Spectroscopic effects on natural occurring O-sulfated flavones from Wissadula periplocifolia

Abstract

Natural occurring O-sulfated flavonoids have been reported from the plant specie Wissadula periplocifolia Presl. (Malvaceae). Sulfated flavonoids are considered uncommon flavonoid derivatives and the biological relevance of sulfation on specialized metabolites are still poorly understood. In the recent years, the interest on O-sulfated flavonoids has aroused since they have shown anticoagulant, larvicidal and antiproliferative potential. Due the low occurrence of O-sulfated flavonoids the literature is scarce regarding specific isolation, identification and spectroscopic data. In this approach, the complete NMR data of a new bi-O-sulfated flavone isolated from W. periplocifolia are presented. Additionally, based on the obtained experimental and theoretical NMR data, the spectroscopic effects caused by the O-sulfate groups in natural occurring falvones were evaluated in order to better understand the additional effects on multiple O-sulfated compounds. The identified substance was purified from aerial parts of W. periplocifolia using Sephadex chromatographic column. The compound was identified by 1D and 2D NMR and HRMS as the unreported compound 4’-O-methyl-7,8-di-O-sulfate isoscutellarein (telescutellarein). The experimental NMR data was complemented by DFT calculations at the ωB97X-D3/def2-TZVP level of theory. The theoretical NMR spectra were simulated using the GIAO approach, and the observed chemical shifts due to the two O-sulfate groups were analyzed using chemical bond descriptors (QTAIM and OP/TOP) to study the bonds between the aromatic ring and their substituent groups. The anticipated electron-withdrawing and electron-donating effects of the O-sulfate and OH substituents on the aromatic ring were explored through a synergic experimental and theoretical analysis.