Isolation and characterisation of bioactive compounds from antidesma venosum E.Mey. ex Tul. and euphorbia cooperi N.E.Br. ex A.Berger.

Abstract

The need for the discovery of new pharmacologically active compounds is a global priority as new diseases and drug resistance mechanisms are emerging and spreading throughout the world. In the past natural products have played an important role in drug discovery. Plantderived natural products are important in the discovery of new drugs for treatment of infectious diseases and cancer. Many biologically active compounds with diverse structures have been isolated from the Euphorbiaceae family. In South Africa, Euphorbiaceae is the largest flowering plant family and some species within the family have not been investigated for unique secondary metabolites. However, many species have not been researched. This project aimed to identify bioactive compounds in South Africa Euphorbiaceae species. Two species within the Euphorbiaceae family, Euphorbia cooperi N.E.Br. ex A.Berger and Antidesma venosum E.Mey. ex Tul. were selected for investigation. The crude extracts were fractionated using column chromatography and centrifugal thin-layer chromatography to yield single compounds. A mixture of hexane, dichloromethane, chloroform, ethyl acetate and methanol were used as eluting solvents. The structures of the compounds isolated were elucidated by 1D and 2D NMR experiments (1H, 13C NMR, DEPT135, COSY, HSQC, HMBC and NOESY). The compounds isolated from A. venosum were identified as loliolide, β-sitosterol 3-O-β-Dglucoside, lutein, 4-hydroxyphenylethyl trans-ferulate as well as pheophytin A and pheophytin B. Although these compounds are known compounds, only pheophytin A has previously been identified in A. venosum. A phytochemical study of the aerial part of E. cooperi has led to the isolation of a novel norsesquiterpenoid named euphorbilactone and its glucoside, arachiside A, along with a triterpenoid, glutinol, a known phorbol ester, 16-angeloyloxy-13α-isobutanoyloxy-4β,9α,20- trihydroxytiglia-1,5-diene-3,7-dione, and a novel phorbol ester, 20-acetoxy-16-angeloyloxy13α-isobutanoyloxy-4β,9α,20-tetrahydroxytiglia-1,5-diene-3-one. This is the first report on the isolation of these compounds from E. cooperi. The crude extracts and loliolide, p-hydroxyphenylethyl trans-ferulate and β-sitosterol 3-O-βD-glucoside, were assayed for antimicrobial activities against one Gram-positive (Staphylococcus aureus ATCC 12600), two Gram-negative (Pseudomonas aeruginosa ATCC ii | P a g e 10145, Klebsiella pneumoniae ATCC 13883), and one drug-resistant Gram-negative (Escherichia coli ATCC 25218) bacteria. The crude extracts of the plants showed moderate but promising antimicrobial activity. The pure compounds did not show any antimicrobial activity. This investigation has shown that South African Euphorbiaceae contains novel bioactive compounds. The two usual phorbol esters isolated from E cooperi warrant further investigation of the bioactivity of these compounds.