Rearranged Phloroglucinol-Monoterpenoid Adducts from <i>Callistemon rigidus</i>

Callisretones A (<b>1</b>) and B (<b>2</b>), two rearranged phloroglucinol-monoterpenoid adducts featuring an unprecedented isopropylcyclopenta­[<i>b</i>]­benzofuran backbone, together with their postulated biosynthetic precursors (<b>3</b>–<b>9</b>), were isolated from <i>Callistemon rigidus</i>. The previously assigned absolute configurations of viminalins H (<b>7</b>), L (<b>8</b>), and N (<b>9</b>) were revised and unequivocally established by X-ray diffraction data. A putative biosynthetic pathway toward callisretones A and B involving the rearrangement of the terpenoid motif is proposed. In addition, <b>1</b> and <b>2</b> showed inhibitory effects on nitric oxide production with IC<sub>50</sub> values of 15.3 ± 1.0 and 17.7 ± 1.1 μM, respectively.