Silencing Tautomerization to Isolate Unstable Physalins from Physalis minima

The inherent structural instability of some physalins has hampered the isolation and identification of these compounds for approximately 50 years, and an effective method to overcome these challenges remains unavailable. In the present study, the unprecedented tautomerization mechanism of unstable physalins was elucidated by performing isotopic labeling experiments and DFT calculations, which led to the successful separation of tautomers and isolation of highly pure products for the first time. As a result, 15 new physalins, physaminins A-O (1-15), as well as 17 known analogues (16-32), were isolated from the whole plants of Physalis minima L. The chemical structures of the new compounds were established by performing a comprehensive analysis of spectroscopic data, and their absolute configurations were confirmed by using computational ECD calculations and/or single-crystal X-ray diffraction analyses. All obtained isolates were evaluated for their antiproliferative effects against four human cancer cell lines (A549, HepG2, MCF-7, and SCG-7901) and two noncancerous cell lines (RAW 264.7 and human normal hepatocytes L02), as well as their anti-inflammatory activities by measuring their abilities to inhibit NO production in LPS-stimulated murine RAW 264.7 cells in vitro. Compounds 1-5, 13, 16, 18, 19, 23, and 30 exerted significant antiproliferative effects on the four human cancer lines, with IC₅₀ values ranging from 0.2(0) to 24.7(2) μM, and these compounds were not toxic to the two noncancerous cell lines at a concentration of 10 μM. Moreover, compounds 7, 10, 11, 12, 14, 17, 22, and 27 significantly inhibited NO production, with IC₅₀ values ranging from 2.9(1) to 9.5(2) μM.