4 research outputs found
Similarity in Dissymmetry Factor Spectra: A Quantitative Measure of Comparison between Experimental and Predicted Vibrational Circular Dichroism
To quantitatively determine the agreement
between experimental
and calculated vibrational circular dichroism (VCD) spectra, a new
approach, based on the similarity of dissymmetry factor spectra has
been developed and implemented. This method, which places emphasis
on robust regions both in the experimental and in the calculated spectra,
has been tested with six chiral compounds of known absolute configurations,
namely, (<i>R</i>)-(+)-3-chloro-1-butyne, (3<i>R</i>)-(+)-methylcyclopentanone, (3<i>R</i>)-(+)-methylcyclohexanone,
(1<i>S</i>)-(−)-α-pinene, (1<i>R</i>)-(+)-camphor, and (<i>S</i>)-(+)-epichlorohydrin. The
criterion of maximum overlap among experimental and calculated dissymmetry
factor spectra is shown to have definite advantages over those using
maximum overlap among VCD or absorption spectra individually. The
new method provides a better assessment of the comparison between
experimental observations and quantum chemical VCD predictions and
improves the confidence in the assignment of absolute configurations
Determination of the Absolute Configurations Using Exciton Chirality Method for Vibrational Circular Dichroism: Right Answers for the Wrong Reasons?
Quantum chemical (QC) predictions
of vibrational circular dichroism
(VCD) spectra for the keto form of 3-benzoylcamphor and conformationally
flexible diacetates of spiroindicumide A and B are presented. The
exciton chirality (EC) model has been briefly reviewed, and a procedure
to evaluate the relevance of the EC model has been presented. The
QC results are compared with literature experimental VCD spectra as
well as with those obtained using the EC model for VCD. These comparisons
reveal that the EC contributions to bisignate VCD couplets associated
with the Cî—»O stretching vibrations of benzoylcamphor, spiroindicumide
A diacetate, and spiroindicumide B diacetate are only ∼30%,
∼3%, and ∼15%, respectively. With such meager EC contributions,
the correct absolute configurations (ACs) suggested in the literature
for spiroindicumide A diacetate and spiroindicumide B diacetate molecules
using the EC concepts can be considered fortuitous. The possibilities
for obtaining wrong AC predictions using the EC concepts for VCD are
identified, and guidelines for the future use of this model are presented
Atropoisomerism in Biflavones: The Absolute Configuration of (−)-Agathisflavone via Chiroptical Spectroscopy
The first natural occurrence in optically
active form of the dimeric
flavonoid agathisflavone and definition of its axial chirality using
chiroptical spectroscopic methods are described. The experimental
electronic circular dichroism, electronic dissymmetry factor, optical
rotatory dispersion, vibrational circular dichroism (VCD), and vibrational
dissymmetry factor spectra of agathisflavone are presented and analyzed
with their corresponding quantum chemical predictions to definitively
assign the axial chirality of (−)-agathisflavone as (a<i>S</i>)
Structure and Stereochemical Determination of Hypogeamicins from a Cave-Derived Actinomycete
Culture extracts from the cave-derived
actinomycete <i>Nonomuraea
specus</i> were investigated, resulting in the discovery of a
new S-bridged pyronaphthoquinone dimer and its monomeric progenitors
designated hypogeamicins A–D (<b>1</b>–<b>4</b>). The structures were elucidated using NMR spectroscopy, and the
relative stereochemistries of the pyrans were inferred using NOE and
comparison to previously reported compounds. Absolute stereochemistry
was determined using quantum chemical calculations of specific rotation
and vibrational and electronic circular dichroism spectra, after an
extensive conformational search and including solute–solvent
polarization effects, and comparing with the corresponding experimental
data for the monomeric congeners. Interestingly, the dimeric hypogeamicin
A (<b>1</b>) was found to be cytotoxic to the colon cancer derived
cell line TCT-1 at low micromolar ranges, but not bacteria, whereas
the monomeric precursors possessed antibiotic activity but no significant
TCT-1 cytotoxicity