Antimicrobial Properties of Plectranthus ornatus Extracts, 11-acetoxyhalima-5, 13-dien-15-oic Acid Metabolite and its Derivatives

An antimicrobial screening of six different extracts of Plectranthus ornatus, revealed anti-Gram positive bacteria activity for the acetone and methanol-water extracts. 11R*-Acetoxyhalima-5,13E-dien-15-oic acid (1), isolated from the acetone extract of P. ornatus, exhibited growth inhibitory activity against five Staphylococcus and five Enterococcus strains. [MIC values 15.62 μg/mL (43.15 μM) - 62.50 μg/mL (172.65 μM)]. Diterpenes with a halimane skeleton are rarely isolated and their pharmacologic properties remain unknown. To study the potential of 1 as antibacterial prototype some derivatives (2-10) were obtained from 1 and the majority were tested against the same ten Gram positive bacteria. The more active was the (11R*,13E)-halima-5,13-diene-11,15-diol (4) with MIC values of 15.62 μg/mL (51.05 μM) to 31.25 μg/mL (102.12 μM). The preparation and spectroscopic data of 2 - 10 are also reported.