| Summary: | A novel macrolactam named oxalactam A (<b>1</b>), three known dipeptides (<b>2</b>–<b>4</b>) as well as other known alkaloids (<b>5</b>–<b>7</b>) were obtained from the endophytic fungus <i>Penicillium oxalicum</i>, which was derived from the tuber of <i>Icacina trichantha</i> (Icacinaceae). All chemical structures were established based on spectroscopic data, chemical methods, ECD calculations, and <sup>13</sup>C-DP4+ analysis. Among them, oxalactam A (<b>1</b>) is a 16-membered polyenic macrolactam bearing a new skeleton of 2,9-dimethyl-azacyclohexadecane core and exhibited potent anti-<i>Rhizoctonia solani</i> activity with a MIC value of 10 μg/mL in vitro. The plausible biosynthetic pathway of <b>1</b> was also proposed via the alanyl protecting mechanism. Notably, three dipeptides (<b>2</b>–<b>4</b>) were first identified from the endophytic fungus <i>P. oxalicum</i> and the NMR data of cyclo(<i>L</i>-Trp-<i>L</i>-Glu) (<b>2</b>) was reported for the first time. In addition, the binding interactions between compound <b>1</b> and the sterol 14<i>α</i>-demethylase enzyme (CYP51) were studied by molecular docking and dynamics technologies, and the results revealed that the 16-membered polyenic macrolactam could be a promising CYP51 inhibitor to develop as a new anti-<i>Rhizoctonia solani</i> fungicide.
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