On the unusual propensity by the red seaweed Laurencia microcladia of II Rogiolo to form C₁₅ oxepanes: Isolation of rogioloxepane A, B, C, and their likely biogenetic acyclic precursor, prerogioloxepane

It is shown here that the red seaweed Laurencia microcladia KÜTZNING, collected in the Mediterranean off the torrent Il rogiolo, contains the C 15 acetogenin rogioloxepane A ( = (+)‐ (2 R, 7 R )‐2‐(1‐bromopropyl)‐7‐[( Z )‐1‐chlorohex‐3‐en‐5‐ynyl]‐2,3,6,7‐tetrahydrooxepin ; (+)‐ 3 ) besides its epoxy derivative rogioloxepane B ((+)‐ 4 ) and the rogioloxepane C ((+)‐ 5 ) derived from (+)‐ 4 by epoxide opening. Co‐occurring is an acyclic C 15 acetogenin, perrogioloxepane ( = (7 R ,4 Z ,9 Z ,12 Z )‐6‐chloropentadeca‐3,9,12‐trien‐1‐yn‐7‐ol; 9 ), which exists preferentially in a folded conformation as required to give, by intramolecular cyclization, rogioloxepane A ( 3TC ) in its favored twist‐chair form. Molecular‐mechanics calculations support this view and help also to assign the relative configurations at the side chains, whereas the configurations at the seven‐membered ring are established via NMR spectroscopy and chemical correlations. The absolute configuration is established by Mosher 's NMR method applied to the MTPA esters of (+)‐ 5 or 9 .