STRUCTURAL AND STEREOCHEMICAL STUDIES OF C-21 TERPENOIDS FROM MEDITERRANEAN SPONGIIDAE SPONGES

The degraded C-21 sesterterpenoid (+)-3, enantiomeric with (-)-untenos pongin B, has been isolated from the Mediterranean sponge Spongia virg ultosa. The absolute stereochemistry of 3 was assigned by applying Mos her's method. On the basis of this work, the absolute stereochemistry at C-11 of nitenin (1) and dihydronitenin (2) has been reanalyzed by a pplying Mosher's method, whereas the R chirality at C-8 of 2 was deter mined by recording NOE spectra. The structures of two known C-21 furan oterpenes, tetradehydrofurospongin-1 (8) and 7, have been revised as ( +)-3 (named ''revised tetradehydrofurospongin-1'') and its acetyl deri vative 5, respectively. Finally, a comparison between the Mosher and H oreau methods was carried out, paying attention to the reliability of the stereochemical predictions obtained by the two approaches applied to compounds (+)-3, nitenin (1), and dihydronitenin (2), containing al iphatic-type alcohols.