New gamma-pyrone propionates from the Indian Ocean sacoglossan Placobranchus ocellatus.

A new gamma-pyrone propionate, compound 9, and its peroxy derivative 10 have been isolated from the sacoglossan Placobranchus ocellatus. The structure and the relative stereochemistry of the new molecules, which displayed an unprecedented carbon skeleton characterised by a bicyclo [4.2.0] octane, have been determined by both spectroscopic methods and comparison with model compounds. By analogy with photodeoxytridachione (2), a sunscreen protective role could be also suggested for compound 9 in living P. ocellatus.