Rhodomollanol A, a Highly Oxygenated Diterpenoid with a 5/7/5/5 Tetracyclic Carbon Skeleton from the Leaves of <i>Rhododendron molle</i>

A novel diterpenoid with an unprecedented carbon skeleton, rhodomollanol A (<b>1</b>), and a new grayanane diterpenoid, rhodomollein XXXI (<b>2</b>), were isolated from the leaves of <i>Rhododendron molle</i>. Their structures were elucidated using comprehensive spectroscopic methods and single-crystal X-ray diffraction. Compound <b>1</b> possesses a unique <i>cis</i>/<i>trans</i>/<i>trans</i>/<i>cis</i>/<i>cis</i>-fused 3/5/7/5/5/5 hexacyclic ring system featuring a rare 7-oxabicyclo[4.2.1]­nonane core decorated with three cyclopentane units. The plausible biosynthetic pathway for <b>1</b> was proposed. Compound <b>1</b> exhibited moderate PTP1B inhibitory activity.