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.