Glycolipids from sponges. 13. Clarhamnoside, the first rhamnosylated alpha-galactosylceramide from Agelas clathrodes. Improving spectral strategies for glycoconjugate structure determination.

Reinvestigation of the glycosphingolipid composition of the marine sponge Agelas clathrodes revealed the presence of a new tetraglycosylated alpha-galactoglycosphingolipid (1a), containing an unusual l-rhamnose unit in the sugar head. The structure of the new compound was elucidated using extensive 2D NMR studies. Because of the strong overlapping of the signals of the sugar protons in the (1)H spectrum, (13)C-coupled and (13)C-decoupled phase-sensitive HMQC spectra were used to study the multiplicity of the overlapping signals. In addition, the absolute configuration of sugars was determined using a simple and efficient, yet underutilized CD method.