XESTOBERGSTEROL-C, A NEW PENTACYCLIC STEROID FROM THE OKINAWAN MARINESPONGE IRCINIA SP AND ABSOLUTE STEREOCHEMISTRY OF XESTOBERGSTEROL-A

A new pentacyclic steroid, xestobergsterol C [1], possessing a cis C/D ring junction, has been isolated together with two known compounds, x estobergsterols A [2] and B [3], from the Okinawan marine sponge Ircin ia sp., and the structure determined on the basis of spectral data. Re examination of the nmr data of xestobergsterols A [2] and B [3] result ed in revision of the configuration at C-23 and of the conformation of ring D in 2 and 3. The absolute stereochemistry of xestobergsterol A [2] was established by the cd exciton chirality method.