Structure and synthesis of bioactive natural products.

Abstract

Selected South African medicinal plants were screened in vitro for the anti‐HIV activity using the HIV‐RT colorimetric assay and XTT cell viability assay. In the cell‐based assay the plant extracts screened exhibited no anti‐HIV activity and most plant extracts were not highly toxic, with a few exceptions. In the HIV‐RT assay extracts of Harpephyllum caffrum, Combretum kraussii, Plumbago spp., Berkheya speciosa, Polygala fruticosa, Vernonia glabra, Lippia javanica, Smilax anceps, and Vernonia spp showed inhibition greater than 50% at a concentration of 500 μg/mL. Combretum kraussii stem extract inhibited 70% of the HIV‐RT and the leaf extract inhibited 67%. Because of these results obtained for Combretum kraussii, the leaf extract was investigated further resulting in the isolation of three compounds, combretastatin B‐1, combretastatin B‐5, and combretastatin B‐1 2‐β‐D‐glucoside. These compounds were not investigated further because of the non‐activity observed on the cell‐based assay. The plant metabolites arzanol and lepidissipyrone were chosen for synthesis. Arzanol, a prenylated α‐pyrone‐phloroglucinol, isolated from Helichrysum italicum ssp. microphyllum, exhibits antioxidant, anti‐inflammatory and anti‐HIV activities. Despite failure to complete its total synthesis, its two precursors, 2‐(2‐ethyl‐1,3‐dioxolan‐2‐yl)propanal and ethyl 3‐ (3‐acetyl‐2,4,6‐tribenzoxyphenyl)propanoate were successfully synthesised. Lepidissipyrone, the α‐pyrone flavanone structurally similar to arzanol, was isolated from Helichrysum lepidissimum and from Helichrysum excisum. Both species are endemic to South Africa. The first total synthesis of lepidissipyrone was successfully achieved by a multicomponent Carba‐Betti strategy to couple 6‐ethyl‐4‐hydroxy‐5‐methyl‐α‐pyrone and 7‐tert‐butyldimethylsilyloxy‐5‐hydroxyflavanone. During the last step of the total synthesis of lepidissipyrone, helipyrone was also synthesised. Various structural analogues of α‐pyrone, i.e. 4‐hydroxy‐5,6‐dimethyl‐α‐pyrone and 4‐hydroxy‐5‐ methyl‐6‐propyl‐α‐pyrone and acylphloroglucinols, i.e. 2,4‐bis(tertbutyldimethylsilyl) phloroacetophenone, 2,4‐bis(tert‐butyldimethylsilyl)‐3‐ prenylphloroacetophenone, and 2,4‐bis(tert‐butyldimethylsilyl)‐ 3‐prenylisobutyrophenone, were synthesised. These analogues could be used to synthesise arzanol derivatives