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.