A new xanthone from Penicillium oxalicum

Abstract

Penicillium oxalicum is a ubiquitous toxigenic fungus found in soil and musty cereal. Previous investigations of this fungus led to the isolation of several biologically active and structurally novel secondary metabolites, such as secalonic acid D [1], oxalicine [2], as well as ergosterol peroxide [3]. In our recent study, a new xanthone, 6,8-dihydroxy-3-methyl-9-oxo-9Hxanthene-1-carboxylic acid (1), together with endocrocin (2) [4], ergochrome EE (3) [1], 5-hydroxy-2-methoxybenzoic acid (4) [5], and 2-phenylacetic acid (5) [6], has been isolated from the EtOAc extract of Penicillium oxalicum. Their structures were determined on the basis of physicochemical and spectrometric data. Compound 1 was obtained as a yellow powder, with a molecular formula C15H10O6 determined by ESI-MS, m/z at 285 [M – H]–. This compound gave a dark green color with methanolic ferric chloride, indicating it to be a phenolic compound. Its UV (CH3OH) spectrum showed absorption bands at max 239, 255, 301, and 371 nm, indicating that 1 is a xanthone derivative. The IR spectrum displayed characteristic absorption bands for hydroxyl (3445 cm–1), and absorption bands for carbonyl (1645–1610 cm–1). The 1H NMR spectrum showed two couples of meta-positioned coupled aromatic doublets at 7.47 (1H, d, J = 1.2 Hz), 7.21 (1H, d, J = 1.2 Hz), and 6.39 (1H, d, J = 1.8 Hz), 6.22 (1H, d, J = 1.8 Hz), one methyl group at 2.44 (3H, s), one carboxylic hydroxyl group at 13.10 (1H br.s), one phenolic hydroxyl group at 11.09 (1H br.s), and one hydrogen-bonded hydroxyl group at 12.54 (1H, s) indicating the -position to the carbonyl function. The 13C NMR spectrum revealed a total of 15 carbon singals (Table 1). The presence of one methyl group, two carbonyl carbons, four oxygenated aromatic carbons, four protonated aromatic carbons, and four aromatic quaternary carbons was supported by HSQC spectral data.