Semi-synthesis of new antimicrobial esters from the natural oleanolic and maslinic acids

In this article, we report an effective procedure for the selective isolation of oleanolic acid 1 and maslinic acid 2 (3.4 and 8.5 mg/g DW, respectively) from pomace olive (Olea europaea L.) using an ultrasonic bath, and the synthesis of a series of new triterpenic acid esters. The compounds were characterized by their spectral data and were evaluated for their antimicrobial activity. Among the compounds tested, those having sulfur and chlorine atoms were found to be antibacterial. They showed activity against two Gram-positive bacteria Staphylococcus aureus and Enterococcus faecalis and two Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa (MICs within a range of 5-25 μg/mL). The fungus Penicillium italicum was found to be the most sensitive to both sulfur derivatives: (3β)-3-((thiophene-2-carbonyl)oxy)-olean-12-en-28-oic acid (1a) (IZ = 22 mm) and (2α,3β-2,3-bis((thiophene-2-carbonyl)oxy)olean-12-en-28-oic acid (2a) (IZ = 24 mm).