Three New Abietane-Type Diterpenoids from Plectranthus africanus and Their Antibacterial Activities

Three new abietane-type diterpenoids, plectranthroyleanones A-C (1-3), together with five known compounds (48) were isolated from the methanol extract of the whole plant of Plectranthus africanus using column chromatography techniques. The structures of the new compounds were elucidated using a combination of 1D and 2D NMR and HRESIMS analyses. Compound 1 exhibited weak activities with minimal inhibitory concentration values of 75 mu g/mL against gram-positive bacteria, Bacillus subtilis and Staphylococcus aureus, and 150 mu g/mL against two gram-negative bacteria, Pseudomonas aeruginosa and Klebsiella pneumoniae, respectively, while 2 and 3 had moderate antibacterial activity against K. pneumoniae with a minimal inhibitory concentration value of 37.5 mu g/mL.