Cardenolides and dihydro-β-agarofuran sesquiterpenes from the seeds of Salacia staudtiana

Abstract

Phytochemical studies of the seeds of the Cameroonian medicinal plant, Salacia staudtiana, resulted in the isolation and identification of five new cardenolides (1–5) as well as a new dihydro-β-agarofuran (9), along with eight known compounds. The structures of all compounds were elucidated by 1D/2D NMR, ESI-HRMS data and comparison with literature data. The relative configurations of the new compounds were defined by X-ray crystallography analysis, NOESY correlations and coupling constants. We evaluated their antibacterial efficacy against two commonly dispersed environmental strains of Escherichia coli and Bacillus subtilis, and two pathogenic strains of Staphylococcus aureus and Pseudomonas aeruginosa, compared to the standard antibiotics, streptomycin and gentamicin. Moreover, we assessed the antibacterial activity of the crude extract of the seeds in parallel to evaluate the plausible synergistic effects of the compounds in chemical defense of the seeds during germination and plant reproduction. The isolated compounds showed moderate antibacterial activities against the tested organisms. Compounds 1 and 3 and the crude extract exhibited distinct antibacterial activities against B. subtilis and S. aureus. The isolated compounds showed weak DPPH radical scavenging properties compared to the reference standard (Trolox). Our study lends evidence to the antibacterial chemical defense of S. staudtiana seeds by seed-borne compounds.

Introduction

Plants of the genus Salacia (Family: Celastraceae) are broadly dispersed around the world, mainly in the tropical and subtropical regions, and contain >200 species [1]. A survey of literature on Salacia species has shown that several types of triterpenes have been isolated earlier from this genus [[2], [3], [4], [5]]. Interestingly, plants in the family Celastraceae are a rich source of cardiac glycosides, particularly those belonging to the genera Elaeodendron [[6], [7], [8]] and Euonymus [9,10]. Some cardenolides from this family contain an unusual sugar linkage dioxane-type six-membered ring and a methylendioxy moiety, for example elaeodendroside A isolated from the seeds of Elaeodendron glaucum [11].

Recently, we reported a plethora of cardenolides from the stem bark of Salacia staudtiana, one of the species native to Cameroon [12]. As part of our ongoing search for bioactive metabolites from this medicinal plant, we report herein the isolation, structure elucidation, and antimicrobial and antioxidant activities of five new cardenolides (1–5), a new dihydro-β-agarofuran (9), as well as eight known compounds from the seeds of the plant. Their structures were determined by application of HRMS, 1D and 2D NMR techniques and X-ray crystallographic analysis. The antibacterial efficacies of the compounds and the seed crude extract were evaluated against the clinically important risk-group 2 (RG2) pathogenic bacteria Staphylococcus aureus and Pseudomonas aeruginosa, as well as two environmental strains of Escherichia coli and Bacillus subtilis. The DPPH radical scavenging activities of the compounds were further evaluated.