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Nortriketones: Antimicrobial Trimethylated Acylphloroglucinols from Ma̅nuka (Leptospermum scoparium)

Killeen, Daniel P., Larsen, Lesley, Dayan, Franck E., Gordon, Keith C., Perry, Nigel B., van Klink, John W.
Journal of natural products 2016 v.79 no.3 pp. 564-569
4-hydroxyphenylpyruvate dioxygenase, Gram-positive bacteria, Leptospermum scoparium, Myrica gale, Myrtus communis, Raman spectroscopy, antibacterial properties, herbicides, ketones, leaves
Four trimethylated acylphloroglucinols (5–8) have been isolated from ma̅nuka (Leptospermum scoparium) foliage. Apart from myrigalone A (8), which has previously been isolated from European bog myrtle (Myrica gale), these compounds have not been characterized before. The nortriketones are structurally similar to the bioactive tetramethylated β-triketones from ma̅nuka, but have one less ring methyl group. Two oxidized trimethylated compounds, 9 and 10, were also isolated, but these are likely isolation artifacts. When evaluated for antibacterial activity against Gram-positive bacteria, myrigalone A (8) was slightly less potent (MIC 64 μg/mL) than the corresponding tetramethylated compound, grandiflorone (4) (MIC 16–32 μg/mL). Unlike their tetramethylated analogues, the nortriketones were inactive against the herbicide target enzyme p-hydroxyphenylpyruvate dioxygenase. The Raman spectra of leaf oil glands in different ma̅nuka varieties can be used to distinguish plants that contain nortriketones from those that accumulate triketones.