Photochemical isomerisation studies of rubifolide and bipinnatin J. Unravelling some of the biosynthesis interrelationships between macrocyclic and polycyclic cembranoids found in corals

Irradiation of natural rubifolide (1) and bipinnatin J (2) for short periods of time using a 400 W sunlamp results in photochemical Z- to E-isomerisations to their corresponding E-isomers 10 and 16, respectively, neither of which has yet been discovered in Nature. Prolonged irradiation of rubifolide (1) produces the ring-contracted compound kallolide B (11a), and photooxidation of E-rubifolide (10) leads to epilophodione (12); both 11a and 12 are found in corals. The photoisomerisation studies provide an insight into the most likely origins of more complex cembranoids, for example, 4, 5, 18 and 19 from the archetypal cembranoids 1, 2, and 3 in vivo. The studies also suggest a likely route to the unusually rearranged cembranoid sarcofuranocembrenolide B (23) via a cascade of photo-initiated radical cyclisations and rearrangements from the biradical intermediate 24 produced from cembranoid 22.