Rogiolenyne D, the likely immediate precursor of rogiolenyne A and B, branched C15 acetogenins isolated from the red seaweed Laurencia microcladia of II Rogiolo. Conformation and absolute configuration in the whole series

It is shown that in the red seaweed Jaurencia microcladia, collected in the Mediterranean off the torrent Il Rogiolo, the new branched C15 acetogenin rogiolenyne D ( = (+)‐(2S,3S,7R))‐3‐(bromomethyl)‐7‐[(Z)‐1‐chlorohexen‐3‐en‐5‐nyyl)]‐2‐ethyl‐2,3,6,7‐tetrahydrooxepin; (+)‐−3 co‐occurs with the already reported rogiolenyne A ((−)‐1) and B ((−)‐2a, suggesting the lineage (+)‐3→(−)‐1(−)‐2a,Which is realized here chemically. The relative configurations are established via NMR analysis and chemical transformations as regards the seven‐membered ring, while recourse is made to conformational analysis for the side chain. The absolute configuration is established via the Mosher's NMR method applied to the MPTA esters of (−)‐2a.