Absolute Stereochemistry of Raspacionin, the Main Triterpenoid from the Marine Sponge Raspaciona aculeata

Raspacionin [1] is an unusual triterpenoid recently isolated from the red encrusting sponge Raspaciona aculeata and characterized by X-ray diffraction studies. The absolute stereochemistry of raspacionin [1] has been established by applying high field super(1)H-nmr to the Mosher method, as recently suggested by Kakisawa's group. The stereochemistry at C-4 of raspacionin was inverted to obtain a less hindered alcohol 4. After esterification with (R)- and (S)- alpha -methoxy- alpha -trifluoromethylphenylacetyl (MTPA) chloride, the MTPA esters were submitted to careful super(1)H- and super(13)C-nmr measurements which assigned the S absolute stereochemistry at C-4 of raspacionin.