Chiroptical Studies on Brevianamide B: Vibrational and Electronic Circular Dichroism Confronted

Chiroptical spectroscopies, such
as electronic circular dichroism
(ECD) and vibrational circular dichroism (VCD), are highly sensitive
techniques to probe molecular conformation, configuration, solvation,
and aggregation. Here we report the application of these techniques
to study the fungal metabolite brevianamide B. Comparison of the experimental
ECD and VCD spectra with the density functional theory simulated counterparts
establishes that VCD is the more reliable technique to assign absolute
configuration due to the larger functional and dispersion dependence
of computed ECD spectra. Despite a low amount of available material
and a relatively unusual example of using VCD carbonyl multiplets,
the absolute configuration could be reliably predicted, strengthening
the case for application of VCD in the study of complex natural products.
Spectral and crystallographic evidence for or against the formation
of a dimeric aggregate is discussed; in solution, the VCD spectra
strongly suggest only monomeric species are present.