Flexibanone, the first cembranoid possessing an -methylene-8-lactone linking with a tetrahydronfuran ring by C-3/C-4 single bond, from the soft coral Sinularia flexibilis

A new cembranoid, flexibanone (1), and a known related diterpene sinulariolone (2) were isolated from a Taiwanese soft coral Sinularia flexibilis. Compound 1 is the first cembranoid possessing an alpha-methylene-8-lactone linking with a tetrahydronfuran ring by C-3/C-4 single bond. The structures of 1 and 2 were established by using extensive spectroscopic analysis, including 2D NMR methods. Both compounds were suggested to be biosynthesized from the same cembranoid precursor. Further, the cytotoxicity against human oral cancer Ca9-22, breast adenocarcinoma MDA-MB-231 and non-small cell lung cancer A549 cell lines of 1 and 2 were also assayed.