Isolation, Structure, and Biological Activities of Long‐Chain Catechols of Plectranthus sylvestris (Labiatae)

Antioxidant activity guided fractionation of extracts of the aerial parts of the title plant and HPLC separation yielded a series of oxygenated long‐chain alkylcatechols. Their structures were inferred by spectroscopic methods and chemical transformations to be the novel 4‐[(2S,4R,6S)‐4‐(acetyloxy)tetrahydro‐6‐pentyl‐2H‐pyran‐2‐yl]benzene‐1,2diol (1a), 4‐[(2S,4R6S)‐tetrahydro‐4‐hydroxy‐6‐pentyl‐2H‐pyran‐2‐yl]benzene‐1,2‐diol (1b), 4‐[(3S,5S)‐5‐(acetyloxy)‐3‐hydroxydecyl]benzene‐1,2‐diol (2a), 4‐[(3S,5S)3‐(acetyloxy)‐5‐hydroxydecyl]benzene‐1,2‐diol (2b), (3S,13Z)‐1‐(3,4‐dihydroxyphenyl)‐3‐hydroxydocos‐13‐en‐5‐one (3a), (Z)‐1‐(3,4‐dihydroxyphenyl)docos‐13‐en‐5‐one (4), besides the known l‐(3,4‐dihydroxyphenyl)icosan‐5‐one (5). The absolute configurations of the optically active compounds which are structurally related to the [n]‐gingerols (6) and ‐diols (7) were established by the high‐field 1H‐NMR application of Mosher's method. All compounds are in vitro potent antioxidants, inhibiting the Fe2+‐catalysed autoxidation of linoleic acid in the same order of magnitude as the commercial antioxidant 2,6‐di(tert‐butyl)‐4‐methylphenol (BHT). The dose‐dependent inhibitory effects on soybean‐lipoxygenase are in the μmol range, that of the most effective compound (3a) in the nmol range, hence being significantly more potent than the Known anti‐inflammatory and analgesic drugs indomethacin and nordihydroguaiaretic acid.