A New ent-kaurene Diterpenoid Isolated from Leaves of Espeletia semiglobulata Cuatrec. and its Potential Antimicrobial Activity

The fraction of the neutral extraction of the leaves of Espeletia semiglobulata Cuatrec. was subjected to chromatographic separation, it yielded four ent-kaurene type diterpenoids: three known [ent-kaur-16-en-19-al (I), ent-kaur-18-nor-16-en-4-ol (III), ent-kaur-16-en-19-ol (IV)] and a new one elucidated as ent-kaur-3-acetoxy-15-ene (II), based on the physicochemical and spectroscopic data of FTIR, GC-MS, and 1D and 2D NMR. These compounds were subjected to antimicrobial bioassay studies. This new ent-kaurene showed a significant inhibition potential against the growth of gram negative bacterial strains [Escherichia coli (ATCC 25922): 8 mm, Klebsiella pneumoniae (ATCC 23357): 10 mm, Pseudomonas aeruginosa (ATCC 27853): 8 mm], it also showed inhibition against the growth of fungal strain (Candida krusei: 8 mm), at a 2 mg/mL concentration. The compounds (I), (III) and (IV) failed to show any significant results in the antimicrobial screening against five bacterial strains [Staphylococcus aureus (ATCC 25923), Enterococcus faecalis (ATCC 29212), Klebsiella pneumoniae (ATCC 23357), Escherichia coli (ATCC 25922) y Pseudomonas aeruginosa (ATCC 27853)] and one fungal strain [Candida krusei (ATCC 6558)]. These results reveal a remarkable natural structure-activity relationship of the ent-kaurene core regarding the C-3 position (A ring of perhydrophenanthrene unit), whose oxygenation or addition of a hydrogen bond acceptor or donor group, improves the antimicrobial activity.