Antimycobacterial metabolites from Plectranthus: Royleanone derivatives against Mycobacterium tuberculosis strains

The antimycobacterial activities of eight diterpenes, 1-8, isolated previously from Plectranthus and eleven esters, 9-19, of 7α-acetoxy- 6β,12-dihydroxyabieta-8,12-diene-11,14-dione (5) were evaluated against the MTB strains H₃₇Rv and MDR. Only diterpenoids with a quinone framework revealed anti-MTB activity. Abietane 5 and its 6,12-dibenzoyl, 12-methoxybenzoyl, 12-chlorobenzoyl, and 12-nitrobenzoyl esters, 9, 11, 12, and 13, respectively, showed potent activities against the MDR strain with MIC values between 3.12 and 0.39 μg/ml. Cytotoxic activities towards 3T3 and Vero cells were also evaluated. Compound 11, with the best selectivity index, may be a suitable lead for further chemical modifications. The complete structural elucidation of the new esters, 9-14, 16, 18, and 19, as well as the NMR data of known derivatives 15 and 17 are reported.