Involucratustones A–C: Unprecedented Sesquiterpene Dimers Containing Multiple Contiguous Quaternary Carbons from Stahlianthus involucratus

Involucratustones A–C (1–3), three cadinane dimers containing multiple contiguous quaternary carbons, were isolated from the rhizomes of Stahlianthus involucratus. Their structures were determined by a combination of NMR spectroscopy, chemical conversion, and X-ray diffraction analysis. Compounds 1 and 2 are rearranged homodimers of cadinane sesquiterpene fused with a unique fully substituted 1-oxaspiro[4.4]nonane core observed for the first time in natural products, and 3 is a novel 3′,4′-seco-cadinane-dimer. Compounds 1 and 2 exhibited potent cytotoxic activities, and 3 showed notable anti-inflammatory effect.