New bioactive altersolanol derivatives from the endophytic fungus Stemphylium globuliferum isolated from Mentha pulegium

Extracts of the fungus Stemphylium globuliferum, an endophyte of the Moroccan medicinal plant Mentha pulegium, exhibited considerable cytotoxic and antimicrobial activities when tested in vitro. Chemical investigation of the extracts yielded four new secondary metabolites, including altersolanol K (1), altersolanol L (2), altersolanol M (3), and altersolanol N (4), together with the known compounds 6-O-methylalaternin (5), macrosporin (6), altersolanol A (7), tetrahydroaltersolanol B (8), altersolanol C (9), altersolanol J (10), stemphypyrone (11) and indole-3-carboxaldehyde (12). The structures of all compounds were determined on the basis of one- and two-dimensional NMR spectroscopy and mass spectrometry. Among the altersolanol derivatives tested, compound (3) was the most active congener against L5178Y cell line with an EC₅₀ value of 1.14 µM. Moreover, all anthraquinones exhibited antibiotic activity against several pathogenic microbes. Interestingly, the new altersolanol N together with altersolanol C showed considerable antiviral activity against HRV39, whereas tetrahydroaltersolanol B was very active against HRV2, HRV8 and HRV16.