Computing chiroptical properties with first-principles theoretical methods: Background and illustrative examples

Synthesis, enzymatic resolution, and stereochemical characterization of isoparaconic acid derivatives: a combined experimental and theoretical investigation

Enantiomerically enriched isoparaconic acid derivatives were obtained by kinetic enzymatic resolution. To explain the solvent dependence observed for their optical rotatory power a computational investigation of their chiroptical properties was performed.

Accurate molecular structures and infrared spectra of trans-2,3-dideuterooxirane, methyloxirane, and trans-2,3-dimethyloxirane

Oxirane derivatives are the most used benchmarks for chiroptical spectroscopies in view of their small size and relative rigidity. The molecular structure, vibrational harmonic and anharmonic frequencies, and infrared intensities of the ground electronic states are analyzed in this paper. Equilibrium structure and harmonic force fields have been evaluated by means of high-level quantum-chemical calculations at the coupled-cluster level including single and double excitations together with a perturbative treatment of triples (CCSD(T)). Extrapolation to the complete basis-set limit as well as core-correlation effects have also been taken into account. Anharmonic contributions have been computed at the CCSD(T)/cc-pVTZ level for trans-2,3-dideuterooxirane. These data can serve as references to evaluate the accuracy of less expensive computational approaches rooted in the density functional theory (DFT). The latter have been used within hybrid CC/DFT approaches, which have been applied to simulate fully anharmonic infrared (IR) spectra. Finally, the best theoretical estimates of the equilibrium structures and vibrational wavenumbers are compared to the most accurate experimental data and show in all cases very good agreement, i.e., within 0.001 Å, 0.1 deg, 10 cm(-1), and 0.5 km mol(-1), for bond lengths, angles, wavenumbers, and IR intensities, respectively.

Enantioresolution, stereochemical characterization and biological activity of a chiral large-conductance calcium-activated potassium channel opener

A number of large-conductance calcium-activated potassium (BK) channel openers based on the 2-aryl-1,4-benzothiazine scaffold were previously synthesized, and 2-(5-bromo-2-methoxyphenyl)-6-trifluoromethyl-2H-1,4-benzothiazin-3(4H)-one (1) was identified as the most active compound. Since a stereoselective activation of BK channels was demonstrated for arylindolone derivatives, the effect of the absolute configuration at the C-2 position on the vasorelaxing potency of 2-aryl-1,4-benzothiazines is investigated in this article. Compound 1 was initially evaluated as a racemate: subsequently, the "racemic approach" was used to isolate its enantiomers. The excellent enantioresolution obtained using the Sepapak-4 column (CSP 4, cellulose tris(4-chloro-3-metylphenylcarbamate); RS=8.36; α=2.03) allowed to collect highly pure enantiomeric fractions, with enantiomeric excess (e.e.) values higher than 97% and 98% for the first- and second-eluted enantiomer, respectively. Electronic circular dichroism (ECD) studies on the two isolated enantiomers, combined with time-dependent density functional theory (TD-DFT) calculations allowed to characterize the configuration of the enantiomers and determine a (R), (S) elution order. Results from biological assays indicated that the racemate and the isolated enantiomers are endowed with comparable vasorelaxing potency.

Optically active tetra-tert-butyl-P(5)-deltacyclene epimers: preparation, spectroscopy, dynamic equilibriums, H/D exchange, and transition-metal complex chemistry

On the basis of isolated diastereomeric triorganylstannyl-P5 -deltacyclenes 7' and 7'', almost pure enantiomers of their destannylation products 8' and 8'' are now available. These stereochemically inert cage chiral species contain a configurationally labile P1H1 group that defines two epimers 8 a and 8 b of each of the enantiomers, which are connected by a rapid equilibrium. Mirror-symmetric circular dichroism (CD) spectra of the enantiomeric cages are compatible with the identification of epimers. A simulation of the CD spectrum of the major epimer 8'a relates the cage chirality of the system to the observed chiroptical effects. Both cage epimers and two of the phosphorus cage atoms are active as ligands with respect to [M(CO)5 ] fragments of Cr, Mo, and W. Four almost isoenergetic regio- and stereoisomers of the resulting mononuclear complexes are formed for these metals, but only one of the isomers per metal crystallized in the case of the racemic series of the complexes. The enantiopure versions of cages and cage complexes, however, did not crystallize at all, a well-known phenomenon for chiral compounds. CD spectra of the optically active complex isomer mixtures are close to identical with the CD spectra of the related free cages and point again to the chiral cages as the dominant source of the CD effects of the complexes. [(Benzene)RuCl2 ] complexes of the cage ligand 8 behave totally differently. Only a single species 12=[(benzene)RuCl2 ⋅8 b] is formed in almost quantitative yield and the minor epimer 8 b plays the role of the ligand exclusively. The reaction works as well for the separated enantiomeric cage versions to yield the highly enriched enantiomers 12' and 12'' separately. An efficient kinetic resolution process was identified as the main reason for this finding. It is based on a high stereo- and regiochemical flexibility of the PC cage ligand that is capable of adjusting to the specific requirements of a suitable transition-metal complex fragment. Such ligand flexibility is regularly observed in metalloenzymes, but is a very rare case in classical and organometallic complex chemistry.

Chiroptical study and absolute configuration of (-)-O-DMA produced from daidzein metabolism

To elucidate the hitherto unknown absolute configuration of (-)-O-desmethylangolensin ((-)-O-DMA), an intestinal bacterial metabolite produced from daidzein, chiroptical study, including specific optical rotation and electronic circular dichroism (ECD), of (R)-O-DMA was carried out by Time-Dependent Density Functional Theory (TD-DFT) calculations. Intramolecular hydrogen bonding between 2'-OH and carbonyl oxygen at 1-C of O-DMA was a governing factor for O-DMA to form the stable conformations. Total energy values of four possible conformers were calculated in the framework of DFT using the B3LYP exchange correlation functional at the 6-31++G basis set level. The theoretical specific rotation and ECD spectra of all conformers in ethanol were obtained by TD-DFT calculation using B3LYP functional at the 6-311++G basis set level, and compared to the experimental data. Chiroptical properties of (R)-O-DMA showed a good agreement with the biological (-)-O-DMA. Therefore, the stereospecific biosynthetic pathway of (-)-O-DMA was proposed as daidzein → (R)-dihydrodaidzein ↔ (S)-dihydrodaizein → (R)-O-DMA.

Mycoleptones A-C and polyketides from the endophyte Mycoleptodiscus indicus

Three new azaphilones with an unusual methylene bridge, named mycoleptones A, B, and C (2, 4, and 5), were isolated from cultures of Mycoleptodiscus indicus, a fungus associated with the South American medicinal plant Borreria verticillata. Additionally, four known polyketides, austdiol (1), eugenitin (3), 6-methoxyeugenin (6), and 9-hydroxyeugenin (7), were also isolated. The structural characterization of compounds was carried out by nuclear magnetic resonance spectroscopy, high-resolution mass spectrometry, electronic circular dichroism spectroscopy, time-dependent density functional theory calculations, and X-ray crystallography. Compounds 1-9 were weakly active when tested in antileishmanial and cytotoxicity assays.

Strong intermolecular exciton couplings in solid-state circular dichroism of aryl benzyl sulfoxides

A series of 13 enantiopure aryl benzyl sulfoxides () with different substituents on the two aromatic rings has been previously analyzed by means of electronic circular dichroism (CD) spectroscopy. Most of these compounds are crystalline and their X-ray structure is established. For almost one-half of the series, CD spectra measured in the solid state were quite different from those in acetonitrile solution. We demonstrate that the difference is due to strong exciton couplings between molecules packed closely together in the crystal. The computational approach consists of time-dependent density functional theory (TDDFT) calculations run on "dimers" composed of nearest neighbors found in the lattice. Solid-state CD spectra are well reproduced by the average of all possible pairwise terms. The relation between the crystal space group and conformation, and the appearance of solid-state CD spectra, is also discussed.

Absolute configurations of four resorcylic acid lactones, paecilomycins J-M, by CD/TDDFT calculations

The absolute configurations of four resorcylic acid lactones (RALs), paecilomycins J-M (1-3 and 5), were assigned by Time-Dependent Density-Functional Theory (TDDFT) calculations of their electronic circular dichroism (CD) spectra. The previously reported structure 4 for paecilomycin M was found to be incorrect and should be changed to structure 5. Analysis of structure-spectrum relationship for this group of RALs suggested that V'-shape conformations give type I CD spectra (two negative Cotton effects around 300 and 260 nm, a positive Cotton effect around 220 nm) while V-shape conformations yield type II spectra (signs of three Cotton effects were opposite to those in type I).

Automatic baseline correction of vibrational circular dichroism spectra

A three-phase, computational method for the baseline correction of vibrational circular dichroism (VCD) spectra has been proposed. In the first phase the raw spectrum is subdivided into m segments (or regions) resulting in p rough estimates of the baseline. A second phase uses gradient characteristics to discriminate between baseline and band response for each baseline, in turn. In the final phase all baselines are interrogated simultaneously by assigning the median estimate of each differential response’s distribution to the true baseline. Using VCD spectra of (R)-camphor as test cases, this work demonstrated that the accurate removal of baseline components is readily achievable with minimal user intervention. Baseline correction also demonstrated flexibility in that prior information, such as the symmetry of a baselinefree VCD spectrum, is readily used during the correction protocol. Although three adjustable parameters are present in the base algorithm, optimal performance and full automation were attainable following the use of analysis of variance (ANOVA) to analyze simulated bipolar spectra. These ANOVAs suggested that band point discrimination could be discarded and the remaining two default parameters adopted.

SpecDis: quantifying the comparison of calculated and experimental electronic circular dichroism spectra

This article outlines theory and practice of the comparison of calculated and experimental electronic circular dichroism (ECD) curves to determine the absolute configuration of chiral molecules. The focus is on the evaluation of excited-state calculations giving hints at the identification of the correct bandwidth and the application of the so-called "UV shift" as a correction factor. A similarity factor is introduced, which helps to quantify the degree of matching of curves. In addition, a few common errors are described that can be made during the measurements of ECD and UV spectra-and advice is given of how to avoid these mistakes. All equations mentioned in the article are implemented in our SpecDis software, which has been developed to rapidly compare calculated ECD and UV curves with experimental ones, and to produce graphics in publication quality.

Agarsenone, a Cadinane Sesquiterpenoid from Commiphora erythraea

Agarsenone (1), a new cadinane sesquiterpenoid, was isolated from the resin of Commiphora erythraea. The structures of 1 and its decomposition products agarsenolides (2a and 2b) and myrrhone (3) were established by extensive NMR spectroscopic analysis. The absolute configuration of 3 and the relative and absolute configurations of 1 were assigned by comparison of experimental and calculated optical rotatory dispersion and electronic circular dichroism spectra.

Walking in the woods with quantum chemistry--applications of quantum chemical calculations in natural products research

This Highlight describes applications of quantum chemical calculations to problems in natural products chemistry, including the elucidation of natural product structures (distinguishing between constitutional isomers, distinguishing between diastereomers, and assigning absolute configuration) and determination of reasonable mechanisms for their formation.

Absolute configurations of fungal and plant metabolites by chiroptical methods. ORD, ECD, and VCD studies on phyllostin, scytolide, and oxysporone

The absolute configuration (AC) of the bioactive metabolites phyllostin (1) and scytolide (2), two hexahydro-1,4-benzodioxines produced by Phyllosticta cirsii, and oxysporone (3), a dihydrofuropyranone recently isolated from a strain of Diplodia africana, has been assigned by computational analysis of their optical rotatory dispersion (ORD), electronic circular dichroism (ECD), and vibrational circular dichroism (VCD) spectra. Computational prediction of ORD, ECD, and VCD allowed us to assign (3S,4aR,8S,8aR) AC to naturally occurring (-)-1, while (4aR,8S,8aR) AC was assigned to (-)-2 employing only ECD and VCD, because in this case ORD analysis turned out to be unsuitable for AC assignment. Theoretical prediction of both ORD and ECD spectra of 3 led to assignment of (4S,5R,6R) AC to (+)-3. In this case a satisfactory agreement between experimental and calculated VCD spectra was obtained only after taking into account solvent effects. This study shows that in the case of flexible and complex natural products only a concerted application of more than a single chiroptical technique permits unambiguous assignment of absolute configuration.

Circular dichroism and optical rotation of lactamide and 2-aminopropanol in aqueous solution

The performance of implicit and explicit solvent models (polarizable continuum model (PCM) and microsolvation with positions of water molecules obtained either from molecular dynamics (MD) simulations or quantum mechanical geometry optimization) for calculations of electronic circular dichroism (CD) and optical rotation (OR) is examined for two polar and flexible molecules: lactamide and 2-aminopropanol. The vibrational structure of the CD spectrum is modeled for lactamide. The results are compared with the newly obtained experimental data. The signs of the bands are correctly reproduced using all three methods under investigation and the CAM-B3LYP functional for the CD spectrum of lactamide, but not for 2-aminopropanol. The sign of the calculated optical rotation is correctly predicted by means of PCM, but its magnitude is somewhat underestimated in comparison with experiment for lactamide and overestimated for 2-aminopropanol. To some extent it is rectified by employing explicit hydration. Overall, microsolvation with geometry optimization seems more cost-effective than classical MD, but this is likely to be a consequence of inadequate classical potential and electronic structure model.

Control of the helical chirality of enantiopure sulfinyl (Z)-azobenzene-based photoswitches

A new class of enantiopure ortho,ortho-disubstituted azobenzene photoswitches has been synthesized from (S)-2-(p-tolylsulfinyl)benzoquinone and arylhydrazines. The sulfoxide acts as a unidirectional controller of the helical chirality that arises in the Z isomer after photoisomerization. Highly congested E-azobenzenes 5 c showed two atropisomeric diastereoconformers in the solid state that converged upon irradiation into a unique Z isomer with defined helicity (M), as evident in the X-ray structure. The chiroptical properties of this three-state enantiopure switch can be externally tuned both photochemically and/or thermally. Theoretical CD spectra calculated by using time-dependent DFT methods support the existence of two atropoisomeric E isomers and only one Z isomer with (M) helicity. Complementary to the classical azobenzene-based switches, the photoswiching event is promoted under green/blue light and do not occur under UV irradiation.

Chiral discrimination by vibrational spectroscopy utilizing local modes

Chiral discrimination of homochiral and heterochiral H-bonded complexes is a challenge for both experimentalists and computational chemists. It is demonstrated that a two-pronged approach based on far-infrared vibrational spectroscopy and the calculation of local mode frequencies facilitates the chiral discrimination of H-bonded dimers. The local H-bond stretching frequencies identify the strongest H-bonds and by this the dominating chiral diastereomer. This is shown in the case of peroxide, trioxide, hydrazine, glycidol, and butan-2-ol dimers as well as propylene oxide···glycidol complexes investigated with the help of second-order Møller-Plesset perturbation, coupled cluster, and density functional theory calculations where, in the latter case, the ωB97X-D functional was used for an improved description of H-bonding. In some cases, additional intermolecular interactions overrule the important role of H-bonding, which is found by calculating chirodiastaltic energies.

Nudicaulins, yellow flower pigments of Papaver nudicaule: revised constitution and assignment of absolute configuration

The constitution of the aglycon of nudicaulin has been revised to be a pentacyclic indole alkaloid. The relative and absolute configurations of the two diastereomers, nudicaulins I (3a) and II (3b), have been assigned by NMR, conformational analyses, and interpretation of the experimental ECD spectra by quantum-chemical calculations.

Synthesis and chiroptical properties of arylimines of cholest-4-ene-3,6-dione

In the hope of generating a new class of materials with large optical rotations, various arylamines were condensed with cholest-4-ene-3,6-dione (2) to give 3-arylimino steroids. These compounds possess moderately high specific rotations ([α](D) ~300-800) and strong circular dichroism. One such derivative, 3-(4-methoxyphenylimino)cholest-4-en-6-one (3), crystallizes as the (E)-imine and upon dissolution undergoes mutarotation to an equilibrium mixture of (E)- and (Z)-isomers with a half-life of approximately 1 h at room temperature, as judged by both NMR spectroscopy and polarimetry.

Chiral discrimination of amines by anisotropic NMR parameters using chiral polyacrylamide-based gels

A new chiral alignment medium for dimethyl sulfoxide, methanol, and water as solvents was developed. Because both enantiomers of the gel are available, it is possible to enantiodiscriminate natural products such as strychnine HCl that naturally occurs as single enantiomer. With the two methods of achieving anisotropy, namely stretching and confinement, the degree of alignment can be adjusted, and the director changed from horizontal to vertical. This increases the applicability. Three compounds were enantiodiscriminated on the basis of residual dipolar coupling data: mefloquine HCl, strychnine HCl, and menthylamine HCl.

Chiroptical, linear, and second-order nonlinear optical properties of binaphthol derivatives

We have investigated the chiroptical, linear, and second-order nonlinear optical (NLO) properties of seven binaphthol derivatives and elucidated structure-property relationships from the micromechanism for the first time. The excitation energies, oscillator strengths, and rotational strengths of the 150 lowest energy electron excitations for the most stable conformers have been calculated at TDB3LYP/cc-pVDZ level of theory. The experimental UV-vis absorption energies were reproduced well by our calculations. The simulated circular dichroism (CD) spectra and calculated optical rotation (OR) values are in reasonable agreement with experimental ones. These results demonstrate that TDDFT calculations can not only describe the electron transition property but also can be used to assign the absolute configurations (ACs) of binaphthol derivatives with high confidence. Whereas OR values are more sensitive to the molecular structures than CD spectra. The electron transition property and chiroptical origin have been assigned and analyzed. These derivatives possess remarkably large molecular first hyperpolarizabilities, especially compound 7 which has a value of 241.65 × 10(-30) esu. This value is about 60 times as large as that of highly π-delocalized phenyliminomethyl ferrocene complex. Moreover, compound 6 exhibits pronounced different second-order NLO response values from neutral state to the two cationic states (6(2+) and 6(4+)), which indicates that this compound could act as a potential NLO switch material. The cooperativity of intramolecular charge transfer of the studied compounds was also discussed.

Advantages of electronic circular dichroism detection for the stereochemical analysis and characterization of drugs and natural products by liquid chromatography

The need for analytical methods for the determination of the enantiomeric excess of chiral compounds increased significantly in the last decades, and enantioselective separation techniques resulted particularly efficient to this purpose. Moreover, when detection systems based on chiroptical properties (optical rotation or circular dichroism) are employed in high-performance liquid chromatography (HPLC), the stereochemistry of a chiral analyte can be fully determined. Indeed, the coupling of HPLC with chiroptical detection systems allows the simultaneous assessment of the absolute configuration of stereoisomers and the evaluation of the enantiomeric/diastereomeric excess of samples. These features are particularly important in the study of drugs and natural products provided with biological activity, because the assignment of their absolute stereochemistry is essential to establish reliable structure-activity relationships. The following review aims to discuss the analytical advantages arising from the employment of electronic circular dichroism (ECD) detection systems in stereochemical analysis by HPLC upon chiral and non-chiral stationary phases and their use for the stereochemical characterization of chiral drugs and natural compounds. The different methods for the correlation between absolute stereochemistry and chiroptical properties are critically discussed. Relevant HPLC applications of ECD detection systems are then reported, and their analytical advantages are highlighted. For instance, the importance of the concentration-independent anisotropy factor (g-factor; g=Δɛ/ɛ) for the determination of the stereoisomeric composition of samples upon non-chiral stationary phases is underlined, since its sensitivity makes ECD detection very well suited for the enantioselective analysis of large libraries of chiral compounds in relatively short times.

Chiroptical spectroscopy of natural products: avoiding the aggregation effects of chiral carboxylic acids

Determination of the absolute configurations and predominant conformations of chiral natural products, occurring as carboxylic acids, using chiroptical spectroscopic methods becomes challenging due to the formation of solute aggregates (in the form of dimers, etc.) and/or solute-solvent complexes resulting from intermolecular hydrogen bonding with solvent. A hypothesis that such aggregation effects can be avoided by using corresponding sodium salts or acid anhydrides for chiroptical spectroscopic measurements has been tested. For this purpose, vibrational circular dichroism, electronic circular dichroism, and optical rotatory dispersion spectra for disodium salts of two natural products, hibiscus acid and garcinia acid, and the anhydride of acetylated garcinia acid have been measured. These experimental spectra are analyzed in combination with quantum chemical calculations of corresponding spectra. The spectral analysis for sodium salts and anhydride turned out to be simpler, suggesting that the conversion of carboxylic acids to corresponding salts or anhydride can be advantageous for the application of chiroptical spectroscopy.

Density functional study of tetraphenylporphyrin long-range exciton coupling

The performance of time-dependent density functional theory (TDDFT) for calculations of long-range exciton circular dichroism (CD) is investigated. Tetraphenylporphyrin (TPP) is used as a representative of a class of strongly absorbing chromophores for which exciton CD with chromophore separations of 50 Å and even beyond has been observed experimentally. A dimer model for TPP is set up to reproduce long-range exciton CD previously observed for a brevetoxin derivative. The calculated CD intensity is consistent with TPP separations of over 40 Å. It is found that a hybrid functional with fully long-range corrected range-separated exchange performs best for full TDDFT calculations of the dimer. The range-separation parameter is optimally tuned for TPP, resulting in a good quality TPP absorption spectrum and small DFT delocalization error (measured by the curvature of the energy calculated as a function of fractional electron numbers). Calculated TDDFT data for the absorption spectra of TPP are also used as input for a 'matrix method' (MM) model of the exciton CD. For long-range exciton CD, comparison of MM spectra with full TDDFT CD spectra for the dimer shows that the matrix method is capable of producing very accurate results. A MM spectrum obtained from TPP absorption data calculated with the nonhybrid Becke88-Perdew86 (BP) functional is shown to match the experimental brevetoxin spectrum 'best', but for the wrong reasons.