitterionic stationary phase. The polar-ionic eluent system consisting of ACN:MeOH:water—49.7:49.7:0.6 (v/v/v) with formic acid (4.0 mM) and diethylamine (2.5 mM), allowed the successful separation of the four acid stereoisomers: α d , d -/ d , l - 1 = 1.08; α d , l -/ l , d - 1 = 1.08; α l , d -/ l , l - 1 = 1.40. According to the in-house developed synthetic procedure and the recorded electronic circular dichroism spectra, the following stereoisomeric elution order was readily established in the optimal chromatographic conditions: d , d - 1 < d , l - 1 < l , d - 1 < l , l - 1 . With the aim of better understanding the molecular basis of the retention behaviour of the four stereoisomers in the employed chromatographic system and conditions, a computational protocol consisting in molecular dynamics simulations was applied. The use of the three descriptors INTER (in kcal mol −1 , encoding for the interaction energy between the selector SO unit and the whole system), INTER_SA (in kcal mol −1 , encoding for the interaction energy between SO and the sole selectand SA), and SELF (in kcal mol −1 , encoding for the conformational energy of SA relative to its minimum energy registered by the collected snapshots) revealed the active role of achiral sub-structural elements of the chiral stationary phase and eluent components in the overall stereorecognition mechanism. [ABSTRACT FROM AUTHOR]

Abstract: An asymmetric synthetic strategy was designed for the preparation of the four possible diastereoisomers of 3,6-dimethyl-1-(2-methylphenyl)-4-(4-phenoxyphenyl)-4,8-dihydro-1H-pyrazolo[3,4-e][1,4]thiazepin-7-one, a non-steroidal FXR agonist, we recently discovered following a virtual screening approach. The results obtained from an AlphaScreen assay clearly demonstrated that only the isomer endowed with 4R,6S absolute configuration is responsible for the biological activity. A deep investigation of the different putative binding modes adopted by these enantiomerically pure ligands using computational modeling studies confirmed the enantioselectivity of FXR towards this class of molecules. [Copyright &y& Elsevier]

In the present study, the "Inverted Chirality Columns Approach (ICCA)" was applied to follow an asymmetric synthetic reaction, namely, the addition of butan-2-one to trans -β-nitrostyrene, catalysed by (S)-proline, leading to the formation of 3-methyl-4-phenyl-5-nitro-2-pentan-2-one. The ICCA method was applied to overcome the lack of pure enantiomeric standards. The two widely employed (R , R)- and (S , S)-Whelk-O1 chiral stationary phases (CSPs), incorporating fully synthesized enantiomeric chiral selectors, were profitably used for this purpose. The enantioselective analysis with the two CSPs was performed under optimized reversed-phase conditions with a water/acetonitrile (60/40, v/v) eluent. In the probe reaction under investigation, a diastereomeric excess >90% was found according to a well-established reaction mechanism, thus affording the enantiomer couple (3 S ,4 R)-3-methyl-4-phenyl-5-nitropentan-2-one and (3 R ,4 S)-3-methyl-4-phenyl-5-nitropentan-2-one as the main product. Therefore, the attention was exclusively focused on this enantiomers pair. Rather similar retention and separation factor [1.12 with (R , R)-Whelk-O1 and 1.13 with (S , S)-Whelk-O1] values as well as resolutions [2.06 with (R , R)-Whelk-O1 and 2.30 with (S , S)-Whelk-O1] were produced by the two enantiomeric CSPs. Applying the ICCA concept allowed to identify the two enantiomers-related peaks in the chromatograms, ultimately indicating a 65-to-35 enantiomeric per cent ratio. Electronic circular dichroism (ECD) and high-resolution mass spectrometry analyses of the two peaks collected during the enantioselective analyses further confirmed the enantiomeric nature of the identified compounds. The (3 S ,4 R) < (3 R ,4 S) enantiomer elution order with the (R , R)-Whelk-O1 was fully disclosed thanks to ECD studies coupled with in silico quantum mechanical simulations. As expected, reversed elution order turned out with (S , S)-Whelk-O1. Image 1 • The ICCA was applied to monitor an enantioselective synthesis without enantiomeric standards. • For the enantioseparation, the (R,R)- and (S,S)-Whelk-O1 CSPs were used under RP conditions. • Experimental and in silico ECD spectra defined the enantiomer elution order. • HRMS analysis facilitated the identification of the enantiomers on the chromatogram. [ABSTRACT FROM AUTHOR]

Chiral ligand‐exchange chromatography is one of the elective strategies for the direct enantioresolution of small chelating compounds: amino acids, diamines, amino alcohols, diols, small peptides, etc. Unlike other methods, the interaction between chiral selector and analyte enantiomers is mediated by a cation, thus producing diastereomeric ternary complexes. Two main approaches are conventionally applied in chiral ligand‐exchange chromatography. The first relies upon chiral stationary phases where the chiral selector is either covalently immobilized or physically adsorbed onto suitable packing materials (coated phases). In the second approach, chiral molecules are added to the eluent, thus generating chiral eluent systems. Among the advantages of chiral ligand‐exchange chromatography, the generation of UV/vis‐active metal complexes, and the use of commercially available or easy‐to‐synthesize chiral selectors, in combination to rather inexpensive achiral columns for coated phases and chiral eluents, are noteworthy. Besides amino acids and amino alcohols, other species have proven suitable for chiral ligand‐exchange chromatography applications. Recently, the use of either chiral ionic liquids or micellar liquid chromatography systems as well as the successful off‐column formation of diastereomeric complexes have expanded the selectivity profiles and application fields. All of these issues are touched in the review, shedding light to the contributions appeared in the last decade. [ABSTRACT FROM AUTHOR]

In the present paper we describe the optimization and the application of a chromatographic method suitable to get all four diastereoisomers of C24–C25 cyclopropyl dafachronic acid derivatives in sufficient amount for their biological appraisal towards the nuclear hormone receptor transcription factor DAF-12. A preliminary column screening of six anion exchange type Cinchona alkaloid-based chiral stationary phases (CSPs) allowed to identify the one with a quinine scaffold and carrying a O -9-(3,5-bis(trifluoromethyl)phenyl) moiety at C9 position as the best CSP. Few modifications of the experimental conditions revealed that a content of 18 mM acetic acid used as counterion and displacer in acetonitrile and a column temperature fixed at 35 °C were optimal for the simultaneous discrimination of all four diastereoisomers with a 1.0 mL/min flow rate. With such conditions, transoid ( S , S ) and ( R , R ) diastereoisomers were resolved with R S > 1.4. With non chiral reversed-phase columns, neither the cisoid nor the transoid diastereoisomers could be resolved. This way, ca. 1.0 mg of each stereoisomer was isolated with a diastereomeric purity >98%, suitable for the following biological tests. The indirect stereochemical assignments of the four diastereoisomers, and hence the corresponding chromatographic elution order (24 R ,25 R ) < (24 S ,25 S ) < (24 R ,25 S ) < (24 S ,25 R ) were made in an analogy manner on the basis of the resolution of fully assigned and structurally very similar ursodeoxycholic acid derivatives. As support of this indirect way of assigning the absolute configuration of the C24 and C25 chiral centre a molecular modeling procedure based on dynamic simulation was successfully applied. [ABSTRACT FROM AUTHOR]

The enantiomers of trans -paroxetine were separated on four chiral stationary phases (CSPs) based on chiral zwitterionic Cinchona alkaloids fused with ( R,R )- or ( S,S )- trans -2-aminocyclohexanesulfonic acid. The enantioseparations were carried out in polar-ionic or in hydro-organic mobile phases with MeOH/THF, MeCN/THF, MeCN/THF/H 2 O and MeOH/MeCN/THF containing organic acid and base additives, in the temperature range 0–50 °C. The effects of the mobile phase composition, the natures and concentrations of the additives and temperature on the separations were investigated. Thermodynamic parameters were calculated from plots of ln α vs 1/ T . Δ(Δ H °) ranged between −3.0 and +1.5 kJ mol −1 , and Δ(Δ S °) between −8.8 and +5.9 J mol −1 K −1 . The enantioseparation was generally enthalpically controlled, the retention factor and separation factor decreasing with increasing temperature, but entropically controlled separation was also observed. The elution sequences of the paroxetine enantiomers on the two pairs of pseudo-enantiomeric CSPs were investigated, and an attempt was made to explain the observed anomalies in silico in order to gain an insight into the underlying molecular recognition events between the four chiral selectors and the analyte enantiomers. [ABSTRACT FROM AUTHOR]

Abstract: With the use of a chiral ligand-exchange chromatography (CLEC) system operating with the O-benzyl-(S)-serine [(S)-OBS] [1,2] as the chiral mobile phase (CMP) additive to the eluent, the effect of the copper(II) anion type on retention (k) and separation (α) factors was evaluated, by rationally changing the following experimental conditions: salt concentration and temperature. The CLEC–CMP analysis was carried out on ten amino acidic racemates and with nine different cupric salts. While the group of analytes comprised both aliphatic (leucine, isoleucine, nor-leucine, proline, valine, nor-valine, and α-methyl-valine) and aromatic (1-aminoindan-1,5-dicarboxylic acid, phenylglycine, and tyrosine) species, representative organic (formate, methanesulfonate, and trifluoroacetate) and inorganic (bromide, chloride, fluoride, nitrate, perchlorate, and sulfate) Cu(II) salts were selected as the metal source into the eluent. This route of investigation was pursued with the aim of identifying analogies among the employed Cu(II) salts, by observing the variation profile of the selected chromatographic parameters, upon a change of the above experimental conditions. All the data were collected and analyzed through a statistical approach (PCA and k-means clustering) that revealed the presence of two behavioral classes of cupric salts, sharing the same variation profile for k and α values. Interestingly, this clustering can be explained in terms of ESP (electrostatic surface potential) balance (ESPbal) values, obtained by an ab initio calculation operated on the cupric salts. The results of this appraisal could aid the rational choice of the most suitable eluent system, to succeed in the enantioseparation of difficult-to-resolve compounds, along with the eventual scale-up to a semi-preparative level. [Copyright &y& Elsevier]