Abstract
Racemic new cyclohexenone and cyclopentenone derivatives, (±)-(4R*,5S*,6S*)-3-amino-4,5,6-trihydroxy-2-methoxy-5-methyl-2-cyclohexen-1-one (1) and (±)-(4S*,5S*)-2,4,5-trihydroxy-3-methoxy-4-methoxycarbonyl-5-methyl-2-cyclopenten-1-one (2), and two new xanthone derivatives 4-chloro-1,5-dihydroxy-3-hydroxymethyl-6-methoxycarbonyl-xanthen-9-one (3) and 2,8-dimethoxy-1,6-dimethoxycarbonyl-xanthen-9-one (4), along with one known compound, fischexanthone (5), were isolated from the culture of the mangrove endophytic fungus Alternaria sp. R6. The structures of these compounds were elucidated by analysis of their MS (Mass), one and two dimensional NMR (nuclear magnetic resonance) spectroscopic data. Compounds 1 and 2 exhibited potent ABTS [2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid)] scavenging activities with EC50 values of 8.19 ± 0.15 and 16.09 ± 0.01 μM, respectively. In comparison to Triadimefon, compounds 2 and 3 exhibited inhibitory activities against Fusarium graminearum with minimal inhibitory concentration (MIC) values of 215.52 and 107.14 μM, respectively, and compound 3 exhibited antifungal activity against Calletotrichum musae with MIC value of 214.29 μM.
Highlights
Marine microbes are recognized as an abundant source of biologically active natural products with structural diversity [1]
The 13C NMR spectrum (Table 1) revealed the presence of one α,β-unsaturated ketone at δC 185.62, 156.36 and 126.93, one methoxyl at δC 58.13, one methyl at δC 13.39, two oxygenated methines at δC 76.99 and 71.55, and one quarternary carbon bearing an oxygen atom at δC 77.97. These data presumed compound 1 to be a cyclohexenone with one amino, three hydroxyls, one methoxyl, and one methyl substituents
In the HMBC spectrum, the correlations from the proton at δH 5.94(4-OH) to the carbon at δc 156.36 (C-3), from the proton at δH 3.46 (2-OCH3) to the carbon at δc 126.93 (C-2), and from the proton at δH 4.22 (H-4) to the carbons at δc 126.93 (C-2) and 156.36 (C-3), suggested that the –OH at δH 5.94 and the –OCH3 at δH 3.46 were respectively attached to C-4 at δc 71.55 and C-2 at δc 126.93
Summary
Marine microbes are recognized as an abundant source of biologically active natural products with structural diversity [1]. (collection No R6), isolated from the root of Myoporum bontioides A Gray, showed both antifungal and ABTS [2,2′-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid)] radical scavenging activities. This prompted us to investigate the corresponding metabolites produced by the fungus. A large number of xanthones had shown inhibitory activities against different fungi, such as Fusarium oxysporum vasinfectum, Alternaria tenuis, Dreschlera oryzae [12], Aspergillus flavus [13], Gloeophyllum Candida krusei and Cryptococcus neoformans [14]. A few monocyclic cyclohexenone and cyclopentenone derivatives had been reported to have antifungal activities. Details of the isolation, structure elucidation, and the results of antifungal and ABTS radical scavenging activities of the isolated compounds are reported
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