Molecular Networking‐Guided Discovery of a New Antitumor Pyranonaphthoquinone from Streptomyces tanashiensisDSM 731: Insights from Single‐Molecule Stretching Assays

Guided by molecular networking based on single-molecule stretching assay, an unprecedented pyranonaphthoquinone, methyl kalafunginate (1) and five known compounds 2-6 were isolated from Streptomyces tanashiensis DSM 731. Compound 1 was characterized through a combination of spectroscopic techniques, including 1D and 2D NMR analysis, ECD calculation, and X-ray crystallography. Interestingly, we discovered that compound 1 was spontaneously converted from kalafungin (4) in methanol solution. All isolated compounds, except for compound 3, were assessed for their cytotoxic potential against a panel of five human cancer cell lines: A549, HepG2, BxPC-3, SW620, and C4-2B. Compounds 1, 2, 4, and 5 exhibited remarkable cytotoxicity with IC