Structural revision of halipeptins: synthesis of the thiazoline unit and isolation of halipeptin C

The structural revision of the anti-inflammatory marine metabolites halipeptin A (1) and B (2) along with the isolation of the new related product halipeptin C (3) are reported. In particular, the heterocyclic portion of the molecule, incorrectly assigned as an oxazetidine ring, has now been characterised as a thiazoline unit by comparison of the spectral data of the natural products (1–3) with an appropriate synthetic model (10). GIAO calculated 13C NMR chemical shifts for oxazetidine and thiazoline model compounds provide additional support to the revised structure.