Structural and stereochemical studies of C-21 terpenoids from Mediterranean Spongiidae sponges

The degraded C-21 sesterterpenoid (+)-3, enantiomeric with (-)-untenospongin B, has been isolated from the Mediterranean sponge Spongia virgultosa. The absolute stereochemistry of 3 was assigned by applying Mosher's method. On the basis of this work, the absolute stereochemistry at C-11 of nitenin (1) and dihydronitenin (2) has been reanalyzed by applying Mosher's method, whereas the R chirality at C-8 of 2 was determined by recording NOE spectra. The structures of two known C-21 furanoterpenes, tetradehydrofurospongin-1 (8) and 7, have been revised as (+)-3 (named ''revised tetradehydrofurospongin-1'') and its acetyl derivative 5, respectively. Finally, a comparison between the Mosher and Horeau 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 aliphatic-type alcohols.