Chiral action at a distance: remote substituent effects on the optical activity of calyculins A and B

Configurational Study of an Aporphine Alkaloid from Annona purpurea

Purpureine (1), norpurpureine (2), and 3-hydroxyglaucine (4) were isolated from the leaves of Annona purpurea. A vibrational circular dichroism study for the absolute configuration determination of 1 provides evidence for the mutually dependent atropisomerism, local chirality of the sole stereogenic center, and the geometry of the tetrahedral nitrogen atom in aporphine alkaloids. The observed change in the optical rotation sign between 2 and its hydrochloride 3 might explain why this compound has been reported as dextrorotatory and levorotatory from the same botanical source. Furthermore, 1H and 13C NMR spectra of 1, 2 and 4 were fully assigned for the first time.

Antibacterial secondary metabolites from an endophytic fungus, Eupenicillium sp. LG41

Two new compounds containing the decalin moiety, eupenicinicols A and B (1 and 2), two new sirenin derivatives, eupenicisirenins A and B (3 and 4), and four known compounds, (2S)-butylitaconic acid (5), (2S)-hexylitaconic acid (6), xanthomegnin (7), and viridicatumtoxin (8), were isolated from an endophytic fungus, Eupenicillium sp. LG41, harbored in the roots of the Chinese medicinal plant Xanthium sibiricum. Their structures were confirmed through combined spectroscopic analysis (NMR and HRMS(n)), and their absolute configurations were deduced by ECD calculations or optical rotation data. Since the endophytic fungus was isolated from the roots, the antibacterial efficacies of the compounds 1-6 were investigated against Bacillus subtilis and Acinetobacter sp. BD4, which typically inhabit soil, as well as the clinically important Staphylococcus aureus and Escherichia coli. (2S)-Butylitaconic acid (5) and (2S)-hexylitaconic acid (6) exhibited pronounced efficacy against Acinetobacter sp., corroborating the notion that root-endophytes provide chemical defense to the host plants. Compound 2 was highly active against the clinically relevant S. aureus. By comparing 1 with 2, it was revealed that altering the substitution at C-11 could drastically increase the antibacterial efficacy of 1. Our study reveals plausible ecological roles of the endophyte and its potential pharmaceutical use as a source of antibacterial compounds.

Synthesis and in vitro evaluation of analogues of avocado-produced toxin (+)-(R)-persin in human breast cancer cells

A structure-activity study of several new synthetic analogues of the avocado-produced toxin persin has been conducted, with compounds being evaluated for their cytostatic and pro-apoptotic effects in human breast cancer cells. A 4-pyridinyl derivative demonstrated activity comparable to that of the natural product, suggesting future directions for exploration of structure-activity relationships.

Optical signatures of molecular dissymmetry: combining theory with experiments to address stereochemical puzzles

Modern chemistry emerged from the quest to describe the three-dimensional structure of molecules: van't Hoff's tetravalent carbon placed symmetry and dissymmetry at the heart of chemistry. In this Account, we explore how modern theory, synthesis, and spectroscopy can be used in concert to elucidate the symmetry and dissymmetry of molecules and their assemblies. Chiroptical spectroscopy, including optical rotatory dispersion (ORD), electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA), measures the response of dissymmetric structures to electromagnetic radiation. This response can in turn reveal the arrangement of atoms in space, but deciphering the molecular information encoded in chiroptical spectra requires an effective theoretical approach. Although important correlations between ECD and molecular stereochemistry have existed for some time, a battery of accurate new theoretical methods that link a much wider range of chiroptical spectroscopies to structure have emerged over the past decade. The promise of this field is considerable: theory and spectroscopy can assist in assigning the relative and absolute configurations of complex products, revealing the structure of noncovalent aggregates, defining metrics for molecular diversity based on polarization response, and designing chirally imprinted nanomaterials. The physical organic chemistry of chirality is fascinating in its own right: defining atomic and group contributions to optical rotation (OR) is now possible. Although the common expectation is that chiroptical response is determined solely by a chiral solute's electronic structure in a given environment, chiral imprinting effects on the surrounding medium and molecular assembly can, in fact, dominate the chiroptical signatures. The theoretical interpretation of chiroptical markers is challenging because the optical properties are subtle, resulting from the strong electric dipole and the weaker electric quadrupole and magnetic dipole perturbations by the electromagnetic field. Moreover, OR arises from a combination of nearly canceling contributions to the electronic response. Indeed, the challenge posed by the chiroptical properties delayed the advent of even qualitatively accurate descriptions for some chiroptical signatures until the past decade when, for example, prediction of the observed sign of experimental OR became accessible to theory. The computation of chiroptical signatures, in close coordination with synthesis and spectroscopy, provides a powerful framework to diagnose and interpret the dissymmetry of chemical structures and molecular assemblies. Chiroptical theory now produces new schemes to elucidate structure, to describe the specific molecular sources of chiroptical signatures, and to assist in our understanding of how dissymmetry is templated and propagated in the condensed phase.

Determination of the Absolute Configuration of Flexible Molecules byab InitioORD Calculations: A Case Study with Cytoxazones and Isocytoxazones

Ab initio calculations of the optical rotatory power of the natural cytokine modulator cytoxazone 1 and its trans-diastereomer 2, as well as the structural isomers cis-3 and trans-4 isocytoxazones, have been performed at four different wavelengths (589, 546, 435, and 405 nm) by Density Functional Theory. The calculation of ORD curves provides a reliable method for the assignment of absolute configuration of these conformationally flexible molecules. The absolute configurations of isocytoxazones has been established as (+)-(4R,5S)-cis-3 and (+)-(4S,5S)-trans-4.

Determination of absolute configurations of chiral molecules using ab initio time-dependent Density Functional Theory calculations of optical rotation: How reliable are absolute configurations obtained for molecules with small rotations?

The absolute configuration (AC) of a chiral molecule can be determined via calculation of its specific rotation. Currently, the latter is most accurately carried out using the TDDFT/GIAO methodology. Here we examine the reliability of this methodology in determining ACs of molecules with small specific rotations. We report TDDFT/GIAO B3LYP/aug-cc-pVDZ//B3LYP/6-31G* calculations of the sodium D line specific rotations, [alpha]D, of 65 conformationally rigid chiral molecules whose experimental [alpha]D values are small (<100). The RMS deviations, sigma, of calculated and experimental [alpha]D values is 28.9. The distribution of deviations is approximately Gaussian, i.e., random. For eight molecules, more than 10% of the set, the sign of the predicted [alpha]D is incorrect. In determining an AC of a rigid molecule from [alpha]D with 95% confidence, the calculated [alpha]D value must lie within +/-2sigma of the experimental [alpha]D for one, but not both, of the possible ACs. For the 65 molecules of this study +/-2sigma is 57.8. For conformationally flexible molecules, the error bar is +/- >57.8.

Ab Initio Calculation of Optical Rotatory Dispersion (ORD) Curves: A Simple and Reliable Approach to the Assignment of the Molecular Absolute Configuration

In this paper, both Hartree-Fock (HF) and density functional theory (DFT) methods have been used to make ab initio calculations of the optical rotatory power of selected molecules at several wavelengths; that is, part of the optical rotatory dispersion (ORD) curve has been predicted. This approach constitutes a new, simple, and reliable method to assign the molecular absolute configuration, at least for rigid molecules such as those studied in the present work. In fact, in this way, it is possible to overcome the difficulties connected to some relevant cases, in particular that of (-)-beta-pinene, for which even a very high-level (DFT/B3LYP/6-311++G(2d,2p)) calculation affords the wrong sign of the optical rotation at 633 nm. On the contrary, the predicted ORD curve, even using small basis sets, reproduces (below 400 nm) the experimental trend well, allowing for the correct configurational assignment. This result clearly shows that to have a reliable configurational assignment the comparison between experimental and predicted rotation values must be carried out at different wavelengths and not at a single frequency. The reason for this is that working at wavelengths approaching the absorption maximum the [alpha](lambda) values become larger and their prediction becomes more reliable. Coupling the use of an inexpensive instrument (a polarimeter working at a few wavelengths) with the use of a DFT-calculation package can also allow the experimental organic chemist to arrive, quickly and reliably, at the assignment of the molecular absolute configuration.

Assignment of the molecular absolute configuration through the ab initio Hartree-Fock calculation of the optical rotation: can the circular dichroism data help in reducing basis set requirements?

In this paper the calculation of the optical rotation (OR) of some rigid organic molecules, using the Hartree-Fock method with small (6-31G, DZP) basis sets, has been studied thoroughly to carefully evaluate the scope and limitations of this method, previously introduced by other authors. Calculations on test molecules (compounds 1-13) together with a careful analysis of their CD spectra allow a simple criterion for the reliability of this approach to be formulated: for unsaturated and/or aromatic (i.e., absorbing in the near-UV region) molecules, if the [alpha](D) is quantitatively determined by the lowest energy Cotton effect (at wavelengths >220 nm), then the HF/6-31G result is reliable. The usefulness of this method for the experimental organic chemist has been further demonstrated because the OR (sign and order of magnitude) of compounds 14-19 (i.e., large molecules having considerable interest in organic chemistry), which fulfill the above criterion and for which an extended basis set treatment is not feasible owing to their size, is correctly predicted.

Optical rotation: recent advances in determining the absolute configuration

The field of optical rotations is currently undergoing a renaissance, which is a direct result of the advances in quantum mechanics and the availability of faster desktop computers. In the last few years, the field of optical rotations has taken an important role for determining the three-dimensional molecular structure (absolute configuration and conformations) with confidence. The purpose of this review is to present the latest advances in this area so that practicing chemists can utilize them in their respective areas of research and development.