Bioactive metabolites from the endophytic fungus Alternaria alternata

Two altenuene derivatives (1-2) and one isocoumarin (3), together with six known compounds (4-9) were isolated from solid cultures of an endophytic fungus Alternaria alternata, obtained from the fresh branches of Camellia sinensis. Chiral analysis revealed the racemic nature of 1 and 2, which were subsequently resolved into two pairs of enantiomers [(+)-1 and (-)-1, (+)-2 and (-)-2]. Structures of all the isolates were identified through spectroscopic data. Absolute configurations of the two pairs of enantiomers were determined by electronic circular dichroism (ECD) calculation and the chiral center of C-10 in 3 was deduced via [Rh2(OCOCF₃)₄]-induced CD experiment. All the isolates were evaluated for their antimicrobial abilities against the pathogenic bacteria and fungi as well as cytotoxic activities against two human tumor cell lines. Compound 5 was the most active against Bacillus subtilis with MIC₈₀ of 8.6 μg/ml, and compounds 1-3, 6-7 and 9 exhibited moderate to weak inhibition towards the test pathogenic microorganism. Compound 4 showed mild cytotoxic activity against human osteosarcoma cells U2OS with IC₅₀ of 28.3 μM.