Flavone C-glycosides from Vaccaria pyramidata: Structure elucidation by spectroscopy and theoretical calculations

The flavone C-glycosides namely apigenin-7-O-β-D-glucopyranosyl-8-C-β-D-glucopyranoside (1) and apigenin-8-C-α-L-rhamnopyranosyl-(1→2)-O-β-D-glucopyranoside (2), along with p-methoxycinnamic acid (3), vaccaroside A (4), vaccaroside D (5), vaccaroside E (6) and abscisic acid glucose ester (7) were isolated from the methanol extract of the aerial parts of Vaccaria pyramidata. Structure elucidation of compounds 1-7 was accomplished by the extensive use of 1D- and 2D NMR experiments along with HR-ESI-MS. The structures of 1-2 have been confirmed by theoretical study of the 1H, 13C chemical shifts and UV data. In addition, Ionization Potential (IP) and Bond Dissociation Enthalpy (BDE) were used to evaluate the theoretical antioxidant activity of compounds 1 and 2. TD-DFT method (time-dependent density functional) was used to calculate the electronic absorption spectra of compounds 1 and 2 from optimized structures in methanol and the orbitals involved in the electronic transition were elucidated.