Solution conformations of cyclosporins and magnesium-cyclosporin complexes determined by vibrational circular dichroism

Vibrational circular dichroism (VCD) spectroscopy was used to investigate the solution conformations of cyclosporins A, C, D, G, and H in CDCl(3), in the amide I and NH/OH-stretching regions, and their corresponding magnesium complexes in CD(3)CN, in the amide I region. VCD spectra are sensitive to the chiral arrangement of Cdbond;O and NH bonds in this cyclic undecapeptide. Calculations of molecular geometries, as well as IR and VCD intensities of model cyclosporin fragments that include the intramolecular hydrogen bonds of the crystal conformations of cyclosporins A and H (CsA and CsH), were carried out at the density functional theory (DFT; BPW91 functional/6-31G* basis set) level. The good agreement between IR and VCD spectra from experiment and DFT calculations provides evidence that the crystal conformation of CsA is dominant in CDCl(3) solution; CsH, however, assumes both an intramolecularly hydrogen-bonded crystal conformation and more open forms in solution. Comparisons of the experimental and calculated VCD spectra in the NH/OH-stretching region of the noncomplexed cyclosporins indicate that conformers with both free and hydrogen-bonded NH and OH groups are present in solution. Differences between the IR and VCD spectra for the metal-free and magnesium-complexed cyclosporins are indicative of strong interactions between cyclosporins and magnesium ions.     

Determination of the absolute configuration and solution conformation of a novel disubstituted pyrrolidine acid A by vibrational circular dichroism

Compound A, a novel disubstituted pyrrolidine acid, is a member of a new class of agents that are potentially useful for the treatment of diabetes and dyslipidemia. The absolute configuration of this compound was determined by using vibrational circular dichroism (VCD). The results are in agreement with the assignments based on both X-ray analysis and the stereo-selective chemical synthesis. During VCD analysis, the solution conformation for a portion of compound A in CDCl(3) was also established. The compound is found to associate as an H-bonded carboxylic acid "dimer" in CDCl(3) solution, and VCD calculations on a model dimer fragment were required to establish the absolute configuration.     

Assessment of configurational and conformational properties of naringenin by vibrational circular dichroism

The electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) spectra of both enantiomers of naringenin (4',5,7-trihydroxyflavanone) in acetonitrile solution have been measured. The enantiomers were obtained by chiral HPLC separation of the racemic sample. DFT calculations have been performed for relevant conformers and subsequent evaluations of VCD spectra are compared with VCD experiments: safe assignment of the absolute configuration is provided, based in particular on the VCD data. The relevance of the rotational conformers of the hydroxyl groups and of the mobility of phenol moiety is studied: based on this, we provide a first interpretation of the observed intense and broad couplet at 1325/1350 cm(-1). Four conformers contribute to this pattern with different sign and amplitude as shown by DFT calculations. Time dependent DFT calculations have been performed and compared with ECD experimental data, under the same assumption of conformational properties and mobilities investigated by VCD.     

Absolute configurations of brominated sesquiterpenes determined by vibrational circular dichroism

Two brominated sesquiterpenes, majapolene B (1) and acetylmajapolene B (2), isolated from the red algal genus Laurencia were investigated using vibrational circular dichroism (VCD). The ab initio theoretical VCD and IR calculations of 1 and 2 were performed by density functional theory (DFT) using the B3PW91/6-31G(d,p) basis set. The experimental VCD spectra and corresponding population-weighted theoretical VCD spectra were found to be in excellent agreement in CCl(4) solution in the 1800-850 cm(-1) region, which allowed unambiguous determination of the absolute configurations of (-)-1 and (-)-2 as 7S,10S and 7S,10S, respectively.     

Absolute configuration and conformational analysis of a degradation product of inhalation anesthetic Sevoflurane: A vibrational circular dichroism study

1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane, compound B, is a product obtained in the degradation of the anesthetic Sevoflurane. Enantiopure (+)-B was investigated using vibrational circular dichroism (VCD). Experimental absorption and VCD spectra of (+)-B in CDCl(3) solution in the 2,000-900 cm(-1) region are compared with the ab initio predictions of absorption and VCD spectra obtained from density functional theory using B3LYP/6-31G* basis set for different conformers of (S)-1,1,1,3,3-pentafluoro-2-(fluoromethoxy)-3-methoxypropane. This comparison indicates that (+)-B is of the (S)-configuration in CDCl(3) solution, in agreement with previous literature results. Our results also indicate that this compound adopts six predominant conformations in CDCl(3) solution.     

Absolute configuration determination of donor-acceptor [2.2]paracyclophanes by comparison of theoretical and experimental vibrational circular dichroism spectra

Vibrational circular dichroism (VCD) spectra were obtained for the assignments of the absolute configurations of diastereomeric 4,7-bis(methoxycarbonyl)-12,15-dimethoxy-[2.2]paracyclophanes (1 and 2), and density functional theory (DFT) calculations were performed at the B3LYP/6-31G(d) level for all possible conformations of both 1 and 2 to give the theoretical VCD spectra. Comparisons of the experimental and theoretical VCD spectra obtained unambiguously established the absolute configurations of the dextrorotatory (+)-enantiomers as (4S(p);12S(p))-1 and (4S(p);12R(p))-2, respectively.     

Absolute configuration of eremophilanoids by vibrational circular dichroism

The absolute configurations (AC) of natural occurring 6-hydroxyeuryopsin (1), of its acetyl derivative 2, and of eremophilanolide 8 were confirmed by comparison of the experimental vibrational circular dichroism (VCD) spectra with theoretical curves generated from density functional theory (DFT) calculations. Initial analyses were carried out using a Monte Carlo searching with the MMFF94 molecular mechanics force field. All MMFF94 conformers were further optimized using DFT at the B3LYP/6-31G(d) level of theory, followed by calculations of their vibrational frequencies at the B3LYP/6-31G(d,p); the VCD spectra of 2 and 8 were also calculated at the B3PW91/DGDZVP level of theory. Good agreement between theoretical and experimental VCD curves unambiguously verified the 4S,5R,6S absolute configuration for 1 and 2, and the 1S,4S,5R,6S,8S,10S configuration for 8.     

Chirality of camphor derivatives by density functional theory

Infrared (IR) and vibrational circular dichroism (VCD) spectra of chiral camphor, camphorquinone and camphor-10-sulfonic acid (CSA), known as standard compounds for electronic circular dichroism (ECD) spectroscopy, are measured and their vibrational frequencies, infrared intensities, and rotational strengths are calculated using density functional theory (DFT). The observed IR and VCD spectra of chiral camphor and camphorquinone in carbon tetrachloride solution are reproduced by the DFT calculations, but those of CSA are not. DFT calculations of hydration models, where an anionic CSA specifically binds a few water molecules, are carried out. The average of the simulated VCD spectra in the hydration models is more consistent with the observed spectra. In addition, the wavelengths and dipole and rotational strengths for chiral camphor, camphorquinone, anionic CSA, and the hydration models were calculated by time-dependent DFT. In the region of 280-300 nm, the calculated wavelengths of the ECD bands for chiral camphor and camphorquinone coincide with the observed wavelengths that have been reported, and the calculated wavelengths for the hydration models are closer to the observed wavelengths reported than are those calculated for chiral anionic CSA. Consequently, the analysis combined with VCD and ECD spectroscopy using DFT calculations can elucidate the chirality of optically active molecules, even in an aqueous solution.     

Determination of the absolute configuration of a key tricyclic component of a novel vasopressin receptor antagonist by use of vibrational circular dichroism

We present the results of a study using vibrational circular dichroism (VCD) for (+)-1, which furnished an unambiguous determination of its absolute configuration as S. The most abundant conformation of (+)-1 in CDCl(3) solution was also established.     

Determination of absolute configuration using vibrational circular dichroism spectroscopy: the chiral sulfoxide 1-thiochromanone S-oxide

We reexamined the absolute configuration (AC) of the chiral sulfoxide 1-thiochromanone S-oxide (1) using vibrational circular dichroism (VCD) spectroscopy. The VCD spectrum of 1 was analyzed using density functional theory (DFT). DFT predicts two stable conformations of 1, separated by <1 kcal/mole. Their VCD spectra were calculated using the DFT/GIAO methodology. The VCD spectrum predicted for the equilibrium mixture of the two conformations of (S)-1 is in excellent agreement with the experimental spectrum of (+)-1. The AC of 1 is therefore definitively R(-)/S(+).     

Determination of the absolute configurations of synthetic daunorubicin analogues using vibrational circular dichroism spectroscopy and density functional theory

The absolute configurations of three synthesized anthracycline analogues have been determined using vibrational circular dichroism (VCD) spectroscopy and the density functional theory (DFT) calculations. The experimental VCD spectra of the three compounds have been measured for the first time in the film state, prepared from their CDCl₃ solutions. Conformational searches for the monomers and some dimers of the three compounds have been performed at the DFT level using the B3LYP functional and the 6‐311G** and 6‐311++G** basis sets. The corresponding vibrational absorption and VCD spectra have been calculated. The good agreement between the experimental and the calculated spectra allows one to assign the absolute configurations of the three compounds with high confidence. In addition, the dominant conformers of the three compounds have also been identified. Chirality, 2010. © 2010 Wiley‐Liss, Inc.     

Absolute configuration assignment of (3-phenyloxirane-2,2-diyl)bis(phenylmethanone) via density functional calculations of optical rotation and vibrational circular dichroism

Density Functional Theory (DFT) calculations of optical rotation (OR) and vibrational circular dichroism (VCD) have been used to assign the absolute configuration (AC) of a recently prepared (3-phenyloxirane-2,2-diyl)bis(phenylmethanone), 3, by asymmetric epoxidation of the corresponding 2-arylidene-1,3-diketone. The experimental OR at 589.3 nm and the VCD spectrum of the (+)- and (-)-enantiomer of 3 have been measured. The conformationally-averaged OR value and VCD spectrum of (R)-3 were calculated at B3LYP/6-311G(2d,2p) level of theory. Both approaches provide the same absolute configuration of the stereogenic carbon, i.e. the AC of (+)-3 is (R)-3, thus affording a confident assignment. Only two conformational isomers of 3 have been predicted to be populated at ambient temperature. Their presence is directly observed in the VCD spectrum.     

Conformational analysis of perezone and dihydroperezone using vibrational circular dichroism

The vibrational circular dichroism (VCD) spectra of perezone and dihydroperezone measured from CDCl₃ solutions were quite similar, suggesting analogous conformations for both molecules. Their absolute configurations were confirmed by comparison of the experimental VCD spectrum of each compound with curves generated from theoretical calculations using density functional theory (DFT) at the B3LYP/DGDZVP level of theory taking into account their conformational mobility. Conformational analysis of the 8-(R) enantiomer showed 19 low energy conformers in a 2.4 kcal/mol energy range, while for 8-(R), with the saturated side alkyl chain, 34 conformers were considered in the first 2 kcal/mol. Initial analyses were carried out using a Monte Carlo searching with the MMFF94 molecular mechanics force field, all MMFF94 conformers were geometrically optimized using DFT at the B3LYP/6-31G(d) level of theory, followed by reoptimization and calculations of their vibrational frequencies at the B3LYP/DGDZVP level. Good agreement between the theoretical 8-(R) enantiomers and experimental VCD curves were observed for both.     

Application of chiral technology in a pharmaceutical company. Enantiomeric separation and spectroscopic studies of key asymmetric intermediates using a combination of techniques. Phenylglycidols

Phenylglycidols substituted in the 2-, 3-, and 4- positions with fluorine, chlorine, and trifluoromethyl, and with methoxy in the 3- position, were synthesized from the corresponding E-cinnamic acids and separated into their (R,R)- and (S,S)- enantiomers using subcritical fluid chromatography with mixtures of MeOH in CO(2), on either a Chiralpak AD or AS chiral stationary phase. These compounds and commercially-available (R,R)- and (S,S)-phenylglycidol were analyzed for their vibrational circular dichroism (VCD), electronic circular dichroism (ECD), and optical rotation (OR) properties to exemplify a strategy whereby the absolute stereochemistry of common and key chiral intermediates is established early in the structure-activity and structure-property relationship phase of a drug discovery program in a pharmaceutical company. From this study, substituents in the phenyl group of the synthesized molecules were found not to grossly alter spectroscopic features, and therefore, diagnostic absorption bands in the respective VCD spectra, and the sign and shape of the measured ECD curves could be used to determine and track the absolute stereochemistry of analogs without necessarily requiring time-consuming ab initio calculations of all low energy conformers for all compounds. VCD, OR, and ECD calculations for the determination of absolute configuration carried out at the DFT level with the hybrid B3PW91 functional and the TZVP basis set were found to be especially useful in this study.     

Unusual CD couplet pattern observed for the pi*<--n transition of enantiopure (Z)-8-methoxy-4-cyclooctenone: an experimental and theoretical study by electronic and vibrational circular dichroism spectroscopy and density functional theory calculation

Ultraviolet absorption (UV) and electronic circular dichroism (ECD) spectra of enantiopure (Z)-8-methoxy-4-cyclooctenone (MCO) were measured in hexane to give a normal single UV absorption band at 298 nm, which is assigned to the carbonyl's pi*<--n transition. Unexpectedly, the ECD spectrum exhibited an apparent couplet pattern with vibrational fine structures. Obviously, the conventional CD exciton coupling mechanism cannot be applied to this bisignate CD signal observed for single-chromophoric MCO. Variable temperature-ECD and vibrational circular dichroism (VCD) spectral measurements, simultaneous UV and ECD spectral band resolution, and density functional theory (DFT) calculations of energy and structure revealed that this apparent CD couplet originates from a rather complicated spectral overlap of more than three conformers of MCO, two of which exhibit mirror-imaged ECD spectra at appreciably deviated wavelengths. In the simultaneous band-resolution analysis, the observed UV and ECD spectra were best fitted to four overlapping bands. Two major conformers were identified by comparing the experimental IR and VCD spectra with the simulated ones, and the other two by comparing the observed UV and ECD spectra with the theoretical ones obtained by time-dependent DFT calculations. It was shown that the combined use of experimental ECD and VCD spectra and theoretical DFT calculations can give a reasonable interpretation for the Cotton effects of the conformationally flexible molecule MCO.     

VCD configuration of enantiopure/-enriched tetrasubstituted alpha-fluoro cyclohexanones and their use for epoxidation of trans-olefins

The configurations of three enantiopure tetrasubstituted alpha-fluoro cyclohexanones (-)-5Ia, (-)-5IIa and (-)-6a were determined by VCD and proved to be (-)-(2S,5R)-5Ia, (-)-(2R,5R)-5IIa, and (-)-(2R,5R)-6a. The VCD study also identified the conformers populated in CDCl3 solution, including higher-energy gas-phase conformers with equatorial fluorine for 5Ia and 5IIa that are stabilized in CDCl3 solution. Used as catalysts for epoxidation of trans olefins (beta-methylstyrene, stilbene, methyl p-methoxy cinnamate) by oxone, it was found that (-)-5Ia is the most efficient for all trans olefins (providing, respectively, 62%, 90% and 66% ee) but that all three ketones provide high ee% with stilbene (78-90% ee). Moreover, the configurations predicted from the stereo outcome of the epoxidation reaction are identical to those determined by VCD.     

Conformational studies on chiral rhodium complexes by ECD and VCD spectroscopy

This article reports vibrational circular dichroism (VCD) and electronic circular dichroism (ECD) spectroscopic studies in acetonitrile on the chiral Rh(2)(O-Phe-Cbz)(1)(OAc)(3) and Rh(2)(O-Phe-Ac)(1)(OAc)(3) complexes (abbreviated Rh(2)Z(1) and Rh(2)Ac(1) , respectively; Phe, L-phenylalanine; Cbz, benzyloxycarbonyl; Ac, acetyl) supported by theoretical calculations. The ECD spectra of the complexes depend on temperature that indicates the conformational mobility of the chiral ligands. Calculations of the VCD spectra were performed at ab initio (DFT) level of theory using Gaussian 03 [B3LYP functional combined with the LANL2DZ basis set for the dirhodium core and the 6-31G(d) basis set for other atoms]. The population-weighted sums of the computed VCD spectra of the conformers are in excellent agreement with the experimental VCD spectra. The combination of the VCD and ECD spectroscopic methods led us to the structural characterization of the complexes.     

Absolute configuration of (−)-myrtenal by vibrational circular dichroism

The VCD spectrum of the monoterpene (−)-myrtenal (1) was compared with theoretical spectra using ab initio density functional theory (DFT) calculations at the B3LYP/6-31G(d,p), B3LYP/6-31G+(d,p), B3LYP/6-311G+(d,p), B3LYP/DGDZVP, and B3PW91/DGTZVP levels of theory. Conformational analysis of 1 indicated that the lowest energy conformer was s-trans-C2-C10, which contributes more than 98.5% to the total conformational population regardless of the employed level of theory. The use of a recently developed confidence level algorithm demonstrated that VCD spectra calculated for the main conformer, using the indicated hybrid functionals and basis set, gave no significant changes, from where it follows that B3LYP/DGDZVP calculations provide a superior balance between computer cost and VCD spectral accuracy. The DGDZVP basis set demanded around a quarter the time than the 6-311G+(d,p) basis set while providing similar results. The spectral comparison also provided evidence that the levorotatory enantiomer of myrtenal has the 1R absolute configuration.     

Absolute configuration determination and conformational analysis of (−)‐(3S,6S)‐3α,6β‐diacetoxytropane using vibrational circular dichroism and DFT techniques

The absolute configuration of semisynthetic (?)‐3α,6β‐acetoxytropane 1 , prepared from (?)‐6β‐hydroxyhyoscyamine 2 , has been determined using vibrational circular dichroism (VCD) spectroscopy. The vibrational spectra (IR and VCD) were calculated using DFT at the B3LYP/DGDZVP level of theory for the eight more stable conformers which account for 99.97% of the total relative abundance in the first 10 kcal/mol range. The calculated VCD spectra of all considered conformations showed two distinctive spectral ranges, one between 1300 and 1200 cm^?1, and the other one in the 1150–950 cm^?1 region. When compared with the experimental VCD spectrum, the first spectral region confirmed the calculated conformational preferences, whereas the second region showed little change with conformation, thus allowing the determination of the absolute configuration of 1 as (3S,6S)‐3α,6β‐diacetoxytropane. Also, the bands in the second region showed similarities between 1 and 2 in both the experimental and calculated VCD spectra, suggesting that these bands are mainly related to the absolute configuration of the rigid tropane ring system, since they show conformational independency, no variations with the nature of the substituent, and are composed by closely related vibrational modes. Chirality, 2010. © 2009 Wiley‐Liss, Inc.