The aims of this work were to study the antimicrobial activity of nine monoterpenes and the synergistic or antagonistic associations between them, and to relate water solubility, H-bonding and pKa values with antimicrobial activity. The minimum inhibitory concentrations (MICs) and minimum bactericidal concentration (MBCs) were determined. The MIC of carvacrol against S. aureus was 3.2 g/l and of thymol was 7.5 g/l. E. coli was resistant. Carvacrol and thymol were bactericidal. The associations geraniol/menthol against S. aureus and B. cereus and thymol/menthol against B. cereus were totally synergistic. Eugenol/geraniol displayed partial synergism against B. cereus. The other groups did not show any synergistic eff ect. Eugenol had the lowest pKa, followed by thymol and carvacrol. Eugenol had the highest total area and polar area and intermolecular and intramolecular hydrogen-bonding capacity, while carvacrol and thymol only had intermolecular hydrogenbonding capacity. The terpenes alone and in combination were eff ective against microorganisms. Phenolic compounds were the most active terpenes. Associations between terpenes were related to the chemical structure. Studies on the antimicrobial activity of associations of terpenes will advance the search for new alternatives for food preservation.

The new prenylated phloroglucinol α-pyrones 1-3 and the new dibenzofuran 4, together with the known 23-methyl-6-O-demethylauricepyrone (5), achyrofuran (6), and 5,7-dihydroxy-3,8-dimethoxyflavone (gnaphaliin A), were isolated from the aerial parts of Achyrocline satureioides. Their structures were determined by 1D and 2D NMR spectroscopic studies, while the absolute configuration of the sole stereogenic center of 1 was established by vibrational circular dichroism measurements in comparison to density functional theory calculated data. The same (S) absolute configuration of the α-methylbutyryl chain attached to the phloroglucinol nucleus was assumed for compounds 2-6 based on biogenetic considerations. Derivatives 7-16 were prepared from 1 and 5, and the antimicrobial activities of the isolated metabolites and some of the semisynthetic derivatives against a selected panel of Gram-positive and Gram-negative bacteria, as well as a set of yeast molds, were determined.