Racemic Compound (racemic + compound)

Distribution by Scientific Domains


Selected Abstracts


ChemInform Abstract: From Racemic Compound to Spontaneous Resolution: A Linker-Imposed Evolution of Chiral [MnMo9O32]6- -Based Polyoxometalate Compounds.

CHEMINFORM, Issue 51 2009
Huaqiao Tan
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]


Conformational Effects on Photophysical Characteristics of C5,C5,-linked Dihydrothymine Dimers in Solution,

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2000
T. Ito
ABSTRACT Photophysical characteristics of N-substituted C5,C5,-linked dihydrothymine dimers (1a,b[meso], meso compounds of [5R,5,S]-bi-5,6-dihydrothymines; 1a,b[rac], racemic compounds of [5R,5,R]-bi-5,6-dihydrothymines and [5S,5,S]-bi-5,6-dihydrothymines) in aqueous solution with varying contents of less-polar aprotic solvent such as tetrahydrofuran or dioxane have been investigated by UV-absorption, and steady-state and time-resolved fluorescence spectroscopies. Among the C5,C5,-linked dimers, (5R,5,S)-bi-5,6-dihydro-1-methylthymine (1a[meso]) showed a redshifted weak UV-absorption band at 270,350 nm and excimer fluorescence emission at ,max= 370 nm with a quantum yield (,F) of ,0.1 in phosphate buffer (pH < 10) at 293 K. Racemic compound of 5,6-dihydro-1-methylthymine dimer (1a[rac]), meso and racemic compounds of 5,6-dihydro-1,3-dimethylthymine dimers (1b[meso] and 1b[rac]) in phosphate buffer were nonfluorescent under similar conditions. The UV-absorption and fluorescence spectral characteristics of 1a[meso] in aqueous solution were interpreted in terms of intramolecular stacking interactions between the dihydropyrimidine chromophores leading to a preferential "closed-shell" conformation both in the ground state and the excited singlet state. In basic solutions at pH > pKa= 11.7, the fluorescence quantum yield of 1a[meso] decreased due to a dominant "open-shell" conformation resulting from the electrostatic repulsion between the deprotonated dihydrothymine chromophores of 1a[meso] in a dianion form. [source]


A list of organic kryptoracemates

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2010
László Fábián
A list of 181 organic kryptoracemates has been compiled. This class of crystallographic oddities is made up of racemic compounds (i.e. pairs of resolvable enantiomers) that happen to crystallize in Sohnke space groups (i.e. groups that include only proper symmetry operations). Most (151) of the 181 structures could have crystallized as ordered structures in non-Sohnke groups. The remaining 30 structures do not fully meet this criterion but would have been classified as kryptoracemates by previous authors. Examples were found and checked with the aid of available software for searching the Cambridge Structural Database, for generating and comparing InChI strings, and for validating crystal structures. The pairs of enantiomers in the true kryptoracemates usually have very similar conformations; often the match is near-perfect. There is a pseudosymmetric relationship of the enantiomers in about 60% of the kryptoracemate structures, but the deviations from inversion or glide symmetry are usually quite easy to spot. Kryptoracemates were found to account for 0.1% of all organic structures containing either a racemic compound, a meso molecule, or some other achiral molecule. The centroid of a pair of enantiomers is more likely (99.9% versus 99% probability) to be located on an inversion center than is the centroid of a potentially centrosymmetric molecule. [source]


S,S -1,2-Dicyclohexylethane-1,2-diol and its racemic compound: a striking exception to Wallach's rule

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2006
Brian O. Patrick
The structures of enantiopure S,S -1,2-dicyclohexylethane-1,2-diol and its racemic compound (rac - S,S -1,2-dicyclohexyl­ethane-1,2-diol) have been determined at 295 and 173,K. The crystals of the enantiopure material are more than 4% denser than the crystals of the racemic compound, but the melting points indicate that the crystals of the less dense racemic compound are considerably more stable than those of the racemic conglomerate. This apparent exception to the correlation of crystal density and melting point is explained. The enantiopure crystals have four molecules in the asymmetric unit (Z, = 4). Two of the molecules have the conformation observed for the one independent molecule of the racemic compound and two have a higher energy conformation; the overall P21 structure is a perturbed version of a P212121 structure with Z, = 2. The enantiopure and racemic crystals have the same hydrogen-bonding motif, but the motif in the former appears to be significantly strained. A reason why crystals of enantiopure material might be systematically less dense than crystals of its racemic compound and to be more likely to have Z, > 1 is suggested. [source]


Hydrogen bonding in enantiomeric versus racemic mono-carboxylic acids; a case study of 2-phenoxy­propionic acid

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2003
Henning Osholm Sørensen
The structural and thermodynamic backgrounds for the crystallization behaviour of racemates have been investigated using 2-phenoxypropionic acid (PPA) as an example. The racemate of PPA behaves normally and forms a racemic compound that has a higher melting point and is denser than the enantiomer. Low-temperature crystal structures of the pure enantiomer, the enantiomer cocrystallized with n -alkanes and the racemic acid showed that hydrogen-bonded dimers that form over crystallographic symmetry elements exist in all but the structure of the pure enantiomer. A database search for optically pure chiral mono-carboxylic acids revealed that the hydrogen-bonded cyclic dimer is the most prevalent hydrogen-bond motif in chiral mono-carboxylic acids. The conformation of PPA depends on the hydrogen-bond motif; the antiplanar conformation relative to the ether group is associated with a catemer hydrogen-bonding motif, whereas the more abundant synplanar conformation is found in crystals that contain cyclic dimers. Other intermolecular interactions that involve the substituent of the carboxylic group were identified in the crystals that contain the cyclic dimer. This result shows how important the nature of the substituent is for the crystal packing. The differences in crystal packing have been related to differences in melting enthalpy and entropy between the racemic and enantiomeric acids. In a comparison with the equivalent 2-(4-chlorophenoxy)-propionic acids, the differences between the crystal structures of the chloro and the unsubstituted acid have been identified and related to thermodynamic data. [source]


Potential of different techniques of preferential crystallization for enantioseparation of racemic compound forming systems

CHIRALITY, Issue 8 2009
Daniel Polenske
Abstract Recently the feasibility of preferential crystallization for enantioseparation of racemic compound forming systems has been demonstrated (Lorenz et al., Application of preferential crystallization to resolve racemic compounds in a hybrid process. Chirality 2006;18:828,840; Polenske et al., Separation of the propranolol hydrochloride enantiomers by preferential crystallization: thermodynamic basis and experimental verification. Cryst Growth Des 2007;7:1628,1634). Here, the development and the potential of an efficient separation process operated via two different techniques of preferential crystallization are studied: (1) seeded isothermal preferential crystallization and (2) auto-seeded polythermal preferential crystallization. Both techniques were investigated in the batch and in the cyclic operation mode. On the example of mandelic acid as a typical racemic compound forming system, it is demonstrated that a cyclic auto-seeded polythermal process is feasible and significantly more efficient than the seeded isothermal one. Chirality, 2009. © 2008 Wiley-Liss, Inc. [source]


Application of mixtures of tartaric acid derivatives in resolution via supercritical fluid extraction

CHIRALITY, Issue 6 2007
Ildikó Kmecz
Abstract Racemic N -methylamphetamine (rac -MA) was resolved with 2R,3R -tartaric acid (TA) and its derivatives (O,O,-dibenzoyl-(2R,3R)-tartaric acid monohydrate (DBTA) and O,O,-di- p -toluoyl-(2R,3R)-tartaric acid (DPTTA)), individually and using them in different combinations. After partial diastereomeric salt formation, the free enantiomers were extracted by supercritical fluid extraction using carbon dioxide as solvent. DBTA and DPTTA are efficient resolving agents for rac -MA, the best chiral separation being obtained at a molar ratio of 0.25 resolving agent to racemic compound for both resolving agents (eeE = 82.5% and eeE = 57.9%, respectively). Compared with the two other acids, TA is practically unsuitable for enantiomer separation (eeE < 5%). Applying a mixture of one individually active and one ineffective acid in half the equivalent molar ratio, when the acids are in 1:1 ratio in the mixture, the resolution efficiency values obtained exceeded those obtained by using the components individually. Decreasing the molar ratio of resolving agent mixture to 0.25, at which the individual resolving agents give the best chiral separation, the obtained resolution efficiency values did not differ significantly from those expected. The outcome of the resolution process depended only on the amount of the individually active resolving agents in the mixture. Chirality, 2007. © 2007 Wiley-Liss, Inc. [source]


Preferential enrichment: Full crystallographic analysis of the unusual phenomenon in the mixed crystals' version

CHIRALITY, Issue 7 2002
Hiroki Takahashi
Abstract Full characterization of the crystal structures of the racemate, nonracemate (20% ee), and pure enantiomer of [2-[4-(3-ethoxy-2-hydroxypropoxy)phenylcarbamoyl]ethyl]trimethylammonium p -bromobenzenesulfonate (NBMe3), which has successfully shown an unusual enantiomeric resolution phenomenon, "preferential enrichment," was achieved by means of X-ray crystallographic analysis and construction of the binary melting point phase diagram. The crystalline nature of the racemate is not a racemic compound but a fairly ordered mixed crystal composed of the two enantiomers. The crystal structure of the nonracemate (20% ee) is virtually identical with that of the racemate and similar to that of the pure enantiomer. The binary melting point phase diagram of NBMe3 is consistent with the nature of a mixed crystal composed of the two enantiomers. Chirality 14:541,547, 2002. © 2002 Wiley-Liss, Inc. [source]


Cellulose dimethylphenylcarbamate-immobilized zirconia for chiral separation in reversed-phase CEC

ELECTROPHORESIS, Issue 22 2009
Jurim Gwon
Abstract Cellulose dimethylphenylcarbamate (CDMPC)-immobilized zirconia (CDMPCZ) was used as a chiral stationary phase for enantioseparation of a set of nine racemic compounds in reversed-phase CEC. Influences of the type and composition of organic modifier and the applied voltage on enantioseparation were examined. Separation data on CDMPCZ were also compared with those on CDMPC-immobilized silica (CDMPCS). Enantiomers of the analytes investigated are well separated in ACN/phosphate buffer mobile phases. Better enantioselectivity and resolution were obtained with ACN than MeOH as the organic modifier. Retention was longer but better enantioselectivity and resolution were obtained on CDMPCZ than CDMPCS. [source]


Chromatographic evaluation and comparison of three ,-cyclodextrin-based stationary phases by capillary liquid chromatography and pressure-assisted capillary electrochromatography

ELECTROPHORESIS, Issue 19 2008
Bo Lin
Abstract Enantiomer separations were performed on three ,-cyclodextrin-based chiral stationary phases (CSP) containing the pernaphthylcarbamoylated ,-cyclodextrin (CSP 1), peracetylated ,-cyclodextrin (CSP 2) and permethylated ,-cyclodextrin (CSP 3) as chiral selectors by capillary liquid chromatography and pressure-assisted capillary electrochromatography in this study. Triethylammonium acetate/MeOH or phosphate buffer/MeOH was used as the mobile phase. The experimental factors affecting chiral separations have been examined for each CSP, including pH of the buffers, methanol content and applied voltage. Under optimal separation conditions, a number of racemic compounds were resolved into their enantiomers on three cyclodextrin-based CSP. A comparative study on the performance of three CSP revealed the presence of carbonyl functional groups as well as aromatic rings in the cyclodextrin derivatives, enhanced the interaction between the analytes and CSP, and thus improved enantioselectivity of the CSP. [source]


A Novel Synthesis of Highly Substituted Perhydropyrrolizines, Perhydroindolizines, and Pyrrolidines: Inhibition of the Peptidyl-Prolyl cis/trans Isomerase (PPIase) Pin1

HELVETICA CHIMICA ACTA, Issue 2 2007
Romain Siegrist
Abstract In this paper, we describe the synthesis and biological evaluation of highly substituted perhydropyrrolizines that inhibit the peptidyl-prolyl cis/trans isomerase (PPIase) Pin1, an oncogenic target. The enzyme selectively catalyzes the cis/trans isomerization of peptide bonds between a phosphorylated serine or threonine, and proline, thereby inducing a conformational change. Such structural modifications play an important role in many cellular events, such as cell-cycle progression, transcriptional regulation, RNA processing, as well as cell proliferation and differentiation. Based on computer modeling (Fig.,2), the new perhydropyrrolizinone derivatives (,)- 1a,b, decorated with two substituents, were selected and synthesized (Schemes,1,3). While enzymatic assays showed no biological activity, 15N,1H-HSQC-NMR spectroscopy revealed that (,)- 1a,b bind to the WW recognition domain of Pin1, apparently in a mode that does not inhibit PPIase activity. To enforce complexation into the larger active site rather than into the tighter WW domain of Pin1 and to enhance the overall binding affinity, we designed a perhydropyrrolizine scaffold substituted with additional aromatic residues (Fig.,5). A novel, straightforward synthesis towards this class of compounds was developed (Schemes,4 and 5), and the racemic compounds (±)- 22a,22d were found to inhibit Pin1 with Ki values (Ki,=,inhibition constant) in the micromolar range (Table,2). To further enhance the potency of these inhibitors, the optically pure ligands (+)- 22a and (+)- 33b,c were prepared (Schemes,6 and 7) and shown to inhibit Pin1 with Ki values down to the single-digit micromolar range. According to 15N,1H-HSQC-NMR spectroscopy and enzymatic activity assays, binding occurs at both the WW domain and the active site of Pin1. Furthermore, the new synthetic protocol towards perhydropyrrolizines was extended to the preparation of highly substituted perhydroindolizine ((±)- 43; Scheme,8) and pyrrolidine ((±)- 48a,b; Scheme,9) derivatives, illustrating a new, potentially general access to these highly substituted heterocycles. [source]


Captopril and its synthesis from chiral intermediates

JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 2 2001
R Chirumamilla
Abstract Captopril is an antihypertensive agent that inhibits the angiotensin-converting enzyme of the renin,angiotensin system. Chiral intermediates are used in the synthesis of the drug. These intermediates are obtained by resolution of racemic compounds or by chemical, biocatalytic methods and or by asymmetric synthesis by biocatalytic process. This article reviews the various chemical and biochemical processes involved in the synthesis of the chiral drug, including the pharmacological action of captopril. © 2001 Society of Chemical Industry [source]


Chiral polymers for resolution of enantiomers

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 7 2009
Yoshio Okamoto
Abstract In 1979, the formation of one-handed helical poly(triphenylmethyl methacrylate) (PTrMA) was found through the helix-sense-selective polymerization of methacrylate using chiral anionic initiators, and the existence of a stable helical polymer without chiral side chains was proved. The chiral polymer exhibited unexpected high chiral recognition of various racemic compounds when used as the chiral packing material (CPM) for HPLC, which was commercialized in 1982 as the first chiral column based on an optically active polymer. This success encouraged us to develop further useful commercial chiral packing materials (CPMs) based on polysaccharides, cellulose, and amylose. By using these polysaccharide-based CPMs, particularly phenylcarbamate derivatives, nearly 90% of chiral compounds can be resolved not only analytically but also preparatively, and several chiral drugs have been produced using the CPMs. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1731,1739, 2009 [source]


Urea bonded cyclodextrin derivatives onto silica for chiral HPLC

JOURNAL OF SEPARATION SCIENCE, JSS, Issue 12 2006
I. Wayan Muderawan
Abstract Several structurally well-defined perfunctionalised cyclodextrin chiral stationary phases (CD CSPs) for high performance liquid chromatography have been successfully prepared by immobilisation of perfunctionalised cyclodextrins on silica through urea linkage(s) using the Staudinger reaction. These CSPs show high chiral recognition efficiency and are utilised in the resolution of various types of racemic compounds. This paper reviews the development of sixteen perfunctionalised cyclodextrin-based CSPs, their preparation, and their application to enantioseparation of seventy-seven racemic compounds under a range of separation conditions. [source]


Conformational Effects on Photophysical Characteristics of C5,C5,-linked Dihydrothymine Dimers in Solution,

PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2000
T. Ito
ABSTRACT Photophysical characteristics of N-substituted C5,C5,-linked dihydrothymine dimers (1a,b[meso], meso compounds of [5R,5,S]-bi-5,6-dihydrothymines; 1a,b[rac], racemic compounds of [5R,5,R]-bi-5,6-dihydrothymines and [5S,5,S]-bi-5,6-dihydrothymines) in aqueous solution with varying contents of less-polar aprotic solvent such as tetrahydrofuran or dioxane have been investigated by UV-absorption, and steady-state and time-resolved fluorescence spectroscopies. Among the C5,C5,-linked dimers, (5R,5,S)-bi-5,6-dihydro-1-methylthymine (1a[meso]) showed a redshifted weak UV-absorption band at 270,350 nm and excimer fluorescence emission at ,max= 370 nm with a quantum yield (,F) of ,0.1 in phosphate buffer (pH < 10) at 293 K. Racemic compound of 5,6-dihydro-1-methylthymine dimer (1a[rac]), meso and racemic compounds of 5,6-dihydro-1,3-dimethylthymine dimers (1b[meso] and 1b[rac]) in phosphate buffer were nonfluorescent under similar conditions. The UV-absorption and fluorescence spectral characteristics of 1a[meso] in aqueous solution were interpreted in terms of intramolecular stacking interactions between the dihydropyrimidine chromophores leading to a preferential "closed-shell" conformation both in the ground state and the excited singlet state. In basic solutions at pH > pKa= 11.7, the fluorescence quantum yield of 1a[meso] decreased due to a dominant "open-shell" conformation resulting from the electrostatic repulsion between the deprotonated dihydrothymine chromophores of 1a[meso] in a dianion form. [source]


A list of organic kryptoracemates

ACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2010
László Fábián
A list of 181 organic kryptoracemates has been compiled. This class of crystallographic oddities is made up of racemic compounds (i.e. pairs of resolvable enantiomers) that happen to crystallize in Sohnke space groups (i.e. groups that include only proper symmetry operations). Most (151) of the 181 structures could have crystallized as ordered structures in non-Sohnke groups. The remaining 30 structures do not fully meet this criterion but would have been classified as kryptoracemates by previous authors. Examples were found and checked with the aid of available software for searching the Cambridge Structural Database, for generating and comparing InChI strings, and for validating crystal structures. The pairs of enantiomers in the true kryptoracemates usually have very similar conformations; often the match is near-perfect. There is a pseudosymmetric relationship of the enantiomers in about 60% of the kryptoracemate structures, but the deviations from inversion or glide symmetry are usually quite easy to spot. Kryptoracemates were found to account for 0.1% of all organic structures containing either a racemic compound, a meso molecule, or some other achiral molecule. The centroid of a pair of enantiomers is more likely (99.9% versus 99% probability) to be located on an inversion center than is the centroid of a potentially centrosymmetric molecule. [source]


Potential of different techniques of preferential crystallization for enantioseparation of racemic compound forming systems

CHIRALITY, Issue 8 2009
Daniel Polenske
Abstract Recently the feasibility of preferential crystallization for enantioseparation of racemic compound forming systems has been demonstrated (Lorenz et al., Application of preferential crystallization to resolve racemic compounds in a hybrid process. Chirality 2006;18:828,840; Polenske et al., Separation of the propranolol hydrochloride enantiomers by preferential crystallization: thermodynamic basis and experimental verification. Cryst Growth Des 2007;7:1628,1634). Here, the development and the potential of an efficient separation process operated via two different techniques of preferential crystallization are studied: (1) seeded isothermal preferential crystallization and (2) auto-seeded polythermal preferential crystallization. Both techniques were investigated in the batch and in the cyclic operation mode. On the example of mandelic acid as a typical racemic compound forming system, it is demonstrated that a cyclic auto-seeded polythermal process is feasible and significantly more efficient than the seeded isothermal one. Chirality, 2009. © 2008 Wiley-Liss, Inc. [source]


Molecular homochirality and the parity-violating energy difference.

CHIRALITY, Issue 2 2008
A critique with new proposals
Abstract Previous proposals for the origin of molecular homochirality, based on the effect of the weak neutral current (WNC) on enantiomers, and the amplification of the resultant parity-violating energy difference (PVED), are possibly flawed. The additive amplification of PVED in crystals and polymers ("Yamagata hypothesis") cannot lead to detectable levels of optical activity, the original theory apparently overestimating PVED by a factor equal to Avogadro's number. An alternative theory based on the irreversible and spontaneous evolution of a dynamically fluctuating system is apparently impractical. However, the nonlinear amplification of PVED via autocatalytic polymerization may be possible as indicated by a simplified physico-chemical approach. This may also occur during crystallization and melting, and form the basis of the second order asymmetric transformation. (Thus, reported differences in the melting points of enantiomers in several cases may well be real). Also, the preponderance of racemic compounds over conglomerates may be based on the destabilization of the conglomerate by the action of the WNC on the crystalline lattice. The WNC may also be involved in the anomalous scattering of X-rays, which possibly arises from their circular polarization: the current theory would need to be revised accordingly. Chirality, 2008. © 2007 Wiley-Liss, Inc. [source]