The stereostructure of curdione (1) isolated from Curcuma wenyujin together with curcumol (2a) was determined as 1a on the basis of its direct X-ray crystallography and transformation to 2a. The preferred conformation of 1a in solution was found to be CC/syn (C (5)=O/C (1)-H) (3a) rather than CC/anti (3b) as in crystal based on 1H-NMR variable-temperature and NOE measurements, especially at -70°C. A possible biogenetic pathway from 1a through conformational change (3a to 3b) to 2a and the absolute configuration were proposed.
