Antimicrobial acylphloroglucinols and dibenzyloxy flavonoids from flowers of Helichrysum gymnocomum
Graphical abstract
Four flavonoids and two acylphloroglucinols from Helichrysum gymnocomum are described. Two of the flavonoids are new in Nature and one of acylphloroglucinols is reported for the first time. The six compounds have MIC values below 64 μg/ml against a selection of ten pathogens.
Introduction
Some 600 species of Helichrysum (Asteraceae) occur in Africa, of which some 244 species are found in South Africa (Pooley, 2003). The plants occur as herbs and shrubs. Often the leaves and flowers are pleasantly scented and may be burnt by the indigenous people to fumigate a sick room or to invoke the goodwill of the ancestors (Pooley, 2003). Mixed with fat, the plant produces a soothing ointment. Other applications include the use of various parts of the plant to treat coughs and colds and particularly to combat the infection of wounds (Pooley, 2003, Bremner and Meyer, 2000). Helichrysum gymnocomum, the subject of this investigation, is a perennial herb which grows profusely in the KwaZulu-Natal Drakensburg. The flowers are pleasantly scented. The above properties, coupled with its long flowering season (February–July) (Hilliard, 1997) make the plants an obvious choice for medicinal and ritual purposes.
The chemistry of a large number of Helichrysum species from diverse areas of South Africa has been studied at length by Bohlmann and Mahanta, 1979, Bohlmann and Abraham, 1979, Bohlmann and Hoffmann, 1979). More recent research on the genus has focused on the antimicrobial properties of the phloroglucinol derivatives in the genus (Meyer et al., 1997, Bremner and Meyer, 2000, Drewes et al., 2006).
On the whole researchers have utilized only the leaves, stems and roots of Helichrysum plants. Since flowers have long been known to be particularly rich sources of biologically active compounds (Swerdlow, 2000), this paper reports on the constituents in the flowers of H. gymnocomum. This decision was further reinforced by the findings of a recent paper (Appendino et al., 2007) in which the authors describe the isolation of arzanol, a phloroglucinol α-pyrone having anti-inflammatory and anti-HIV properties. Arzanol was extracted from the bright yellow flowerheads of Helichrysum italicum growing in Sardinia.
Section snippets
Determination of structure
From the yellow flowers of H. gymnocomum, three known compounds were isolated. These were 3-methoxyquercetin 4 (Bouktaib et al., 2002), the 4′-O-glucose derivative of 2′-hydroxy-6′-methoxy chalcone 5 (Wright, 1976), and the acylphloroglucinol derivative 3B, 3-[3′,3′-dimethylallyl-(1′)]-1-isobutyrylphloroglucinol (Bohlmann and Mahanta, 1979). Comparison of spectral data of our compounds with those published in the literature together with high resolution mass spectral analysis of 4, 5 and 3B
General
Melting points were determined on a Kofler hot-stage apparatus and are uncorrected. Mass spectra were recorded on a Kratos MS 80 RF double-focusing magnetic sector instrument at 70 eV. Optical rotations were measured on a Perkin Elmer 127 machine. Nuclear magnetic resonance spectra were recorded on a Varian 500 spectrometer operating at 499.98 for 1H and 125.73 MHz for 13C nuclei. Flash column chromatography was used for initial purification of fractions. Subsequent separations were accomplished
Acknowledgement
The authors wish to thank the University of KwaZulu-Natal Research fund for financial assistance.
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