Micranthanone A, a New Diterpene with an Unprecedented Carbon Skeleton from <i>Rhododendron micranthum</i>

A new diterpene with an unprecedented carbon skeleton, micranthanone A (<b>1</b>), two new grayanane diterpenoids bearing an unusual 5,6-(3,4-dihydroxylbenzylidene acetal) motif, rhodomicranols A (<b>2</b>) and B (<b>3</b>), and three known grayanane diterpenoids (<b>4</b>–<b>6</b>) were isolated from <i>Rhododendron micranthum</i>. Their structures were elucidated by spectroscopic analyses, calculated ECD, and single-crystal X-ray diffraction. The in vitro immunomodulatory activities of <b>1</b>–<b>6</b> were evaluated, and a plausible biogenetic pathway for <b>1</b> is proposed