Separation Of Diastereoisomers Research Articles

Chiral Separation of Diastereoisomers of Cefuroxime Axetil by High-Performance Thin-Layer Chromatography

Conditions for separation and determination of diastereoisomers of cefuroxime axetil by HPTLC on cellulose layers with densitometry were developed. A good separation of the constituents was achieved using a mobile phase composed of 1% aqueous beta-cyclodextrin-methanol (15 + 1, v/v). For detecting spots on the chromatograms, densitometric measurements were used at 285 nm. The method is characterized by high sensitivity; the LOD was 0.04 microg/spot and LOQ was 0.11 microg/spot for both forms of the cefuroxime axetil diastereoisomers. For the individual diastereoisomers, high recovery values from 96.63 to 104.16% were obtained. Identification of the diastereoisomers was performed by 1H-NMR spectrometric analysis.

New monolith technology for automated anion-exchange purification of nucleic acids

Synthetic nucleic acid analysis often employs pellicular anion-exchange (AE) chromatography because it supports very high efficiency separations while offering means to control secondary structure, retention and resolution by readily modifiable chromatographic conditions. However, these pellicular anion-exchange (pAE) phases do not offer capacity sufficient for lab-scale oligonucleotide (ON) purification. In contrast, monolithic phases produce fast separations at capacities exceeding their pellicular counterparts, but do not exhibit capacities typical of fully porous, bead-based, anion-exchangers. In order to further increase monolith capacity and obtain the selectivity and mass transfer characteristics of pellicular phases, a surface-functionalized monolith was coated with pAE nanobeads (latexes) usually employed on the pellicular DNAPac phase. The nanobead-coated monolith exhibited chromatographic behaviors typical of polymer AE phases. Based on this observation the monolithic substrate surface porosity and latex diameters were co-optimized to produce a hybrid monolith harboring capacity similar to that of fully porous bead-based phases and peak shape approaching that of the pAE phases. We tested the hybrid monolith on a variety of previously developed pAE capabilities including control of ON selectivity, resolution of derivatized ONs, the ability to resolve RNA ONs harboring aberrant linkages at different positions in a single sequence and separation of phosphorothioate diastereoisomers. We compared the yield and purity of an 8 mg ON sample purified on both the new hybrid monolith and a benchmark AE column based on fully porous monodisperse beads. This comparison included an assessment of the relative selectivities of both columns. Finally, we demonstrated the ability to couple AE ON separations with ESI-MS using an automated desalting protocol. This protocol is also useful for preparing ONs for other assays, such as enzyme treatments, that may be sensitive to high salt levels.

Toward preparative resolution of chiral alcohols by an organic chemical method

Asymmetric alcohols were resolved as 1-α-O-alkyl-2,3-unsaturated hexosides. After separation of diastereoisomers, the auxiliary and the enantiomeric alcohol were recovered by transglycosidation. Potential applications include resolution of labile secondary and tertiary alcohols, difficult by existing techniques, and enhancement of ees of chiral alcohols produced enzymatically or by synthetic catalytic methods.

Diastereoselective addition of Di‐(trimethylsilyl)phosphite to chiral N‐(R)‐α‐methylbenzyl and N‐(1‐methoxycarbonyl‐iso‐pentyl) Schiff bases of various aldehydes

AbstractThe addition of bis‐(trimethylsilyl) phosphite to chiral imines of several aldehydes was diastereoselective. The separation of predominant diastereoisomers of a majority of formed aminophosphonic acids has been observed. Moreover, the ferrocene‐derived acid 5d occurred in diastereoselectivity up to 100%. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:326–331, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20624

IrIII and RuII Complexes Containing Triazole‐Pyridine Ligands: Luminescence Enhancement upon Substitution with β‐Cyclodextrin

Novel 2-(1-substituted-1H-1,2,3-triazol-4-yl)pyridine (pytl) ligands have been prepared by "click chemistry" and used in the preparation of heteroleptic complexes of Ru and Ir with bipyridine (bpy) and phenylpyridine (ppy) ligands, respectively, resulting in [Ru(bpy)(2)(pytl-R)]Cl(2) and [Ir(ppy)(2)(pytl-R)]Cl (R=methyl, adamantane (ada), beta-cyclodextrin (betaCD)). The two diastereoisomers of the Ir complex with the appended beta-cyclodextrin, [Ir(ppy)(2)(pytl-betaCD)]Cl, were separated. The [Ru(bpy)(2)(pytl-R)]Cl(2) (R=Me, ada or betaCD) complexes have lower lifetimes and quantum yields than other polypyridine complexes. In contrast, the cyclometalated Ir complexes display rather long lifetimes and very high emission quantum yields. The emission quantum yield and lifetime (Phi=0.23, tau=1000 ns) of [Ir(ppy)(2)(pytl-ada)]Cl are surprisingly enhanced in [Ir(ppy)(2)(pytl-betaCD)]Cl (Phi=0.54, tau=2800 ns). This behavior is unprecedented for a metal complex and is most likely due to its increased rigidity and protection from water molecules as well as from dioxygen quenching, because of the hydrophobic cavity of the betaCD covalently attached to pytl. The emissive excited state is localized on these cyclometalating ligands, as underlined by the shift to the blue (450 nm) upon substitution with two electron-withdrawing fluorine substituents on the phenyl unit. The significant differences between the quantum yields of the two separate diastereoisomers of [Ir(ppy)(2)(pytl-betaCD)]Cl (0.49 vs. 0.70) are attributed to different interactions of the chiral cyclodextrin substituent with the Delta and Lambda isomers of the metal complex.

A simple entry into 1,3-diols from ( R)-2,3-cyclohexylideneglyceraldehyde: synthesis of (−)-galantinic acid

Aldehydes 5, 7, and 9 derived from easily accessible ( R)-2,3-cyclohexylideneglyceraldehyde 1 were used as novel substrates to obtain both syn- and anti-1,3-diols in several individual reactions by subjecting each of them to some practically viable metal-mediated Barbier-type allylations under moist conditions. In this regard, a detailed investigation was made regarding the compatibility and stereoselectivity associated with four such metal-mediated allylations of these aldehydes 5–7. Good yields with moderate selectivity in several successful reactions with easy chromatographic separation of diastereoisomers of the products have been elegantly exploited to isolate two pairs of enantiomerically pure syn-1,3 and anti-1,3-diols ( 6a and 6b; 10a and 10b) in substantial amounts. Finally, 10b has been exploited to synthesize (−)-galantinic acid A.

Separation of diastereoisomers of Ara-C phosphotriesters using solid phase extraction and HPLC for the study of their decomposition kinetic in cell extracts

Separations of the diastereoisomers of three nucleoside 5′-phosphotriester derivatives of Ara-C ( tBuSATE, hydroxy tBuSATE and bishydroxy tBuSATE phenylphosphotriester derivatives; pronucleotides) were performed by HPLC using derivatized cellulose and amylose chiral stationary phases. An optimal baseline separation ( Rs > 1.5) was readily obtained with an amylose based chiral column (AD-H) used in normal phase mode. This stereospecific HPLC method has been associated to a solid phase extraction step using a C18 cartridge and an internal standard for the quantification of one nucleoside 5′-phosphotriester derivative in cell extracts. After optimization, this method was validated in terms of specificity, recovery, linearity, precision and accuracy and detection limit. It was applied to the determination of the apparent rate constants of disappearance and half-lives of each diastereoisomer. This enabled us to conclude that the enzymatic activity involved in the first step of the decomposition pathway of the hydroxyl tBuSATE phenylphosphotriester of Ara-C is stereoselective and is related to the nature of the pyrimidic base.

On the Search for NNO-Donor Enantiopure Scorpionate Ligands and Their Coordination to Group 4 Metals

The preparation of new chiral bis(pyrazol-1-yl)methane-based NNO-donor scorpionate ligands in the form of the lithium derivatives [Li(bpzb)(THF)] [1; bpzb = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide] and [Li(bpzte)(THF)] [2; bpzte = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-p-tolylethoxide] or the alcohol ligands (bpzbH) (3) and (bpzteH) (4) has been carried out by 1,2-addition reactions with trimethylacetaldehyde or p-tolualdehyde. The separation of a racemic mixture of the alcohol ligand 3 has been achieved and gave an enantiopure NNO alcohol-scorpionate ligand in three synthetic steps: (i) 1,2-addition of the appropriate lithium derivative to trimethylacetaldehyde, (ii) esterification and separation of diastereoisomers 5, (iii) saponification. Subsequently, the enantiopure scorpionate ligand (R,R)-bpzmmH {6; R,R-bpzmmH = (1R)-1-[(1R)-6,6-dimethylbicyclo[3.1.1]2-hepten-2-yl]-2,2-bis(3,5-dimethylpyrazol-1-yl)ethanol} was obtained with an excellent diastereomeric excess (>99% de) in a one-pot process utilizing the aldehyde (1R)-(-)-myrtenal as a chiral substrate to control the stereochemistry of the newly created asymmetric center. These new chiral heteroscorpionate ligands reacted with [MX(4)] (M = Ti, Zr; X = NMe(2), O(i)Pr, OEt, O(t)Bu) in a 1:1 molar ratio in toluene to give, after the appropriate workup, the complexes [MX(3)(kappa(3)-NNO)] (7-18). The reaction of Me(3)SiCl with [Ti(NMe(2))(3)(bpzb)] (7) or [Ti(NMe(2))(3)(R,R-bpzmm)] (11) in different molar ratios gave the halide-amide-containing complexes [TiCl(NMe(2))(2)(kappa(3)-NNO)] (19 and 20) and [TiCl(2)(NMe(2))(kappa(3)-NNO)] (21 and 22) and the halide complex [TiCl(3)(kappa(3)-NNO)] (23 and 24). The latter complexes can also be obtained by reaction of the lithium compound 1 with TiCl(4)(THF)(2) and deprotonation of the alcohol group of 6 with NaH, followed by reaction with TiCl(4)(THF)(2) in a 1:1 molar ratio, respectively. Isolation of only one of the three possible diastereoisomers of the complexes 19 and 22 revealed that chiral induction from the ligand to the titanium center took place. The structures of these complexes were elucidated by (1)H and (13)C{(1)H} NMR spectroscopy, and the X-ray crystal structures of 3-7, 12, and 24 were also established. Finally, we evaluated the influence that the chiral center of the new heteroscorpionate complexes has on the enantioselectivity of the asymmetric epoxidation of allylic alcohols.

A regio- and stereoisomeric study of allylic alcohol fluorination with a range of reagents

A range of dehydroxyfluorination reagents was reacted with separate diastereoisomers of a chiral allylic alcohol to explore both the regio- and stereoselectivity ratios of direct versus allylic fluorination. The allylic alcohol stereoisomers gave the same predominant fluorinated diastereoisomer indicating that the reaction proceeds with a significant S N1 component via an allylic carbocation intermediate, which is quenched by fluoride ion, predominantly from the least hindered face. None of the reagents displayed very high regio- or stereoselectivity, although in all cases the allylic fluorination products predominated.

Fluorinated Analogues of Amicetose and Rhodinose – Novel Racemic and Asymmetric Routes

AbstractTrifluoroethanol was converted into difluorinated (racemic) analogues of amicetose and rhodinose by metallated difluoroenol acetal chemistry, protection, release of the latent difluoromethyl ketone, stereoselective reduction and ozonolysis in acidic methanol. A fortuitous separation of diastereoisomers allowed the diastereoisomeric pyranoses to be obtained cleanly. Though reductive defluorination allowed a facile entry to the route, the corresponding monofluoro sugar analogues could not be separated. Instead, Sharpless asymmetric epoxidation followed by epoxide ring‐opening with an unusual nucleophilic fluoride source allowed enantiomerically highly enriched and selectively protected fluorodiols to be obtained. Ozonolysis then afforded the methyl pyranosides, which could be transformed in a number of ways. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Simultaneous determination of diastereoisomeric and enantiomeric impurities in SSS-octahydroindole-2-carboxylic acid by chiral high-performance liquid chromatography with pre-column derivatization

SSS-Octahydroindole-2-carboxylic acid ( SSS-Oic) is a key intermediate used in the synthesis of some angiotensin-converting enzyme (ACE) inhibitors. The separation of diastereoisomers and enantiomers of Oic was performed using a pre-column derivatization chiral HPLC method. Phenyl isothiocyanate (PITC) was used as the derivatization reagent. Three PITC derivatives of Oic stereoisomers were separated on an Ultron ES-OVM chiral column (150 mm × 4.6 mm, 5 μm). Derivatization conditions such as reaction temperature, reaction time and derivatization reagent concentration were investigated. The chromatographic conditions for separation of the three PITC-Oic derivatives were optimized. The method was successfully applied in the diastereoisomeric and enantiomeric purity test of SSS-Oic.

Organic base catalyzed carbonyl allylation of methyl trifluoropyruvate with activated alkenes

The carbonyl allylation of methyl trifluoropyruvate (MeTFP) with activated alkenes has been investigated in detail using organic bases as catalysts. Organic bases, such as DMAP, Et 3N, DABCO, NMM, Et 2NH, and quinine, could deprotonate the allylic hydrogen of activated alkenes and furnish nucleophilic species to undergo the addition reaction with methyl trifluoropyruvate and afford versatile homoallylic alcohols with CF 3 group in excellent yields. The 19F NMR monitoring indicated that the isomerization induced by base gave two separable diastereoisomers in an equilibrium ratio of 1:3. The relative configuration of hydroxy and the neighboring alkyl group in the major diastereoisomer was determined as syn-configuration by X-ray diffraction analysis.

FRAGMENTACIÓN DIASTEREOESPECÍFICA EN NCI DE DOS LIGNANOS DIBENCILBUTÁNICOS DE LA CORTEZA DE Dugandiodendron argyrotrichum (MAGNOLIACEAE)

Two dibenzylbutane lignans, dihydroguairetic acid and austrobailigan-6 were isolated as mixture of their diastereoisomers from the bark ethanolic extract of Dugandiodendron argyrotrichum (Magnoliaceae), a colombian endangered species. Characterization was achieved by 1H and 13C NMR, DEPT, COSY, HMQC, HMBC and GCMS analysis. The diastereoisomer separation by GC and the diastereospecific fragmentation patterns in negative chemical ionization mass spectrometry (NCI-MS) spotlights on an interesting tool for stereochemical research.

Synthesis and Characterization of Ruthenium and Rhenium Nucleosides

We report the synthesis and characterization of new ruthenium and rhenium nucleosides [Ru(tolyl-acac)2(IMPy)-T] (tolyl-acac=di(p-methylbenzonatemethane), IMPy=2'-iminomethylpyridine, T=thymidine) (5) and [Re(CO)3(IMPy)-T]Cl (9), respectively. Structural analysis of 9 shows that the incorporation of this metal complex causes minimal perturbation to the sugar backbone and the nucleobase. Eletrochemical (5, E1/2=0.265 V vs NHE; 9, E1/2=1.67 V vs NHE), absorption (5, lambdamax=600, 486 nm; 9, lambdamax=388 nm), and emission (9, lambdamax=770 nm, pi=17 ns) data indicate that 5 and 9 are suitable probes for DNA-mediated ground-state electron-transfer studies. The separation and characterization of diastereoisomers of 5 and bipyridyl-based ruthenium nucleoside [Ru(bpy)2(IMPy)-T]2+ (7) are reported.

Chemical Separation of Bioactive Licorice Compounds Using Capillary Electrophoresis

Four bioactive components derived from licorice root, i.e., glycyrrhizin (GL), 18α‐glycyrrhetinic acid (18α‐GA), 18β‐glycyrrhetinic acid (18β‐GA), and isoliquiritigenin (IQ) were separated by capillary electrophoresis (CE). For the first time, separation of diastereoisomers of 18α‐GA and 18β‐GA by CE using a chiral additive has been achieved. Simultaneous separation of the above four pharmacological active components in one run by CE was reported. Different modes of CE, such as capillary zone electrophoresis (CZE), micellar electrokinetic capillary chromatography (MECC), cyclodextrin‐MECC (CD‐MECC), were employed to optimize the chemical separation. Preliminary experiments started with CZE using a 50 mM sodium tetraborate buffer within a capillary tube of inner diameter 50 µm and length of 60.2 cm. A CE separation voltage of 17 kV was used. Detection was achieved at a distance of 50 cm from the capillary inlet using a diode array UV absorbance detector over the wavelength range from 190 to 300 nm. The optimum separation was achieved by 10 mM sodium tetraborate‐15 mM β‐CD‐25 mM SC. The effects of various experimental parameters, including pH, surfactant concentration, temperature, and organic modifier, on effective separation were investigated. The conditions of pH 8.5 and temperature at 25°C have resulted in more effective separations.

Lithium Bromide-Promoted Three-Component Synthesis of Oxa-Bridged Tetracyclic Tetrahydroisoquinolines

Heating a toluene soln. of an alpha -(isocyano)acetamide deriv., an aldehyde and a 3-(2-aminophenyl)propenoic acid (aniline) deriv. in the presence of lithium bromide at 60 Deg provides an oxa-bridged tetracyclic tetrahydroquinoline in good to excellent yield as a mixt. of two separable diastereoisomers. In this one-pot multicomponent reaction, one C-N, one C-O and three C-C bonds are formed with concomitant creation of five asym. centers and three heterocycles. Fragmentation of epoxy-tetrahydroquinoline an under mild acidic conditions affords a 4,6-phenanthroline deriv. The multicomponent reaction of 2-isocyano-1-(4-morpholinyl)-3-phenyl-1-propanone with heptanal and 3-(2-aminophenyl)-2-propenoic acid Me ester gave (1R,2S,4aS,5S,10bS)-rel-5-hexyl-1,5,6,10b-tetrahydro-2-(4-morpholinyl)-3-(phenylmethyl)-2H-2,4a-epoxybenzo[f][1,7]naphthyridine-1-carboxylic acid Me ester (major product). [on SciFinder (R)]

Asymmetric Synthesis of Bi(thio)xanthylidene Overcrowded Alkenes

AbstractOvercrowded alkenes are a fascinating class of inherent dissymmetric molecules that attract considerable interest for instance as chiroptical molecular switches and unidirectionally rotary motors. A practical synthesis route towards enantiomerically pure overcrowded alkenes is an important goal. We report here the development of an asymmetric synthesis of bi(thio)xanthylidenes. The upper and lower part of the desired alkenes were first coupled to a chiral template. Intramolecular coupling with partial stereocontrol, separation of diastereoisomers and subsequent removal of the templates gave stable enantiomers of bithioxanthylidene. The absolute configuration was determined by X‐ray analysis. In the case of bixanthylidene, stereocontrol during the intramolecular coupling reaction was complete, and it was found that the presence of a binaphthol bridging unit prevents the fast racemization process of the bixanthylidene, which it normally exhibited at ambient temperature. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Chiral α-Branched Benzylic Carbocations: Diastereoselective Intermolecular Reactions with Arene Nucleophiles and NMR Spectroscopic Studies

The chiral benzylic alcohols 1-6 were prepared and subjected to S(N)1-type displacement reactions with various arene nucleophiles in acidic medium. Under optimized conditions (HBF(4).OEt(2), CH(2)Cl(2), -78 degrees C --> r.t.) the corresponding 1,1-diarylalkanes 11-18 and 20 were obtained in good chemical yields (48-99%). The facial diastereoselectivity of the reaction is high (d.r. = 91/9-97/3) when the substrate bears a stereogenic carbon center -CHtBuMe in the alpha-position to the electrophilic carbon atom. If the starting material was enantiomerically pure, no significant racemization was observed (94% ee --> 92% ee). The reactions proceed stereoconvergently as demonstrated by the conversion of the separated diastereoisomers syn-1a and anti-1a in separate reactions to the same product syn-11 (d.r. = 97/3). Further evidence for long-lived chiral benzylic carbocations as reaction intermediates was obtained from NMR studies in superacidic medium. The chiral cation 24 was generated in SO(2)ClF as the solvent at -70 degrees C employing SbF(5) as the Lewis acid and characterized by its (1)H and (13)C NMR spectra. NOE measurements suggest a preferred conformation in which the diastereotopic faces of the cation are differentiated by the two carbon substituents R and Me at the stereogenic carbon center in the alpha-position. The hypothesis is further supported by the observation that the diastereoselectivity of the substitution reaction decreases if the bulky tert-butyl (R = tBu) substituent in the substrate 1a is replaced by a smaller ethyl group (2a, R = Et).

Flow‐through partial‐filling affinity capillary electrophoresis using a crossreactive antibody for enantiomeric separations

It is demonstrated that the separation of diastereoisomers and enantiomers can be accomplished by the flow-through partial-filling affinity CE using a crossreactive mAb. This approach revealed differences in the binding strength of the (+/-)-cis- and (+/-)-trans-benzo[a]pyrene tetrols with the anti-polycyclic aromatic hydrocarbon mAb and demonstrated that (+)-enantiomers are more strongly immunocomplexed than their (-)-counterparts. It is proposed that crossreactive monoclonal antibodies (i.e. mAb raised against achiral molecule and possessing limited selectivity) could be effectively utilized for specific stereoisomeric differentiation and chiral separations.

Chiral separations of mandelic acid by HPLC using molecularly imprinted polymers

Styrene is used in a variety of chemical industries. Environmental and occupational exposuresto styrene occur predominantly through inhalation. The major metabolite of styrene is present in twoenantiomeric forms, chiral R- and S- hydroxy-1-phenyl-acetic acid (R-and S-mandelic acid, MA). Thus, theconcentration of MA, particularly of its enantiomers, has been used in urine tests to determine whetherworkers have been exposed to styrene.This study describes a method of analyzing mandelic acid using molecular imprinting techniques andHPLC detection to perform the separation of diastereoisomers of mandelic acid. The molecularly imprintedpolymer (MIP) was prepared by non-covalent molecular imprinting using (+) MA, (-) MA or (+)phenylalanine, (-) phenylalanine as templates. Methacrylic acid (MAA) and ethylene glycol dimethacrylate(EGDMA) were copolymerized in the presence of the template molecules. The bulk polymerization wascarried out at 4 o C under UV radiation. The resulting MIP was grounded into 25~441⁄4m particles, whichwere slurry packed into analytical columns. After the template molecules were removed, the MIP-packedcolumns were found to be effective for the chromatographic resolution of (±)-mandelic acid. This methodis simpler and more convenient than other chromatographic methods.