Antimicrobial acylphloroglucinols and dibenzyloxy flavonoids from flowers of Helichrysum gymnocomum

From the dichloromethane extract of the flowers of Helichrysum gymnocomum (Asteraceae) two known flavonoids, 4 and 5, and a known acylphloroglucinol, 313, were isolated. In addition to 1 and 2, the 4',6'-dibenzyloxy-2'-hydroxy derivative of 2',4',6'-trihydroxychalcone and 5,7-dibenzyloxy derivative of pinocembrin, respectively, are reported in Nature for the first time. A compound 3A, related to 3B has the structure 2-methyl-1-[2,4,6-trihydroxy-3-(2-hydroxy-3-methyl-3-butenyl)phenyl]-1-propanone. Compounds 1, 2, 3A, 3113, 4 and 5 have MIC values below 64 mu g/ml against a selection of pathogens, with 3B having the highest sensitivity (6.3-45 mu g/ml) for eight of the ten pathogens tested, including Staphylococcus aureus (6.3 mu g/ml) and methicillin and gentamycin resistant strain of S. aureus (7.8 mu g/ml). With the exception of 2, the other compounds had notable activity (45-63 mu g/ml) towards Pseudomonas aeruginosa. (C) 2008 Elsevier Ltd. All rights reserved.