Discovery of the Caged-Vibsane Norditerpenoids with Unprecedented Chemical Architectures and Exploration of Their Various Acid Tolerances

Cyclovibsanones A–D (1–4, respectively), featuring unprecedented caged tricyclo­[5.4.1.05,9]­dodecane and bicyclo[4.2.1]­hexane cores, were isolated from the leaves of Viburnum odoratissimum. Their structures as well as that of one chemical derivative (5), which was transformed from 2, were determined by spectroscopic data, theoretical calculations, and the ML-JDP4/MAEΔΔδ methods. In addition, compounds 1 and 2 were found to possess dissimilarities in acid tolerance during nuclear magnetic resonance (NMR) experiments. The potential mechanism was consequently postulated and further supported through NMR analysis and mechanistic calculations. Biologically, chemical derivative 5 exerted antiproliferative activity against HepG2 cells.