9,9′-epoxylignans from Syringa pinnatifolia: A typical case of stereochemical assignment by a quantum chemical calculation with MAEΔΔδ parameter.

In the current study, twenty-two stereochemical 9,9′-epoxylignans including 19 undescribed ones were isolated from the ethanol extract of Syringa pinnatifolia in our continuing effort to understand the overall chemical spectrum of this species. These isolates were structurally elucidated by extensive spectroscopic data analysis, X-ray diffraction, modified Mosher's method, and quantum chemical calculations. Meanwhile, the utilization of 13C NMR calculation and the MAE ΔΔ δ parameter facilitated the stereochemical assignment of groups of lignan stereoisomers. The 13C NMR data were corrected by the averaged errors at each corresponding carbon position in groups of lignan stereoisomers, which improved the theoretic 13C NMR calculation. The finding of the stereochemical structures of 9,9′-epoxylignans is significant. It is helpful to determine the absolute configurations of molecules with the similar core. In addition, these lignans exhibited potential cardioprotective activities on H9c2 cardiomyocytes in vitro and presented significant antioxidant effect. Nineteen undescribed 9,9′-epoxylignans and three known ones were isolated from the stems of Syringa pinnatifolia. The utilization of 13C NMR calculation and the MAE ΔΔ δ parameter facilitated their stereochemical assignments. [Display omitted] • 21 lignans including 19 undescribed ones were isolated from Syringa pinnatifolia. • Stereochemistry was assigned by NMR calculations with MAE ΔΔδ parameter. • Systematical errors were eliminated by the averaged errors. • These lignans exhibited cadioprotective and antioxidant effects. [ABSTRACT FROM AUTHOR]