TDDFT-ECD and DFT-NMR studies of thaigranatins A-E and granatumin L isolated fromXylocarpus granatum

Abstract

TDDFT-ECD calculations were utilized to explain the mirror image or different ECD spectra of previously reported homochiral natural products thaigranatins A-E and granatumin L, the simple comparison of which would result in a wrong stereochemical conclusion. The configurational assignment was confirmed independently and geometrical parameters of the chromophores governing the ECD spectra were identified in the structurally related natural products by analyzing the ECD spectra and geometries of the low-energy computed conformers obtained by different methods. Different conformations of the furan-2-yl-δ-lactone subunit were found responsible for the mirror image ECD spectra of the homochiral thaigranatins C-E. Two DFT13C NMR chemical shift calculation methods and DP4+ analysis were performed on the C-6 epimers of thaigranatin D, which together with the ECD calculation, could determine the absolute configuration of C-6 as (R).