Antimicrobial monomeric and dimeric diterpenes from the leaves of Helichrysum tenax var tenax

A dimeric diterpene ( 5) was isolated from the sticky leaves of Helichrysum tenax together with known diterpenes showing high antimicrobial activity. The hexane extract of fresh air-dried leaves of Helichrysum tenax (Asteraceae) afforded ent-beyer-15-en-19-ol ( 1), its 4-epimer ent-beyer-15-en-18-ol ( 2), 15β,16β-epoxide- ent-beyeran-19-ol ( 3), as well as ( 4) consisting of two units of ( 1) linked as a diester of malonic acid, and ( 5), a compound. Its constituents are ( 1) and ( 3) also linked as a diester of malonic acid. The leaves of the plant are densely covered in fine glandular trichomes. These are extremely sticky and exude a mixture of the above diterpenes. Antimicrobial tests showed that ( 1), in particular, was highly active (3.1 and 3.6 μg/ml) against Bacillus cereus and Staphylococcus epidermidis, respectively.