ROGIOLENYNE-A, ROGIOLENYNE-B, AND ROGIOLENYNE-C - THE 1ST BRANCHED MARINE C-15 ACETOGENINS - ISOLATION FROM THE RED SEAWEED LAURENCIA-MICROCLADIA OR THE SPONGE SPONGIA-ZIMOCCA OF IL-ROGIOLO

It is shown here that the red seaweed Laurencia microcladia, collected off the torrent Il Rogiolo, south of Livorno, contains rogiolenyne A (= (-)-(1R*,2S*,3R*,5S*,7S*)-2-(bromomethyl)-5-[(Z)-1-chlorohex-3-en-5-ynyl]-3-ethyl-4,8-dioxabicyclo[5.1.0]octane; (-)-1) while the sponge Spongia zimocca, which grows in the same small area, contains rogiolenyne B (= (-)-(2R*,3R*,4R*,5R*,7S*)-3-(bromomethyl)-5-chloro-7-[(Z)-1-chlorohex-3-en-5-ynyl]-2-ethyloxepan-4-ol; (-)-4a) and its acetate, rogiolenyne C((-)-4b). These structures, which are based on extensive NMR and MS data and on chemical transformation, are the first examples of branched marine C-15 acetogenins. Biogenesis of (-)-1 in L. microcladia is thought to involve C(12) extrusion form a C-15 linear tetraen-1-yne precursor via H+-induced cyclopropane ring closure, followed by Br+-induced cyclopropane ring opening, aided by C-O- attack (Scheme 2). It is also proposed that transfer of (-)-1 to S. zimocca is followed by epoxide ring opening by Cl- to give (-)-4a and acetylation to give (-)-4b.