Callistiviminenes A-O: Diverse adducts of β-triketone and sesqui- or monoterpene from the fruits of Callistemon viminalis

Abstract

Callistiviminenes A-O, fifteen adducts of β-triketone with sesqui- or monoterpene, along with a known compound, were isolated from the fruits of Callistemon viminalis. Callistiviminenes A and B are β-triketone-coupled (-)-bicyclosequiphellandrene contained an oxaspiro[5.5]undecene unit, and callistiviminenes C-E are rare adducts of β-triketone and bicyclogermacrene. Their structures and absolute configurations were elucidated by spectroscopic and computational methods, as well as by single-crystal X-ray diffraction experiment. A plausible biosynthetic pathway to these compounds involves a hetero-Diels-Alder reaction. Callistiviminenes C and D exhibited inhibition against lipopolysaccharide-induced nitric oxide production in RAW 264.7 macrophages with IC₅₀ value of 20.3 μM and 32.5 μM, respectively.

Keywords: Callistemon viminalis; Hetero-Diels–Alder; Myrtaceae; β-triketone.