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

Abstract

Callisretones A ( 1 ) and B ( 2 ), two rearranged phloroglucinol-monoterpenoid adducts featuring an unprecedented isopropylcyclopenta[ b ]benzofuran backbone, together with their postulated biosynthetic precursors ( 3 – 9 ), were isolated from Callistemon rigidus . The previously assigned absolute configurations of viminalins H ( 7 ), L ( 8 ), and N ( 9 ) 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, 1 and 2 showed inhibitory effects on nitric oxide production with IC 50 values of 15.3 ± 1.0 and 17.7 ± 1.1 μM, respectively.