Cleistocaltones A and B, Antiviral Phloroglucinol–Terpenoid Adducts from Cleistocalyx operculatus

Two novel phloroglucinol–terpenoid adducts ( 1 and 2 ), featuring a rare 2,2,4-trimethyl-cinnamyl-β-triketone unit, were isolated from the buds of Cleistocalyx operculatus . Their structures with absolute configurations were established by spectroscopic analyses, single-crystal X-ray diffraction, and quantum chemical calculations. Structurally, compound 1 represents a new carbon skeleton possessing a densely functionalized tricyclo[11.3.1.0 3;8 ]heptadecane bridged ring system with an unusual bridgehead enol. Compounds 1 and 2 exhibited significant in vitro antiviral activities against respiratory syncytial virus (RSV).