Rhizochalins C and D from the Sponge Rhizochalina incrustata. A Rare threo -Sphingolipid and a Facile Method for Determination of the Carbonyl Position in α,ω-Bifunctionalized Ketosphingolipids

Rhizochalins C and D ( 1, 2), new representatives of two-headed glycosphingolipids, were isolated from the sponge Rhizochalina incrustata. Rhizochalin D is an unexpected C 29 homologue of the canonical C 28 dimeric sphingolipid structures. Their structures including absolute configurations were established using spectroscopic data, micromolar-scale Baeyer-Villiger oxidation, and LCMS interpretation of the products. Application of the latter method leads to a revision of the structure of oceanapiside and placement of the keto group at C-18 rather than C-11.