Talarergosteroids A-C: Three Unusual Steroid-Polyketone Conjugates with Antifungal Activity from a Kandelia Obovata Derived Fungus Talaromyces sp.

Three previously undescribed steroid-polyketone conjugates, talarergosteroids A-C (1-3), together with talarergosteroid D (4), which was first identified from a natural source, were isolated from a Kandelia Obovata derived fungus Talaromyces sp. SCNU-F0041. Compounds 1 and 2 bear a complicated 6/6/6/5/6/6 hexacyclic ring system characterized by an oxaspiro[5.5]undecane architecture. Compound 3 possesses a benzofuran moiety substituted at C-3 in ergosterol. The structures of the new compounds were identified by comprehensive spectroscopic analysis, X-ray diffraction, and electronic circular dichroism (ECD) calculation. Talarergosteroid B (2) showed significant inhibitory activity against the agricultural plant pathogen Fusarium oxysporum f. sp. lycopersici (MIC = 0.78 μg/mL), outperforming the positive control carbendazim (MIC = 1.56 μg/mL). Preliminary research disclosed that compound 2 may inhibit the spore germination progress, malform the fungal mycelium, and damage the organelle. These results indicate that compound 2 could be a potential fungicidal lead compound against Fusarium oxysporum f. sp. lycopersici.