Isolation of Tetraprenyltoluquinols from the Brown Alga Sargassum thunbergii

Abstract

The brown alga Sargassum thunbergii is distributed widely in the coastal area of the Korean peninsula. Few data on secondary metabolites from this brown alga have been reported although it was chemically investigated. 1-3) Recently we reported the isolation and peroxynitrite-scavenging activities of three known compounds (1-3) from S. thunbergii. 4) In our continuous search for bioactive compounds from marine organisms, we re-encountered the brown alga S. thunbergii along the shore of Busan. The large-scale extraction of organic materials followed by bioactivity-guided separation using combined chromatographic techniques yielded tetraprenyltoluquinol derivatives, sargahydroquinoic acid (1), sargaquinoic acid (2), and sargachromenol (3), together with two additional metabolites of the same structural class as minor constituents. Herein we report the isolation and structure determination of two novel compounds, thunbergols A, B (4, 5) and their antioxidant activities.The structures of three known metabolites, sargahydroquinoic acid (1), 5) sargaquinoic acid (2), 6,7) and sargachromenol (3) 6,7) were readily determined by a combination of spectroscopic analysis and comparison with reported data for these compounds.A closely related metabolite, thunbergol A (4) was isolated as a colorless gum. The molecular formula for this compound was deduced as C 27 H 38 O 5 by HR-FAB-MS and 13 C-NMR analyses. Comparison of the spectral data with those obtained for compound 3 showed that 4 was also a tetraprenyltoluquinol analog of the same class as 3. However, there were significant differences in the 13 C-NMR spectrum. Signal of the olefinic carbons at d 122.8 and 130.5, typical of the C-3 double bond of the chromene ring moiety in 3 was shifted upfield to d 31.5 and 68.7. Corresponding differences were found in the 1 H-NMR spectrum in which the olefinic protons at d 6.21 (1H, d, Jϭ9.6 Hz) and 5.54 (1H, d, Jϭ9.6 Hz) were replaced by signals at d 2. 90 (1H, dd, Jϭ16.8, 4.8 Hz), 2.62 (1H, dd, Jϭ16.8, 4.8 Hz), and 3.73 (1H, t, Jϭ4.8 Hz). These differences could be explained by a hydration of the C-3 double bond of sargachromenol. A combination of 1 H-COSY, HMQC, and HMBC experiments fully supported this interpretation. In addition, the downfield shift of proton signal (d 3.73) at H-3 to d 5.03 (1H, t, Jϭ5.5 Hz) by acetylation of sodium carboxylate of 4 with acetic anhydride in pyridine which yielded the triacetate derivative (4a) also supported this interpretation. Thus, the structure of thunbergol A (4) was determined as 9-(3,4-dihydro-2,8-dimethyl-6-hydroxy-2H-1-benzopyran-2-yl)-6-methyl-2-(4-methyl-3-pentenyl)-(2E,6E)-nonadienoic acid.Another closely related metabolite, thunbergol B (5) was isolated as a colorless gum that was determined to have the same composition C 27 H 38 O 5 as 4 by HR-FAB-MS and 13 C-NMR spectrometry. The NMR spectral data for 5 were almost identical with those derived from 4. Careful examina- Thunbergols A (4) and B (5), tetraprenyltoluquinols, along with three known compounds (1-3) have been isolated f...

Chromanols from Sargassum siliquastrum and Their Antioxidant Activity in HT 1080 Cells

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Protective effect of chromene isolated from Sargassum horneri against UV‐A‐induced damage in skin dermal fibroblasts

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Secondary Metabolites and Biological Activity of Invasive Macroalgae of Southern Europe

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