Structures and absolute configurations of excoecoumarins A and B studied by experimental and theoretical NMR and circular dichroism spectra

Abstract

The structures and absolute configurations of two organic compounds, excoecoumarin A and excoecoumarin B, were studied by recording their electronic circular dichroism (ECD) spectra and performing quantum‐chemistry computations. The optical rotations and the nuclear magnetic resonance (NMR), ECD, vibrational circular dichroism (VCD), and ultraviolet (UV) spectra of candidate structures were computed using the B3LYP approach of the density functional theory. The structures and absolute configurations of excoecoumarins A and B are inferred from the agreement between the experimental and theoretical NMR, ECD, and UV spectra. Although a specific configuration can be assigned to excoecoumarin A, three structures are possible for excoecoumarin B. The computations indicated that the absolute configuration of excoecoumarin B can be further identified by acquiring the VCD spectra. In any case, the present study indicated that the structures reported previously should be revised.

Diterpenoids and Their Glycosides from the Stems of Tinospora crispa with Beta-Cell Protective Activity

“…All the quantum-chemistry computations were performed using the Gaussian 16 packages . The ECD spectra were simulated using the programs developed by Chang et al, in which thermal averaging was performed for the low-lying conformers …”