*Article* **Cyclic Tetrapeptides with Synergistic Antifungal Activity from the Fungus** *Aspergillus westerdijkiae* **Using LC-MS/MS-Based Molecular Networking**

**Junjie Han 1,†, Hanying Wang 1,2,†, Rui Zhang <sup>1</sup> , Huanqin Dai 1,3, Baosong Chen <sup>1</sup> , Tao Wang <sup>1</sup> , Jingzu Sun <sup>1</sup> , Wenzhao Wang <sup>1</sup> , Fuhang Song <sup>4</sup> , Erwei Li <sup>5</sup> , Zhitang Lyu 2,\* and Hongwei Liu 1,3,\***


**Abstract:** Fungal natural products play a prominent role in the development of pharmaceuticalagents. Two new cyclic tetrapeptides (CTPs), westertide A (**1**) and B (**2**), with eight known compounds (**3**–**10**) were isolated from the fungus *Aspergillus westerdijkiae* guided by OSMAC (one strain-many compounds) and molecular networking strategies. The structures of new compounds were unambiguously determined by a combination of NMR and mass data analysis, and chemical methods. All of the isolates were evaluated for antimicrobial effects, synergistic antifungal activity, cytotoxic activity, and HDAC inhibitory activity. Compounds **1**–**2** showed synergistic antifungal activity against *Candida albicans* SC5314 with the presence of rapamycin and weak HDAC (histone deacetylase) inhibitory activity. These results indicate that molecular networking is an efficient approach for dereplication and identification of new CTPs. CTPs might be a good starting point for the development of synergistic antifungal agents.

**Keywords:** cyclic tetrapeptides; synergistic antifungal activity; molecular networking; *Aspergillus westerdijkiae*
