Tautomeric cinnamoylphloroglucinol-monoterpene adducts from Cleistocalyx operculatus and their antiviral activities

Guided by 1H NMR spectroscopic experiments using the characteristic enol proton signals as probes, three pairs of new tautomeric cinnamoylphloroglucinol-monoterpene adducts (1–3) were isolated from the buds of Cleistocalyx operculatus. Their structures with absolute configurations were established by spectroscopic analysis, modified Mosher's method, and quantum chemical electronic circular dichroism calculation. Compounds 1–3 represent a novel class of cinnamoylphloroglucinol-monoterpene adducts featuring an unusual C-4–C-1′ linkage between 2,2,4-trimethyl-cinnamyl-β-triketone and modified linear monoterpenoid motifs. Notably, compounds 1–3 exhibited significant in vitro antiviral activity against respiratory syncytial virus (RSV).