Novel Antifungal Pyridine Alkaloids from Endophytic Fungus Penicillium citrinum VDL118

Three novel alkaloids, penicitrioids A-C (1-3), and two known compounds (4-5) were isolated from the ethyl acetate (EtOAc) extract of the solid fermentation of Penicillium citrinum VDL118, an endophytic fungus harbored in the leaves of Vaccinium dunalianum Wight (Ericaceae), a perennial evergreen shrub native to the southwestern regions of China, Myanmar, and Vietnam. Compounds 1 and 2 are novel pyridine alkaloids characterized by an unprecedented dihydrofuro[3,4-c]pyridine core, while 3 features a distinct pyrrolo[3,4-c]pyridine framework. Their structures were unambiguously established by comprehensive spectroscopic analysis and electronic circular dichroism (ECD) calculations. In vitro antifungal assays revealed that compounds 1-5 exhibited moderate to potent inhibitory effects against five tested phytopathogenic fungi, with minimum inhibitory concentrations (MICs) ranging from 3.1 to 100 μg/mL. Notably, four of them (1-4) displayed broad-spectrum and potent activity against Gloeophyllum trabeum, Coriolus versicolor, Fusarium solani, and Botrytis cinerea, with MIC values as low as 3.1-12.5 μg/mL. Furthermore, a plausible biosynthetic pathway for compounds 1-3 was proposed.