Diastereomeric Forms Research Articles

Diastereomerism in tetranuclear copper(II) complexes of a phenol based “end-off” compartmental ligand

Abstract Two diastereomeric forms of the μ4-oxo-bridged tetranuclear copper(II) complex [Cu4(O)(L)2(Ac)4] (where L = N2,O-donor Schiff-base ligand and Ac = acetate anion), differing for the bridging arrangement of the carboxylate groups, were individually synthesized and structurally characterized. Their formation was rationalized by DFT calculation and their catecholase-like activity was studied.

Chemical resolution of enantiomers of 3,4-dihydropyrimidin-2(1H)-ones using chiral auxiliary approach

Inherently racemic DHPMs have been resolved using chemical resolution methodology by appending enantiopure chiral auxiliary at N-1 or N-3 position of the DHPMs, leading to the formation of both diastereomers, which were separated chromatographically to obtain both enantiomers (ee upto 99.9%) of DHPMs, after removal of the auxiliary. Absolute configuration of the enantiomers has been assigned.

Short-term stability studies of ampicillin and cephalexin in aqueous solution and human plasma: Application of least squares method in Arrhenius equation

A limited number of studies with application of the Arrhenius equation have been reported to drugs and biopharmaceuticals in biological fluids at frozen temperatures. This paper describes stability studies of ampicillin and cephalexin in aqueous solution and human plasma applying the Arrhenius law for determination of adequate temperature and time of storage of these drugs using appropriate statistical analysis. Stability studies of the beta-lactams in human plasma were conducted at temperatures of 20°C, 2°C, −20°C and also during four cycles of freeze-thawing. Chromatographic separation was achieved using a Shimpak C18 column, acetonitrile as organic modifier and detection at 215nm. LC-UV–MS/MS was used to demonstrate the conversion of ampicillin into two diastereomeric forms of ampicilloic acid. Stability studies demonstrated degradation greater than 10% for ampicillin in human plasma at 20°C, 2°C and −20°C after 15h, 2.7days, 11days and for cephalexin at the same temperatures after 14h, 3.4days and 19days, respectively, and after the fourth cycle of freezing–thawing. The Arrhenius plot showed good prediction for the ideal temperature and time of storage for ampicillin (52days) and cephalexin (151days) at a temperature of −40°C, but statistical analysis (least squares method) must be applied to avoid incorrect extrapolations and estimated values out uncertainty limits.

Enhancement of Diastereoselectivity in Photodimerization of Alkyl 2-Naphthoates with Chiral Auxiliaries via Inclusion within γ-Cyclodextrin Cavities

Irradiations of alkyl 2-naphthoates are known to result in four isomeric "cubane-like" photodimers: anti(HH)-2, syn(HH)-2, anti(HT)-2, and syn(HT)-2 where the anti(HH)-2, anti(HT)-2, and syn(HT)-2 consist of pairs of diastereomers. Here, chiral auxiliary and chiral microreactor strategies have been combined to achieve high diastereoselectivity in photodimerizations of an enantiomeric pair of 2-naphthoates with (R)- and (S)-1-methoxycarbonylethyl esters as chiral auxiliaries (1R and 1S). Thus, irradiations of their γ-cyclodextrin (γ-CD) complexes have been conducted. Fluorescence, IR, and NMR spectra of both enantiomers of 1 demonstrate that their γ-CD complexes are mainly 2:2 with the molecules of 1 in head-to-head orientations. Irradiation of the complexes in the solid state mainly resulted in anti(HH)-2. The absolute configuration of each diastereomer of anti(HH)-2 has been established for the first time here. The diastereomeric excesses (de's) of anti(HH)-2 from 1R and 1S were 94% and 86%, respectively. These de's are much higher than those found from irradiations in solution (55% for 1R and 1S), where the opposite diastereomeric form is in excess! Calculations of the energies of various conformations of the head-to-head 2:2 inclusion complexes were performed using the PM3 approach. The predicted major diastereomers based on the calculation are consistent with those found experimentally.

Investigation of isomer formation upon coordination of bifunctional histidine analogues with 99mTc/Re(CO)3

Histidine is a convenient tridentate chelator used in the synthesis of technetium-99m radiopharmaceuticals, as it can be pendantly attached to a biomolecule for molecular imaging applications. Once coordinated, it forms a neutral complex that is capable of forming diastereomers at the alpha amine of the histidine. This is demonstrated through the synthesis and characterization of four different histidine chelators; three small molecule chelators, which consist of a benzylated histidine at the alpha amine, and one modified dipeptide, containing a phenylalanine derivative at the C-terminus and a histidine at the N-terminus. Upon rhenium coordination, two products are observed, each having the desired exact mass of the metal-containing species. The two products have been characterized through LC-MS, (1)H, gCOSY, NOESY and ROESY NMR experiments, and the relative stereochemistry determined. The implications of diastereomer formation when using this chelation system for creating molecular imaging agents is also discussed.

Preparative scale isolation, purification and derivatization of mimosine, a non-proteinogenic amino acid

Focusing on drug discovery non-proteinogenic amino acids have often been used as important building blocks for construction of compound libraries in the filed of combinatorial chemistry and chemical biology. Highly homogeneous L: -mimosine, α-amino-β-(3-hydoxy-4-oxo-1,4-dihydropyridin-1-yl)-propanoic acid, a non-proteinogenic amino acid, has been successfully isolated and purified on an industrial scale from wild leaves of Leucaena (Leucaena leucocephala de Wit) which is a widely distributed legume in Okinawa, a sub-tropical island in Japan. Optical purity determinations used for quality control have been established through diastereomer formation. Physico-chemical properties and biological properties of purified mimosine have been clarified. Mimosine is sparingly soluble in water and organic solvents but can be dissolved in aqueous alkaline solution. The tyrosinase pathway is of particular interest in the cosmetic field, since mimosine is an analog of tyrosine. Thus the present purified mimosine have been tested in tyrosinase inhibitory assays. The IC50 for tyrosinase inhibitory activity of purified Mim was compared with kojic acid. Mimosine shows significant inhibition of melanin production in murine melanoma cells. The derivatization of mimosine has been investigated with a focus on its use in conventional peptide syntheses to generate mimosyl peptides. N-(9-Fluorenylmethoxycarbonyloxy)-mimosine and resin-bound mimosine for solid-phase syntheses have also been performed. Highly homogeneous Mim is a useful material for the development of functional cosmetics or active pharmaceutical ingredients.

ChemInform Abstract: Asymmetric Cyclization Reactions of N‐Benzoyl‐α‐dehydroarylalanine Alkyl Esters via Photoinduced Electron Transfer from Achiral and Chiral Amines.

AbstractThe photoinduced cyclization of chiral auxiliary‐substituted (Z)‐enamides (I) gives the diastereomeric forms of the cis‐ (II)/(III) and trans‐configured (IV)/(V) oxazole derivatives.

Measurement and analysis of diastereomer ratios for forensic characterization of brodifacoum

The highly toxic anticoagulant rodenticide brodifacoum is an organic compound that has two diastereomeric forms. In this paper, we consider the hypothesis that the relative population of the diastereomers is a characteristic of forensic value for the association or source attribution of specimens of brodifacoum. In general, the stereoisomer distribution in an organic compound depends on the reagents, conditions, and methods used for synthesis and purification, and may vary over time due to differential stabilities of the stereoisomers. The stereoisomer distribution may thus serve as an identifier of the production methods and history of samples and provide a basis for comparing recovered specimens. We refer to this novel approach for signature detection as stereoisomer distribution analysis or SDA. If the stereoisomers are diastereomers, quantitative determination of the diastereomer ratio in a specimen can be performed by a number of techniques, notably gas or liquid chromatography or nuclear magnetic resonance (NMR) spectroscopy. This paper describes an NMR spectroscopic analysis of ten commercial technical grade brodifacoum samples from distinct batches originating from three different sources. The results reveal detectable source-to-source and batch-to-batch variations in diastereomer ratios.

MeCAT—new iodoacetamide reagents for metal labeling of proteins and peptides

Besides protein identification via mass spectrometric methods, protein and peptide quantification has become more and more important in order to tackle biological questions. Methods like differential gel electrophoresis or enzyme-linked immunosorbent assays have been used to assess protein concentrations, while stable isotope labeling methods are also well established in quantitative proteomics. Recently, we developed metal-coded affinity tagging (MeCAT) as an alternative for accurate and sensitive quantification of peptides and proteins. In addition to absolute quantification via inductively coupled plasma mass spectrometry, MeCAT also enables sequence analysis via electrospray ionization tandem mass spectrometry. In the current study, we developed a new labeling approach utilizing an iodoacetamide MeCAT reagent (MeCAT-IA). The MeCAT-IA approach shows distinct advantages over the previously used MeCAT with maleinimide reactivity such as higher labeling efficiency and the lack of diastereomer formation during labeling. Here, we present a careful characterization of this new method focusing on the labeling process, which yields complete tagging with an excess of reagent of 1.6 to 1, less complex chromatographic behavior, and fragmentation characteristics of the tagged peptides using the iodoacetamide MeCAT reagent.

ChemInform Abstract: Purine 5′,8‐Cyclonucleoside Lesions: Chemistry and Biology

AbstractReview: 76 refs.

DMAP‐[2.2]paracyclophane: Observation of an Unusual C–C Insertion

AbstractA novel family of nucleophilic catalysts derived from 4‐(dimethylamino)pyridine (DMAP) is described. The aminopyridine moiety is attached to a [2.2]paracyclophane skeleton, giving a catalyst with intrinsic planar chirality. Their synthesis is described starting from aminoparacyclophane 5 in four steps. In the course of this preparation, an unprecedented rearrangement involving a C–C bond insertion was observed, leading to an unexpected quinolone‐phenyl[2.2]cyclophane (8). The target catalyst 16 was obtained in enantiopure form by formation of diastereomers with a chiral secondary amine, allowing simple chromatographic separation. Preliminary catalytic studies were also performed.

Purine 5′,8-cyclonucleoside lesions: chemistry and biology

5',8-Cyclo-2'-deoxyadenosine and 5',8-cyclo-2'-deoxyguanosine in their 5'R and 5'S diastereomeric forms are tandem-type lesions observed among the DNA modifications and identified in mammalian cellular DNA in vivo. These lesions result from the chemistry of the C5' radicals generated by the attack of HO˙ radicals to 2-deoxyribose units. Quantitative determination of these lesions in biological samples as biomarkers of free radical damage is a challenge. Results reported for irradiated samples of calf thymus DNA have been critically reviewed, underlining the need of further research for the potential involvement of these lesions in human health (76 references).

Azithromycin as a new chiral selector in capillary electrophoresis

In capillary electrophoresis (CE), separation of enantiomers of a chiral compound can be achieved through the chiral interactions and/or complex formation between the chiral selector and the enantiomeric analytes on leaving their diastereomeric forms with different stability constants and hence different mobilities. A great number of chiral selectors have been employed in CE and among them macrocyclic antibiotics exhibited excellent enantioselective properties towards a wide number of racemic compounds. The use of azithromycin (AZM) as a chiral selector has not been reported previously. This work reports the use of AZM as a chiral selector for the enantiomeric separations of five chiral drugs and one amino acid (tryptophan) in CE. The enantioseparation is carried out using polar organic mixtures of acetonitrile (ACN), methanol (MeOH), acetic acid and triethylamine as run buffer. The influences of the chiral selector concentration, ACN/MeOH ratio, applied voltage and capillary temperature on enantioseparation are investigated. The results show that AZM is a viable chiral selector in CE for the enantioseparation of the type of chiral drugs investigated.

Methionine sulfoxide reductase A: Structure, function and role in ocular pathology

Methionine is a highly susceptible amino acid that can be oxidized to S and R diastereomeric forms of methionine sulfoxide by many of the reactive oxygen species generated in biological systems. Methionine sulfoxide reductases (Msrs) are thioredoxin-linked enzymes involved in the enzymatic conversion of methionine sulfoxide to methionine. Although MsrA and MsrB have the same function of methionine reduction, they differ in substrate specificity, active site composition, subcellular localization, and evolution. MsrA has been localized in different ocular regions and is abundantly expressed in the retina and in retinal pigment epithelial (RPE) cells. MsrA protects cells from oxidative stress. Overexpression of MsrA increases resistance to cell death, while silencing or knocking down MsrA decreases cell survival; events that are mediated by mitochondria. MsrA participates in protein-protein interaction with several other cellular proteins. The interaction of MsrA with α-crystallins is of utmost importance given the known functions of the latter in protein folding, neuroprotection, and cell survival. Oxidation of methionine residues in α-crystallins results in loss of chaperone function and possibly its antiapoptotic properties. Recent work from our laboratory has shown that MsrA is co-localized with αA and αB crystallins in the retinal samples of patients with age-related macular degeneration. We have also found that chemically induced hypoxia regulates the expression of MsrA and MsrB2 in human RPE cells. Thus, MsrA is a critical enzyme that participates in cell and tissue protection, and its interaction with other proteins/growth factors may provide a target for therapeutic strategies to prevent degenerative diseases.

Complete Enantioseparation through Supramolecular Complex Formation between Tris(1,3-diaminopropane)cobalt(III) Phosphate and β-Cyclodextrin, [Co(tn)3]PO4·β-CDX

The supramolecular complex [Co(tn)(3)]PO(4)·β-CDX (tn = 1,3-diaminopropane; β-CDX = β-cyclodextrin) was prepared through β-CDX, rac-[Co(tn)(3)]Cl(3), and Na(3)PO(4) in a mole ratio of 1:2:2. After one recrystallization process, the circular dichroism spectrum confirmed that complete enantioseparation of [Co(tn)(3)]PO(4) is achieved in this system. X-ray crystal structure analysis reveals the presence of the only Λ-enantiomer in the unit cell and the head-to-head chiral discrimination structure of this system. Thus, we found that the present diastereomer formation with β-CDX affords the very convenient and effective optical resolution procedure of [Co(tn)(3)](3+).

Glutathione pathway in ethylbenzene metabolism: Novel biomarkers of exposure in the rat

Glutathione pathway was specifically studied in rats exposed by inhalation to a range of ethylbenzene vapours (5–2000 ppm). Urines were collected during exposure (6 h) and over the 18 h following the exposure. The potential metabolites coming from either side-chain or ring oxidation were synthesized: 1-, 2-phenylethylmercapturic acids (1-, and 2-PEMA) and 2-, 3- and 4-ethylphenylmercapturic acids (2-, 3-, and 4-EPMA). Their synthesis was fully described and the molecules characterized. Urine samples were analysed using a selective HPLC–fluorescence method. Among the five metabolites, 2-PEMA was never observed in any urine sample. By contrast, 1-PEMA was discovered in its two diastereomeric forms, and it was shown that one of them was mainly present. 2-EPMA, 3-EPMA and 4-EPMA (in the ratio 1:2:6) were also found, and their combined excretion levels were similar to that of 1-PEMA. The atmospheric concentrations and urinary excretions yielded very close correlations which allow us to consider these mercapturic acids as novel ethylbenzene exposure biomarkers.

Stereoselective synthesis of cationic heterobidentate C(NHC)/SR rhodium(I) complexes using stereodirecting N, N-dialkylamino groups

The synthesis of two different types of chiral C/S ligands based upon N-( N, N-dialkylamino)-substituted N-heterocyclic carbenes and thioether functionalities, along with their neutral [RhCl(CNH)(COD)] and cationic [Rh(I)(NHC/S)(COD)] + complexes, has been accomplished. ( S)-2-[(Phenylthio)methyl]pyrrolidine, carrying the thioether moiety, and (2 S,5 S)-2,5-diphenylpyrrolidine, combined with a thioether functionalized side chain, were studied as potential stereodirecting groups. Only the latter provided high selectivity in the formation of the neutral complex, leading to a single atropoisomer (de >98%) of the newly formed, configurationally stable C(NHC)–Rh bond. The synthesis of the corresponding cationic [Rh(I)(NHC/S)(COD)] + complexes, however, resulted in the formation of single ( R a, S S) and ( S a, S S) diastereomers, respectively, of the four possible complexes in each case [combinations of the ( R a/ S a) C(NHC)–Rh axis and the ( S s/ R s) stereogenic S center formed upon coordination]. For the proline derivative, the resolution of the mixture of ( R a/ S a)-[RhCl(CNH)(COD)] neutral complexes proceeds via dynamic kinetic resolution through coordinatively unsaturated Rh(I) intermediates formed after halide abstraction. The absolute configurations of both types of cationic complexes were unequivocally assigned on the basis of X-ray diffraction analysis.

The difference in stability between 5′R and 5′S diastereomers of 5′,8-cyclopurine-2′-deoxynucleosides. DFT study in gaseous and aqueous phase

Abstract Oxidatively generated damage to DNA frequently appears in the human genome as an effect of aerobic metabolism or as the result of exposure to exogenous oxidizing agents. Due to these facts it has been decided to calculate the stability of 5′,8-cyclo-2′-deoxyadenosine/guanosine (cdA, cdG) in their 5′R and 5′S diastereomeric forms. For all points of quantum mechanics studies presented, the density functional theory (DFT) with B3LYP parameters on 6-311++G** basis set level was used. The calculations showed a significant negative enthalpy for glycosidic bond cleavage reaction for cationic forms and slightly negative for neutral ones. The preliminary study of the discussed process has shown the nature of stepwise nucleophilic substitution DN*AD type mechanism. Surprisingly, the different values in free energy, between short-lived oxacarbenium ion intermediates, have been found to lie over a relatively small range, around 1 and 2.8 kcal mol−1. For anions, the decomposition enthalpies were found as positive in aqueous phases. These theoretical results are supported by the formic acid hydrolysis experiments of both diastereomers of cdA, for the first time. (5′S)cdA exhibited higher stability than (5′R)cdA.

Ionisation potential and electron affinity of free 5′,8-cyclopurine-2′-deoxynucleosides. DFT study in gaseous and aqueous phase

Abstract Oxidatively generated damage to DNA frequently appears in the human genome as an effect of aerobic metabolism or as the result of exposure to exogenous oxidizing agents. Due to these facts it was decided to present, for the first time, the electron affinity, ionization potential of 5′,8-cyclo-2′-deoxyadenosine/guanosine (cdA, cdG) in their 5′R and 5′S diastereomeric forms. For all points of quantum mechanics studies presented, the density functional theory (DFT) with B3LYP parameters on 6-311++G** basis set level was used. The zero-point vibrational corrected adiabatic electron affinity (AEA) and adiabatic ionization potential (AIP) were calculated. Additionally the vertical electron affinity (VEA), vertical detachment energy (VDE) and vertical ionization potential were taken into consideration. AEA in eV (gaseous/aqueous phase) are as follows: 0.3/1.81 (5′R)cdA, 0.13/1.76 (5′S)cdA, 0.17/1.49 (5′R)cdG, 0.14/1.53 (5′S)cdG and AIP followed the order 7.43/5.59(5′S)cdG, 7.49/5.60(5′R)cdG, 7.77/5.97(5′R)cdA, 7.84/5.93(5′S)cdA. The obtained AIPs were found to be lower than that for corresponding natural nucleosides. Therefore, even though the 5′,8-cyclopurine-2′-deoxynucleoside level in a cell was judged as low, they can play an important role in the stability, replication and transcription of genes.

Enzyme-assisted synthesis and structural characterization of pure benzodiazepine glucuronide epimers

The three hydroxybenzodiazepines oxazepam, temazepam, and lorazepam used for their anxiolytic, sedative, and anticonvulsant properties are metabolized by glucuronidation, which is the predominant pathway in the clearance mechanism of exogenous and endogenous substances during phase II metabolism. The aim of this study was the synthesis of benzodiazepine-O-glucuronides as analytical reference substances. All benzodiazepines are prescribed clinically as racemic formulations. The resulting conjugates from the coupling reactions with glucuronic acid are epimeric pairs of glucuronides. Due to the importance of stereochemical factors in drug disposition it is necessary to separate the diastereomeric forms after synthesis. An enzyme-assisted synthesis was developed and optimized by using microsomal UGT from fresh swine liver to receive multimilligram amounts of the benzodiazepine glucuronides, which were not accessible by standard synthetic procedures, like the Koenigs–Knorr- and Williamson-ether-synthesis. Swine liver microsomes were prepared by homogenization and differential centrifugation of liver tissue. In the presence of liver microsomes the benzodiazepines and cofactor UDPGA were incubated for 24 h. After incubation the microsomes were removed by protein precipitation and the residual benzodiazepines by liquid–liquid extraction (dichloromethane). The epimeric pairs of benzodiazepine glucuronides were separated by preparative high performance liquid chromatography (HPLC) followed by solid phase extraction (SPE) to obtain the pure benzodiazepine glucuronide epimers. The synthesis products were characterized by mass spectroscopy and nuclear magnetic resonance (NMR) spectroscopy.