Antibacterial serrulatane diterpenes from the Australian native plant Eremophila microtheca

Abstract

Chemical investigations of the aerial parts of the Australian plant Eremophila microtheca resulted in the isolation of three new serrulatane diterpenoids, 3-acetoxy-7,8-dihydroxyserrulat-14-en-19-oic acid (1), 3,7,8-trihydroxyserrulat-14-en-19-oic acid (2), and 3,19-diacetoxy-8-hydroxyserrulat-14-ene (3) as well as the previously reported compounds verbascoside (4) and jaceosidin (5). Acetylation and methylation of the major serrulatane diterpenoid (2) afforded 3,8-diacetoxy-7-hydroxyserrulat-14-en-19-oic acid (6) and 3,7,8-trihydroxyserrulat-14-en-methyl-19-benzoate (7), respectively. The antibacterial activity of 1-7 was assessed against a panel of Gram-positive and Gram-negative bacterial isolates. All the serrulatane compounds exhibited moderate activity against Streptococcus pyogenes (ATCC 12344) with minimum inhibitory concentrations (MICs) ranging from 64-128 姯mL. Serrulatane 1 demonstrated activity against all Gram-positive bacterial strains (MICs 64-128 姯mL) except for Enterococcus faecalis and Enterococcus faecium. This is the first report of natural products from Eremophila microtheca.