Brønsted acid-induced transannulation of the phytochemical zerumbone.

The sesquiterpene zerumbone was treated with HCl in ethyl acetate under the light-protected condition, and the time-dependent conversions were analyzed by gas chromatography. Nine products were isolated, and their structures were revealed by several NMR measurements such as H NMR, C{H} NMR, distortionless enhancement by polarization transfer (DEPT)-135, H-H correlation spectroscopy (COSY), H-C heteronuclear multiple quantum coherence (HMQC), and H-C heteronuclear multiple bond coherence (HMBC). The X-ray crystallography determined the stereochemistries of the three products and the two derivatives.