Przewalskone: a cytotoxic adduct of a danshenol type terpenoid and an icetexane diterpenoid via hetero-Diels–Alder reaction from Salvia przewalskii

Przewalskone (1), an unprecedented adduct of two different terpenoid units via a hetero-Diels–Alder cycloaddition, was isolated from the roots of Salvia przewalskii. The structure and absolute configurations were elucidated by extensive analysis of NMR spectra and crystal X-ray diffractions. Compound 1 exhibited significant cytotoxicities against five human cancer lines in vitro (IC50 0.69–2.35 μM).