Argyrotoxins A-C, a trisubstituted dihydroisobenzofuranone, a tetrasubstituted 2-hydroxyethylbenzamide and a tetrasubstitutedphenyl trisubstitutedbutyl ether produced by Alternaria argyroxiphii, the causal agent of leaf spot on African mahogany trees (Khaya senegalensis)

Abstract

Three previously undescribed metabolites named argyrotoxins A-C, were isolated, together with the well known porritoxinol, its closely related phthalide, a phthalide derivative, zinniol, alternariol and its 4-methyl ether from Alternaria argyroxiphii E.G. Simmons & Aragaki, the causal agent of leaf spot on African mahogany trees, Khaya senegalensis A. Juss. (Meliaceae). The known compounds were identified comparing their physical and spectroscopic properties to those previously reported in literature. Argyrotoxins A-C were characterized essentially by NMR (¹H, ¹³C, COSY, HSQC, HMBC and NOESY NMR spectra) and HRESIMS spectra as 4-(7-methoxy-6-methyl-3-oxo-1,3-dihydro-isobenzofuran-5-yloxy)-2-methyl-butyric acid, 5-but-2-enyloxy-N-(2-hydroxyethyl)-2-hydroxymethyl-3-methoxy-4-methyl-benzamide and 1-(5-(hydroxymethyl)-3-methoxy-4-(methoxymethyl)-2-methylphenoxy)-3-methylbutane-2,3-diol, respectively. The absolute configuration of argyrotoxin A was determined through electronic circular dichroism, by applying the biphenyl chiroptical probe approach. The phytoxicity of all metabolites isolated was evaluated by leaf puncture assay at concentration of 1 mg/mL. Zinniol proved to be the most active compound causing necrotic lesions on young leaves of Hedera elix L., Phaseolus vulgaris L. and Quercus ilex L. Argirotoxins A and B were found active, to a minor extent, on Phaseolus vulgaris L. leaves, while porritoxinol exhibited activity on holm oak leaves. The other secondary metabolites herein reported for A. argyroxiphii were inactive.

Keywords: African mahogany trees; Alternaria argyroxiphii; Benzamide and ethers; Biological acticity; Dihydrobenzofurans; Khaya senegalensis; Phytotoxins.