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.