Callistrilones A and B, Triketone–Phloroglucinol–Monoterpene Hybrids with a New Skeleton from Callistemon rigidus

The first triketone–phloroglucinol–monoterpene hybrids, callistrilones A and B (1 and 2), along with a postulated biosynthetic intermediate (3) were isolated from the leaves of Callistemon rigidus. Compounds 1 and 2 featured a new carbon skeleton with an unprecedented [1]­benzofuro­[2,3-a]­xanthene or [1]­benzofuro­[3,2-b]­xanthene pentacyclic ring system composed of three kinds of building blocks. Their structures and absolute configurations were elucidated by spectroscopic analysis, X-ray diffraction, and electronic circular dichroism (ECD) calculations. A plausible biogenetic pathway for the new compounds is also proposed. Compound 1 exhibited moderate antibacterial activity against Gram-positive bacteria including multiresistant strains.