VCD spectroscopic investigation of enantiopure cyclic beta-lactams obtained through Lipolase-catalyzed enantioselective ring-opening reaction

A direct enzymatic method for the preparation of cyclic beta-lactams and beta-amino acids was recently developed, involving the Lipolase-catalyzed enantioselective hydrolysis of racemic beta-lactams in an organic solvent. Vibrational circular dichroism (VCD) spectroscopy combined with quantum chemical calculations at ab initio (DFT) level of theory has now been applied to determine the absolute configuration and conformation of a series of cyclic beta-lactams (1-10). The absolute configuration of 8 was derived from X-ray crystallography. Only indirect evidence was available for 1, 2, 5, 6, and 7. The absolute configuration of the new lactams 3, 4, 9, and 10 was not known previously. The VCD analysis indicated the homochirality of the studied lactams. The conformation of the flexible beta-lactams was also predicted from the VCD data. Even in the cases where multiple conformers are allowed, the predominance of one conformer was found, with the exception of 2, being present as a mixture of four conformers. Beta-lactams tend to form H-bonded dimers. The fine structure of the amide I VCD band suggested that only a small population of H-bonded dimers is formed in deuterated chloroform.     

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.     

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.     

Absolute configuration determination of isoflavan-4-ol stereoisomers

Elucidation of the correct stereochemistry of the metabolite is essential for the mechanistic study of bioactive compounds. Isoflavan-4-ol has the same chiropical chromophore as THD, the biosynthetic precursor of the potent phytoestrogen S-equol. Interested in the correct absolute configuration of isoflavan-4-ol stereoisomers and to compare the available practical approaches for the absolute configuration determination, complete absolute configuration analysis of isoflavan-4-ol stereoisomers has been carried out with by means of ECD and VCD spectroscopy as well as modified Mosher method. Theoretical TD-DFT computations resulted in a poor simulation of the observed experimental ECD spectra, and thus inconclusive absolute configuration assignments of isoflavan-4-ol stereoisomers were obtained. However, DFT-assisted VCD spectroscopic analyses successfully determined correct absolute configurations, and further confirmed by modified Mosher method.     

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 the absolute configuration and solution conformation of gossypol by vibrational circular dichroism

Vibrational circular dichroism (VCD) measurements and density functional theory (DFT) calculations were used to obtain the first definitive assignment of the absolute configuration for the polyphenolic binaphpthyl dialdehyde gossypol and a determination of the solution conformation in CDCl(3). VCD spectra recorded for the two resolved enantiomers are near mirror images and excellent agreement between the observed IR and VCD spectra and intensity calculations carried out at the DFT (B3LYP/6-31G*) level establish the absolute configurations of (+)-gossypol as P and (-)-gossypol as M, with two conformations in CDCl(3) solution that differ in isopropyl group orientation.     

Twisted paddlewheel rhodium complexes: Contribution of central and axial chirality to ECD,VCD, and NMR spectra

Dirhodium complexes bearing N-substituted chiral amino acid ligands are investigated. These complexes have an unusual twisted paddlewheel structure, showing inherent chirality. We would like to demonstrate that parallel application of chiroptical spectroscopic methods (ECD and VCD) and NMR spectroscopy combined with quantum chemical calculations constitutes a powerful tool to determine the configuration of the complexes unequivocally. Two chiroptical methods are needed to determine the absolute configuration: ECD for the coordinated nitrogen atom and VCD for the rhodium core. A quick to use NMR method is also presented: Upon the coordination of small molecules in the axial position, the relative configuration of both the rhodium core and the nitrogen atom can be determined simultaneously by studying spatial proximities provided by 1D NOE spectra.     

Isotopic difference spectra as an aide in determining absolute configuration using vibrational optical activity: vibrational circular dichroism of 13C- and 2H-labelled nonamethoxy cyclotriveratrylene

The use of isotopic difference spectra in vibrational optical activity is demonstrated as a supplemental aide in determining the absolute configuration of chiral molecules. It is shown that IR and VCD difference spectra associated with isotopic substitution observed in experimental spectra can be accurately reproduced by density functional theory calculations when the IR and VCD spectra of the original isotopomer are calculated to reasonable accuracy. Results for isotopically substituted nonamethoxy cyclotriveratrylene are presented to illustrate the degree of agreement between measured and calculated IR and VCD difference spectra for several isotopomers of this molecule. These findings highlight the utility of isotoptic substitution as an aide to verifying the determination of absolute configuration using vibrational optical activity.     

Stereochemical analysis of β‐keto sulfoxides by circular dichroism

Three β‐keto sulfoxides ( 1–3 ) were synthesized in enantiopure form and investigated by means of circular dichroism (CD) spectroscopy, both in electronic and vibrational range (ECD, VCD), in combination with quantum chemical calculations. For compound 2 , the X‐ray structure was available; thus, the ECD in the solid state was also considered to reveal the differences between the molecular species in both states. Despite the simplicity of all β‐keto sulfoxides under investigation (29 atoms), reproducing even the major spectral VCD features failed for two compounds, making the use of VCD not ideal to assign their absolute configuration in a reliable way. We demonstrated, however, that the use of ECD spectroscopy, both in solution and solid state, can easily, unambiguously, and without any complication simulate all bands by applying the standard protocol for calculations. This study may stimulate the debate on the need of the use of two chiroptical methods simultaneously in the determination of absolute configurations.     

Absolute configuration assignment of (+)‐fluralaner using vibrational circular dichroism

The absolute configurations of the separated enantiomers of fluralaner, a racemic animal health product used to prevent fleas and ticks, have been assigned using vibrational circular dichroism (VCD). The crystallographic structure of the active enantiomer (+)‐fluralaner has previously been shown to have the (S ) configuration using small molecule crystallography. We sought a faster analytical method to determine the absolute configuration of the separated enantiomers. When comparing the measured IR (infrared) and VCD spectra, it is apparent that the amide carbonyl groups appear in the IR but are nearly absent in the VCD. Computational work to calculate the VCD and IR using in vacuo models, implicit solvation, and explicitly solvated complexes has implicated conformational averaging of the carbonyl VCD intensities.     

Vibrational and electronic circular dichroism monitoring of copper(II) coordination with a chiral ligand

Novel copper(II) coordination compounds with chiral macrocyclic imine ligands derived from R-/S-camphor were asymmetrically synthesized and characterized with the aid of chiroptical spectroscopies. Crystal structures of both enantiomers were determined by single crystal X-ray diffraction analysis. Circular dichroism (CD) spectra were analyzed using a simplified exciton model as well as quantum chemical computations. The absolute configuration of the copper(II) coordination compounds determined from CD was found consistent with the crystal data. The copper(II) complexes were further investigated by vibrational CD (VCD) measurement combined with density functional theory calculation. The complex formation was evidenced by spectral shifts of the characteristic bands in the CD and VCD spectra.     

Chiral cruciferous phytoalexins: preparation, absolute configuration, and biological activity

Synthesized by an efficient one-pot spirocyclization method, two chiral cruciferous phytoalexins, 1-methoxyspirobrassinin (2) and 1-methoxyspirobrassinol methyl ether (4a), were prepared through optical resolution using the chiral HPLC method of corresponding racemates. The absolute configuration of natural (+)-2 was elucidated as R by using the direct comparison of ECD and VCD spectra with those of known (S)-(-)-spirobrassinin (1). Another chiral phytoalexin, (-)-4a, had its absolute configuration 2R,3R elucidated through the comparison of observed and calculated VCD. Interestingly, the absolute configurations of natural (S)-(-)-spirobrassinin (1) and (R)-(+)-1-methoxyspirobrassinin (2) were opposite of each other, even though their structures are almost similar, with the exception of an N-methoxy group. A significant difference in the antiproliferative activity between (2R,3R)-(-) and (2S,3S)-(+)-4a was observed.     

Towards the Limits of Vibrational Circular Dichroism Spectroscopy: VCD Spectra of Some Alkyl Vinylethers

Three alkyl vinylethers from our collection of chiral samples were investigated through VCD spectroscopy, in combination with Density Functional Theory (DFT) calculations. Despite the simplicity of the compounds, reproducing all the spectral features is an involved task, since the many significantly populated conformers contribute to the total VCD spectrum with bands which often have opposite signatures. Nevertheless, we show that certain bands can be satisfactorily reproduced by calculation and therefore they may be employed for the determination of absolute configuration in these and similar compounds, for which no simple alternative method is available. Chirality 28:143–146, 2016. © 2015 Wiley Periodicals, Inc.     

Vibrational circular dichroism spectra of three conformationally distinct states and an unordered state of poly(L-lysine) in deuterated aqueous solution

Fourier transform ir vibrational circular dichroism (VCD) spectra in the amide I′ region of poly(L-lysine) in D₂O solutions have confirmed the existence of three distinct conformational states and an unordered conformational state in this homopolypeptide. Characteristic VCD spectra are presented for the right-handed α-helix, the antiparallel β-sheet, an extended helix conformation previously referred to as the so-called “random coil,” and a completely unordered conformation characterized by the absence of any amide I′ VCD. VCD for the antiparallel β-sheet in solution and the unordered chain conformation are presented for the first time. Each of the four different VCD spectra is unique in appearance and lends weight to the view that VCD has the potential to become a sensitive new probe of the secondary structure of proteins in solution.     

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.     

Chemoenzymatic Approach to Optically Active 4‐Hydroxy‐5‐alkylcyclopent‐2‐en‐1‐one Derivatives: An Application of a Combined Circular Dichroism Spectroscopy and DFT Calculations to Assignment of Absolute Configuration

A series of representative optically active derivatives of 4‐hydroxy‐5‐alkylcyclopent‐2‐en‐1‐one were prepared from the respective 2‐furyl methyl carbinols via the Piancatelli rearrangement followed by the enzymatic kinetic resolution of racemates. Applicability of chiroptical methods (experimental and calculated electronic circular dichroism [ECD] and vibrational circular dichroism [VCD] spectra) to determine the absolute configuration of both stereogenic centers in 4‐hydroxy‐5‐methylcyclopent‐2‐en‐1‐one was demonstrated. It was also demonstrated that the concurrent application of ECD and VCD spectroscopy can be used for the determination of the configuration of two stereogenic centers. Chirality 26:300–306, 2014. © 2014 Wiley Periodicals, Inc.     

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(+).     

Vibrational circular dichroism studies of epidermal growth factor and basic fibroblast growth factor.

Vibrational circular dichroism (VCD) studies are reported for two unrelated recombinant growth factor proteins: epidermal growth factor and basic fibroblast growth factor (bFGF). NMR, electronic CD, and bFGF X-ray studies indicate that these two proteins are primarily composed of beta-sheet and loop secondary structure elements with no detectable alpha-helices. Two reports on solution conformation of these proteins using FTIR absorption spectroscopy with subsequent resolution enhancement confirmed the presence of a large fraction of a beta-sheet conformation but in addition indicated the presence of large absorption bands in the 1650-1656 cm-1 region, which are typically assigned to alpha-helices. The VCD spectra of both proteins have band shapes that strongly resemble those of other high beta-sheet fraction proteins, such as the trypsin family of proteins. Quantitative analysis of the VCD spectra also indicates that these proteins are predominantly in beta-sheet and extended ("other") conformations with very little alpha-helix fraction. These results agree with the CD interpretation and affirm that the FTIR peaks in the region 1650-1656 cm-1 can be assigned to loops. This study provides an example of the limitations of using FTIR frequencies alone for examination of protein secondary structure.     

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.     

Mini review: Instrumentation for vibrational circular dichroism spectroscopy,still a role for dispersive instruments

Vibrational circular dichroism (VCD) has become a standard method for determination of absolute stereochemistry, particularly now that reliable commercial instrumentation has become available. These instruments use a now well‐documented Fourier transform infrared‐based approach to measure VCD that has virtually displaced initial dispersive infrared‐based designs. Nonetheless, many papers have appeared reporting dispersive VCD data, especially for biopolymers. Instrumentation designed with these original methods, particularly after more recent updates optimizing performance in selected spectral regions, has been shown still to have advantages for specific applications. This article presents a mini‐review of dispersive VCD instrument designs and includes sample spectra obtained for various biopolymer (particularly peptide) samples. Complementary reviews of Fourier transform‐VCD designs are broadly available.