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

Guided by 1 H 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).