Chaetosemins A-E, new chromones isolated from an Ascomycete Chaetomium seminudum and their biological activities

Fifteen aromatic polyketide metabolites, including four new chromones, chaetosemins B-E (2-5), with compound 4 bearing a new skeleton, and two new natural products, chaetosemin A (1) and (+)-(S)-chaetoquadrin J (14), together with nine known ones (6-13, 15), were isolated from the organic extract of a solid fermented culture of an Ascomycete filamentous fungus, Chaetomium seminudum. The structures of 1-5 were determined by spectroscopic analysis, and the absolute configuration of chaetosemin A (1) was confirmed by X-ray crystallography, whereas the stereochemistry of 1-4 and 14 was assigned by comparison of their specific optical rotations with reported data for structurally related compounds. All compounds were isolated for the first time from Chaetomium seminudum. Compounds 1 and 2 consisted of L-cysteine and D-cysteine units, respectively. Most of them were evaluated for in vitro antifungal activities against some phytopathogenic fungi. Among them, chaetosemin B (2) was the most active against Magnaporthe oryzae and Gibberella saubinettii with MICs of 6.25 and 12.5 mM, respectively. In addition, chaetosemin C (3) showed antioxidant activity with 50.7% DPPH free radical scavenging activity at 50 mM, and chaetoquadrin J (14) displayed weak inhibition of soluble epoxide hydrolase (sEH).