Isolation and biomimetic synthesis of (+/-)-calliviminones A and B, two novel Diels-Alder adducts, from Callistemon viminalis

(+/-)-Calliviminones A (1) and B (2), two Diels-Alder adducts of polymethylated phloroglucinol and myrcene with unprecedented spiro-[5.5] undecene skeleton, were isolated from the fruits of Callistemon viminalis. Structural elucidation was accomplished by NMR spectra studies, and the biomimetic synthesis of compounds 1 and 2 confirmed the pivotal role of Diels-Alder reaction in the plausible biosynthetic pathway. Compounds 1 and 2 were also the first example of Carbon Diels-Alder adducts between phloroglucinol and terpene. Bioactivity scan indicated that I and 2 showed moderate inhibition on NO production on lipopolysaccharide-induced RAW264.7 macrophages. (C) 2014 Elsevier Ltd. All rights reserved.