PINNATINS A-E - MARINE DITERPENES OF THE RARE GERSOLANE CLASS DERIVEDFROM A PHOTOCHEMICALLY INDUCED REARRANGEMENT OF A CONJUGATED 2,5-BRIDGED FURANOCEMBRANE PRECURSOR

A group of five highly functionalized polycyclic isubstituted-alpha,be ta-unsaturated-gamma-lactones (3-7) featuring a unique bicycle [11.1.0 ] carbon skeleton joined in a trans fashion have been isolated from a Caribbean gorgonian; Pseudopterogorgia bipinnata. These rare marine di terpenoids can be regarded as representatives of the uncommon gersolan e ring system. Two of the new metabolites, pinnatin A (3) and pinnatin B (4), showed significant differential antitumor activity in the Nati onal Cancer Institute's 60-cell-line tumor panel. The biogenesis of th e pivotal metabolite pinnatin A (3) appears to involve a photochemical ly allowed [(sigma)2(a) + (pi)2(a)] cycloaddition process of a conjuga ted 2,5-bridged furanocembrane precursor. Structural assignments were accomplished through extensive spectroscopic analysis including 2D NMR , accurate mass measurements (HREIMS), X-ray crystallography, and chem ical interconversions.