Enantiomerization Process Research Articles

Synthesis, Conformational Stability, and Asymmetric Transformation of Atropisomeric 1,8-Bisphenolnaphthalenes

Highly congested, axially chiral 1,8-bisphenolnaphthalenes have been synthesized in 75% overall yield by palladium-catalyzed Suzuki coupling of 1,8-diiodonaphthalene and 4-methoxy-2-methylphenylboronic acid followed by regioselective formylation and deprotection. The C(2)-symmetric anti-stereoisomers of 1,8-bis(2'-methyl-4'-hydroxy-5'-formylphenyl)naphthalene, 5, and its diimine analogues 9 and 10 were found to be significantly more stable than the corresponding syn-isomer. Crystallographic analysis revealed that this stereochemical preference results from a unique intramolecular hydrogen bonding motif and concomitant minimization of steric repulsion. Triaryl 5 proved stable to rotation about the chiral axes at room temperature and the enantiomers were isolated via formation of diastereomeric diimines with (R)-2-amino-1-propanol and (R)-2-amino-3-methyl-1-butanol, respectively, chromatographic separation, and mild hydrolysis. Slow syn/anti-interconversion of 5, 9, and 10 was observed at enhanced temperatures and the diastereomerization and enantiomerization processes were monitored by CD and NMR spectroscopy. The Gibbs activation energy, ΔG(‡), for the isomerization of 5 was determined as 103.7 (102.4) kJ/mol for the conversion of the anti-(syn-) to the syn-(anti-)isomer at 45.0 °C. Condensation of 5 with two chiral amino alcohols generates diimines that undergo quantitative asymmetric transformation of the first kind toward the thermodynamically favored (P,P,R,R)- or (M,M,S,S)-atropisomer, respectively. The incorporation of two imino alcohol units controls the outcome of this unidirectional atropisomerization, i.e. the central chirality of the amino alcohol used induces a rigid, axially chiral triaryl scaffold with perfect stereocontrol. Accordingly, the rotational energy barrier for the conversion of (M,M,S,S)-9 to its syn-isomer is significantly increased and was determined as 115.7 kJ/mol at 58.0 °C.

Lanthanide dota-like Complexes Containing a Picolinate Pendant: Structural Entry for the Design of LnIII-Based Luminescent Probes

In this contribution we present two ligands based on a do3a platform containing a picolinate group attached to the fourth nitrogen atom of the cyclen unit, which are designed for stable lanthanide complexation in aqueous solutions. Potentiometric measurements reveal that the thermodynamic stability of the complexes is very high (log K = 21.2-23.5), being comparable to that of the dota analogues. Luminescence lifetime measurements performed on solutions of the Eu(III) and Tb(III) complexes indicate that the complexes are nine coordinate with no inner-sphere water molecules. A combination of density functional theory (DFT) calculations and NMR measurements shows that for the complexes of the heaviest lanthanides there is a major isomer in solution consisting of the enantiomeric pair Λ(δδδδ) and Δ(λλλλ), which provides square antiprismatic coordination (SAP) around the metal ion. Analysis of the Yb(III)-induced paramagnetic shifts unambiguously confirms that these complexes have SAP coordination in aqueous solution. For the light lanthanide ions however both the SAP and twisted-square antiprismatic (TSAP) isomers are present in solution. Inversion of the cyclen ring appears to be the rate-determining step for the Λ(δδδδ) ↔ Δ(λλλλ) enantiomerization process observed in the Lu(III) complexes. The energy barriers obtained from NMR measurements for this dynamic process are in excellent agreement with those predicted by DFT calculations. The energy barriers calculated for the arm-rotation process are considerably lower than those obtained for the ring-inversion path. Kinetic studies show that replacement of an acetate arm of dota by a picolinate pendant results in a 3-fold increase in the formation rate of the corresponding Eu(III) complexes and a significant increase of the rates of acid-catalyzed dissociation of the complexes. However, these rates are 1-2 orders of magnitude lower than those of do3a analogues, which shows that the complexes reported herein are remarkably inert with respect to metal ion dissociation.

Conformational Enantiomerization and Estrogen Receptor α Binding of Anti-Cancer Drug Tamoxifen and Its Derivatives

The anticancer drug tamoxifen (TAM) displays two chiral vinyl propeller structures, which interconvert so rapidly that the process is undetectable on the NMR time scale. In the present work, the enantiomerization processes were investigated with molecular modeling techniques. The threshold mechanisms probed at the different rings were shown to be identical, i.e., involving a synchronous three-ring flip, with a correlated rotation of the rings. In order to reveal the pharmacological profiles of the two chiral forms, we performed structural studies on the ligand binding domain of estrogen receptor α (ERα LBD) and associated ligands. The enantiomers, with opposite torsional twist, were found to be discriminated by ERα. For TAM and its main metabolites, the effects of the stereoselectivity of ERα are overcome by the low energy cost for helical inversion between the two torsional enantiomers, estimated to be ∼3 kcal/mol.

Towards Fluoride Sensing with Positively Charged Lanthanide Complexes

AbstractHerein we report a series of [LnL(H2O)]Cl3 complexes, in which L is a 1,4,7,10‐tetrazacyclododecane framework (cyclen) functionalized with 2‐methylpyridyl pendants on the nitrogen atoms at the 1‐ and 7‐positions and with N‐[2‐(2‐hydroxyethoxy)ethyl]acetamide groups on those at the 4‐ and 10‐positions, and Ln stands for Eu, La, and Tb. The complexes were isolated and characterized by elemental analysis, mass spectrometry, infrared spectroscopy, and 1H NMR spectroscopy for the La and Eu complexes. 1H NMR spectroscopy indicates that the complexes display a major twisted square antiprismatic (TSAP) geometry in solution, which was confirmed by using DFT calculations. 1H and 13C variable‐temperature NMR spectroscopic experiments on the La complex provided an activation free energy of ΔG≠ = (58 ± 3) kJ mol–1 for the enantiomerization process of the TSAP isomer, which requires both the inversion of the five‐membered chelate rings formed upon coordination of the cyclen moiety and the rotation of the pendant arms. The photophysical properties of the complexes were studied by means of absorption, steady‐state, and time‐resolved luminescence spectroscopy. Upon ligand excitation, the Eu and Tb complexes displayed the typical lanthanide‐centered emission spectra with weak emission efficiencies (Φov = 0.11 and 0.68 %, respectively, for Eu and Tb in H2O). Luminescence lifetime measurements in water and heavy water allowed the determination of the hydration number of the complexes; these were (1.0 ± 0.2) for Eu and Tb. Spectrophotometric titrations of the [LnL(H2O)]Cl3 complexes (Ln = Eu and Tb) with F–, Cl–, and Br– anions (in buffered water solutions, 0.01 M tris(hydroxymethyl)aminomethane (TRIS)/HClO4, pH = 7.0) showed the complexes to interact selectively with fluoride anions, whereas no interactions were observed with Cl– and Br–. The interaction with F– was monitored by luminescence spectroscopy with [EuL(H2O)], which evidenced dramatic changes in the shape of the luminescence spectra, and an association constant of (2.86 ± 0.06) log units was determined. In the case of the Eu complex, time‐resolved and high‐resolution emission spectroscopy showed that the interaction resulted in the displacement of the coordinated water molecule, which led to an increase of the overall luminescence quantum yield, with almost no impact on the ligand‐to‐metal sensitization efficiency. Investigation of the Eu and La complexes by potentiometry (0.1 M NaCl) allowed the determination of the protonation constants of the complexes as well as the association constants with fluoride anions. DFT calculations on the fluoride adduct of the Eu complex confirmed the higher stability of the TSAP isomer and evidenced large structural changes upon F– binding, in agreement with spectrophotometric observations in solution.

Determination of kinetic parameters of enantiomerization of benzothiadiazines by DCXplorer

Benzothiadiazines differently substituted at the sulfonamidic nitrogen atom, at the stereogenic carbon atom and at the anilinic nitrogen atom have been synthesized and fully characterized. Enantioseparation of these compounds has revealed rapid on-column enantiomerization. The recently developed software DCXplorer has been successfully applied to calculate enantiomerization kinetic parameters. Enantiomerization barriers of 3-phenyl substituted benzothiadiazines, calculated in this work, have indicated a higher enantiomerization rate suggesting that the aromatic substituent exerts a strong effect on the enantiomerization process. Methyl substitution on N(2) position led to higher free energy barriers of enantiomerization, suggesting negative influence of methyl in the N(2) position on enantiomerization kinetics. However, methylation on N(4) position increases the enantiomerization rates significantly. The results obtained have been employed to postulate an enantiomerization mechanism for chiral benzothiadiazine type compounds.

Synthesis, Structural Analysis, and Chiral Investigations of Some Atropisomers with EE-Tetrahalogeno-1,3-butadiene Core

The atropenantiomers of stable 1,2,3,4-tetrahalo-1,3-butadiene derivatives (where halogeno stands for bromine or iodine) were separated with use of chiral HPLC. The barriers for the enantiomerization process were determined on-line by dynamic HPLC (DHPLC) or off-line by classical kinetic measurements. In the case of the tetrachloro compound, the barrier was too low for DHPLC and its value was obtained by dynamic NMR experiments. The obtained barriers for chloro, bromo, and iodo derivatives correlate with the van der Waals radii of the halogens. The absolute configuration of the isolated enantiomers of the tetraiodo and tetrabromo compounds was assigned by comparison of the experimental and conformations averaged calculated VCD spectra. The identification of a signature band of the absolute configuration of the butadiene core, the sign and location of which are independent from the different conformations and substituents, allowing the safe assignment of the absolute configuration of the enantiomers of chiral 1,3-butadienes, is also reported.

Enantiomerization of Chiral Uranyl−Salophen Complexes via Unprecedented Ligand Hemilability: Toward Configurationally Stable Derivatives

In the search for configurationally stable inherently chiral uranyl-salophen complexes, the newly synthesized compound 3 featuring a dodecamethylene chain was expected to be a promising candidate. Unexpectedly, dynamic HPLC on a enantioselective column showed that it still undergoes enantiomerization at high temperature. By comparison with the dynamic behavior of compounds 4 and 5, it was found that the enantiomerization rate is independent of the size of the ligand. This finding definitely rules out a jump rope-type mechanism for the enantiomerization process and points to reaction pathways involving preliminary rupture of one of the O...U coordinative bonds. This provides unprecedented evidence of the occurrence of ligand hemilability in metal-sal(oph)en complexes. Such findings inspired the synthesis of compound 6 endowed with a more rigid spacer, i.e., that derived from 4,4'-(1,4-phenylenediisopropylidene)bisphenol. DHPLC investigations showed that the new structural motif imparts a higher configurational stability, thus raising the half-life for the enantiomerization to more than 2 months at room temperature. This clearly establishes that this compound represents the first member of a new class of inherently chiral receptors, whose potential in chiral recognition and catalysis now can be feasibly explored.

High-performance liquid chromatography separation of enantiomers of flavanone and 2′-hydroxychalcone under reversed-phase conditions

A reversed-phase high-performance liquid chromatography (HPLC) method was developed for evaluating the chiral discrimination ability of Chiralpak IA chiral stationary phase (CSP) towards flavanone. The effect of the nature and pH buffer as well as nature of alcohol modifier on enantioselectivity was investigated. Comparative study of enantioseparation in reversed-phase and polar organic conditions indicated a significative improvement in resolution when aqueous-based eluents were used. The developed reversed-phase chromatographic method was able to separate the enantiomers of flavanone from its isomeric form, the 2′-hydroxychalcone. The stereochemical stability of flavanone was studied by classical off-column HPLC kinetic procedures in aqueous and non-aqueous media. It was clearly demonstrated that the 2′-hydroxychalcone was involved as intermediate in the on-column and off-column enantiomerization process of flavanone.

Unprecedented Detection of Enantiomerization π-Barriers Due to Restricted Aryl Torsion: Case of 1,8-Di-arylbiphenylenes

Dynamic NMR spectroscopy allowed, for the first time, the determination of the pi-barriers responsible for the enantiomerization processes in derivatives bearing two aryl substituents bonded to a planar framework: this could be achieved in the case of 1,8-di-m-tolylbiphenylene (1) and 1,8-di-m-xylylbiphenylene (2). In derivative 1, the three possible conformers predicted by DFT computations (anti-in, syn, and anti-out) were detected, and in addition, the steric barrier responsible for their interconversion could be measured. The barriers predicted by DFT calculations were found in satisfactory agreement with experimental data.

Energy barrier determination of enantiomerization of chiral 3,4-dihydro-1,2,4-benzothiadiazine 1,1-dioxide type compounds by enantioselective stopped-flow HPLC

The synthesis and enantioseparation of chiral 3,4-dihydro-1,2,4-benzothiadiazine 1,1-dioxide derivatives are reported herein. A HPLC stopped-flow procedure was applied to the determination of rate constants and free energy barriers of enantiomerization of the compounds synthesized in the presence of achiral stationary phase. The individual enantiomers of the studied compounds were isolated in parallel by preparative HPLC on a Chiraspher NT column. Rate constants and free energy barriers of enantiomerization were determined in the mobile phase. The results were used to determine the influence of the chiral stationary phase on the enantiomerization process.

The unified equation for the evaluation of degenerated first‐order reactions in dynamic electrophoresis

An analytical solution for the unified equation for degenerated (pseudo-) first-order reactions, e.g., enantiomerization processes, in dynamic CE is presented, and validated with a dataset of 31 250 elution profiles covering typical experimental parameters. The unified equation was applied to determine the enantiomerization barrier of the hypnotic glutarimide derivative thalidomide (Contergan(R)) by dynamic capillary electrokinetic chromatography (DEKC). The enantiomer separation of thalidomide was performed in an aqueous 50 mM sodium borate buffer at pH 9.3 in the presence of the chiral mobile phase additive carboxymethyl-beta-CD. Interconversion profiles featuring pronounced plateau formation were observed. Activation parameters DeltaH( not equal) and DeltaS( not equal) were obtained from temperature-dependent measurements between 20.0 and 37.5 degrees C in 2.5K steps. From the activation parameters the enantiomerization barrier of thalidomide at 37 degrees C under basic conditions were calculated to be DeltaG( not equal) = 93.2 kJ/mol. Comparison of the kinetic data with results obtained at pH 8.0 reveals the catalytic influence of the base on the enantiomerization barrier.

Ab inito study and NBO analysis of the enantiomerization energy profile of tetra- peri-substituted overcrowded naphthalenes

Ab initio molecular orbital (MO) and density functional theory methods were used to investigate the structural and conformational properties of 1,4,5,8-tetra- tert-butyl naphthalene ( 1), 1,4,5,8-tetrakis(trimethylsilyl)naphthalene ( 2), 1,4,5,8-tetrakis(trimethylgermyl)naphthalene ( 3) and 1,4,5,8-tetrakis(trimethylstannyl)naphthalene ( 4). HF/3-21G*//HF/3-21G*, MP2/3-21G*//HF/3-21G*, B3LYP/3-21G*//HF/3-21G*, HF/LANL2DZ*//HF/LANL2DZ*, MP2/LANL2DZ*//HF/LANL2DZ* and B3LYP/LANL2DZ*//HF/LANL2DZ* results revealed that the most stable form of these compounds has D 2 symmetry, and, therefore, they should present optical activity. The enantiomerization processes for compounds 1– 4 could take place via plane symmetrical intermediates (with C 2 h symmetry). MP2/3-21G*//HF/3-21G* results show that the plane-symmetrical ( C 2 h symmetry) geometries of compounds 1– 4 are less stable (about 8.40, 5.18, 8.01, and 6.37 kcal mol −1, respectively) than the axial-symmetrical geometries ( D 2 symmetry). Also, in compounds 1– 4, the plane symmetrical intermediate ( C 2 h symmetry) is less stable than the axial-symmetrical ( D 2 symmetry) geometry, about 10.07, 6.80, 9.49 and 7.33 kcal mol −1, respectively, as calculated by B3LYP/3-21G*//HF/3-21G* level of theory. The barrier heights for interconversion of the D 2 to C 2 h geometries for compounds 1– 4 are 29.32, 14.89, 12.28 and 9.24 kcal mol −1, as calculated by MP2/3-21G*//HF/3-21G* level of theory. Also, B3LYP/3-21G*//HF/3-21G* results show that the required energy for ring flipping ( D 2 to D 2 ′ ) for these compounds is 28.41, 13.48, 11.52, 8.33 kcal mol −1. In both axial and plane symmetrical ( D 2 and C 2 h ) geometries of compounds 1– 4, the naphthalene rings are puckered and their aromatic characters are relatively perturbed. For compounds 1– 3, the calculations were also performed at B3LYP/6-311+G**//HF/6-31G*, MP2/6-31G*//HF/6-31G* and HF/6-31G*//HF/6-31G* levels of theory. However, the comparison showed that the results at HF/3-21G*//HF/3-21G*, B3LYP/3-21G*//HF/3-21G* and MP2/3-21G*//HF/3-21G* methods correlated well with those obtained at HF/6-31G*//HF/6-31G*, B3LYP/6-311+G**//HF/6-31G* and MP2/6-31G*//HF/6-31G* levels of theory. Further, NBO analysis, based on the HF/3-21G* optimized ground state geometries, revealed that in compounds 1– 4, the resonance energy associated with σ C aryl – M to σ C 9 – C 10 * delocalization is 1.43, 3.53, 3.96 and 4.62 kcal mol −1, respectively. These resonance energy values could explain the easiness of ring flipping processes from compound 4 to 1. The NBO results are in good agreement with the calculated energy barriers for ring flipping in compounds 1– 4, as calculated by MP2, B3LYP and HF methods, using all electron (3-21G*) and pseudopotential (LANL2DZ*) basis sets.

Conformational Behavior of Tris(pentafluorophenyl)borane−Benzotriazole Adducts

The stereomutations in a class of tris(pentafluorophenyl)borane−benzotriazole adducts are investigated by means of variable-temperature 19F NMR and X-ray crystallography. At low temperature the presence of a pair of conformational enantiomers is confirmed by NMR spectra obtained in a chiral medium. Two different energy barriers, corresponding to an enantiomerization process and a B−N rotation, were observed and their values measured (ΔG⧧ = 10.7 and 12.8 kcal mol-1 in the case of 1). In the case of the bis-adduct 3, two conformational diastereoisomers with different populations were detected at low temperature, while only one diastereoisomer is present in the crystalline state.

Density Functional Theoretical Study on Enantiomerization of 2,2‘-Biphenol

The S-R enantiomerization processes of 2,2'-biphenol (biphenol) have been investigated using density functional theory (DFT). Five isomers for biphenol were identified: I0, which is the most stable isomer; I1a and I1b, which are formed by a restricted rotation of one OH group; and I2a and I2b, which are formed by a restricted rotation of the two OH groups where a and b denote cis and trans configurations, respectively. Each isomer has R- and S-enantiomers. The energies relative to the most stable isomer I0 are 1.6, 3.3, 5.3, and 5.5 kcal mol(-1) for I1a, I1b, I2a, and I2b, respectively. The direct enantiomerization of I0, in which the phenol-ring rotation is considered to be the reaction coordinate while the OH rotations are frozen, is forbidden because of the repulsion between the two OH groups. The transition states for isomerizations of I0 to other isomers (I1a, I1b, I2a, or I2b) were calculated as well as those for the other direct enantiomerizations except for that of I0. From the viewpoint of the least number of the transition states and their low energy levels, the probable S-R enantiomerization of I0 is expressed as a sequential process of isomerization: I0,S --> I1a,S, a direct enantiomerization induced by one of the two OH rotations, I1a,S --> I1a,R, and another isomerization, I1a,R --> I0,R, that is, I0,S --> I1a,S --> I1a,R --> I0,R as the whole process. This process is effective in quantum control of the enantiomerization of biphenol and can be carried out by a sequence of a pump-dump IR laser-pulse scheme.

Formation of a Dynamic η2(O,N)-Hydroxylaminato Zirconocene Complex by Nitrosoarene Insertion into a Zr−C σ-Bond

The reaction of dimethylzirconocene with nitrosobenzene gave the hydroxylaminatozirconium complex 6. Its structure in the solid state was characterized as the N-inside isomer [η2(O,N)-ON(CH3)Ph](CH3)ZrCp2 (6a) by X-ray diffraction. Dynamic 1H NMR spectroscopy indicated a rapid enantiomerization process of the chiral system 6a in solution, probably taking place via a rapid equilibration with a reactive η1(O) isomer.

Equilibrium and NMR studies on GdIII, YIII, CuIIand ZnIIcomplexes of various DTPA–N,N″-bis(amide) ligands. Kinetic stabilities of the gadolinium(iii) complexes

Three DTPA-derivative ligands, the non-substituted DTPA-bis(amide) (L(0)), the mono-substituted DTPA-bis(n-butylamide) (L(1)) and the di-substituted DTPA-bis[bis(n-butylamide)] (L(2)) were synthesized. The stability constants of their Gd3+ complexes (GdL) have been determined by pH-potentiometry with the use of EDTA or DTPA as competing ligands. The endogenous Cu2+ and Zn2+ ions form ML, MHL and M(2)L species. For the complexes CuL(0) and CuL(1) the dissociation of the amide hydrogens (CuLH(-1)) has also been detected. The stability constants of complexes formed with Gd3+, Cu2+ and Zn2+ increase with an increase in the number of butyl substituents in the order ML(0) < ML(1) < ML(2). NMR studies of the diamagnetic YL(0) show the presence of four diastereomers formed by changing the chirality of the terminal nitrogens of their enantiomers. At 323 K, the enantiomerization process, involving the racemization of central nitrogen, falls into the fast exchange range. By the assignment and interpretation of 1H and 13C NMR spectra, the fractions of the diastereomers were found to be equal at pH = 5.8 for YL(0). The kinetic stabilities of GdL(0), GdL(1) and GdL(2) have been characterized by the rates of the exchange reactions occurring between the complexes and Eu3+, Cu2+ or Zn2+. The rates of reaction with Eu3+ are independent of the [Eu3+] and increase with increasing [H+], indicating the rate determining role of the proton assisted dissociation of complexes. The rates of reaction with Cu2+ and Zn2+ increase with rising metal ion concentration, which shows that the exchange can take place with direct attack of Cu2+ or Zn2+ on the complex, via the formation of a dinuclear intermediate. The rates of the proton, Cu2+ and Zn2+ assisted dissociation of Gd3+ complexes decrease with increasing number of the n-butyl substituents, which is presumably the result of steric hindrance hampering the formation or dissociation of the intermediates. The kinetic stabilities of GdL(0) and GdL(1) at pH = 7.4, [Cu2+] = 1 x 10(-6) M and [Zn(2+)] = 1 x 10(-5) M are similar to that of Gd(DTPA)2-, while the complex GdL2 possesses a much higher kinetic stability.

A new enantiomerization mechanism for tripodal penta-coordinate ZnII(nta) complexes. Theoretical clarification of the observed 1H NMR spectrum

Based on DFT calculations (RB3LYP/LANL2DZp), the unexpected single-line 1H NMR spectrum of Zn(II)(nta), nta = 2,2',2''-nitrilotriacetate, can be ascribed to a non-dissociative enantiomerization process (deltadeltadelta<=>lambdalambdalambda) from C3viaC3v to C3 symmetry. The energy barrier is rather low and depends to a lesser extent on the nature of the co-ligand in [Zn(nta)X]2- (X: H-, CH3- NH2-, OCH3-, F-, Cl-, Br-, I-) and [Zn(nta)Y]- (Y: NCH, CO, N2, O(CH3)2), but more so on the overall charge of the complex. The energy barrier for enantiomerization of [Zn(nta)X]2- is between 5.7 and 6.7 kcal mol-1, and for [Zn(nta)Y]- between 2.2 and 3.1 kcal mol-1.

Analysis of lorazepam and its 3O‐glucuronide in human urine by capillary electrophoresis: Evidence for the formation of two distinct diastereoisomeric glucuronides

Lorazepam (LOR) is a 3-hydroxy-1,4-benzodiazepine that is chiral and undergoes enantiomerization at room temperature. In humans, about 75% of the administered dose of LOR is excreted in the urine as its 30-glucuronide. CE-MS with negative ESI was used to confirm the presence of LOR-30-glucuronide in urines that stemmed from a healthy individual who ingested 1 or 2 mg LOR, whereas free LOR could be detected in extracts prepared from enzymatically hydrolyzed urines. As the 30-glucuronidation reaction occurs at the chiral center of the molecule, two diastereoisomers can theoretically be formed, molecules that can no longer interconvert. The stereoselective formation of LOR glucuronides in humans and in vitro was investigated. MEKC analysis of extracts of the nonhydrolyzed urines suggested the presence of the two different LOR glucuronides in the urine. The formation of the same two diastereoisomers was also observed in vitro employing incubations of LOR with human liver microsomes in the presence of uridine 5'-diphospho-glucuronic acid as coenzyme. The absence of other coenzymes excluded the formation of phase I or other phase II metabolites of LOR. Both results revealed a stereoselectivity, one diastereoisomer being formed in a higher amount than the other. After enzymatic hydrolysis using beta-glucuronidase, these peaks could not be detected any more. Instead, LOR was monitored. Analysis of the extracts prepared from enzymatically hydrolyzed urines by MEKC in the presence of 2-hydroxypropyl-beta-CD revealed the enantiomerization process of LOR (observation of two peaks of equal magnitude connected with a plateau zone). The data presented provide for the first time the evidence of the stereoselectivity of the LOR glucuronidation in humans.

Stereomutations of Atropisomers of Sterically Hindered Salophen Ligands

[structure: see text] The stereomutations in nonsymmetrical salophen ligands 1-4 were studied by means of dynamic NMR and HPLC methods. DNMR experiments showed that in DMSO-d(6) hindered ligands 2-4 exist in two chiral conformations, depending on whether the imine carbon atoms are in a cis or trans disposition with respect to the plane of the central o-phenylenediamine ring, the latter being more stable by 1.0 kcal mol(-1). Owing to its higher dipole moment, in the apolar solvent C(6)D(6) the cis conformer is destabilized with respect to the trans one, in agreement with the results of ab initio calculations. In DMSO-d(6) solution the two conformers are in equilibrium through the less hindered rotation about the C6-N7 bond aligned to the a(6,7) axis, and the interconversion barriers range from 18.4 to 19.3 kcal mol(-1). The enantiomerization process is a two step-process that implies sequential rotations around the C6-N7 and the C1-N8 bonds, so that the rate determining step is the slower rotation around the more hindered C1-N8 bond aligned to the a(1,8) axis, and the energy barriers range from 21.4 to 21.9 kcal mol(-1). These values compare well with those determined by chromatography on an enantioselective HPLC column at low temperature, thus confirming that DNMR and DHPLC can be conveniently employed as complementary techniques.

Evaluation and prediction of stereoisomerizations in comprehensive two-dimensional chromatography.

Conformational and configurational changes such as isomerizations, epimerizations, diastereomerizations and, enantiomerizations are important for the investigation of a large variety of processes ranging from protein folding to the stereostability of drugs. Under optimized conditions, these processes lead to an elution profile characterized by a plateau formation between the two interconverting species in chromatographic separations in a certain temperature range. By temperature-dependent measurements and subsequent computer simulation of the experimental chromatograms, the forward and backward rate constants k1 and k(-1), the Gibb's energy DeltaG++, activation enthalpy DeltaH++, and entropy DeltaS++ can be obtained. Due to its high efficiency two-dimensional chromatography is able to resolve the time-dependent distribution of the two species in the second dimension, thereby yielding the precise ratio of stereoisomers. An algorithm for the simulation and evaluation of two-dimensional chromatographic experiments has been developed, based on the theoretical plate model, which allows the determination of rate constants and barriers of isomerization, epimerization, and enantiomerization processes from two-dimensional chromatographic experiments. In the present article a detailed description of the extended theoretical plate model required for the simulation, the methods available, and examples for the evaluation of complex experimental data and the prediction of the separation conditions to observe isomerization, epimerizations, and enantiomerizations in two-dimensional chromatography are given.