MARINE TERPENES AND TERPENOIDS .14. ABSOLUTE-CONFIGURATION AND ACID-CATALYZED TRANSFORMATION OF (-)-12,13-DIDEHYDROFUROSPONGIN-1 ISOLATED FROM AN ARABIAN SEA SPONGE, FASCIOSPONGIA-CAVERNOSA SCHMIDT

Three furanoterpenes, one having a furanocyclohexane ring (1) and two methyl ethers (2 and 3), and 12,13-didehydrofurospongin-1 (4a) were isolated from the lipid extract of the sponge, Fasciospongia cavernosa SCHMIDT. The absolute configuration of 4a at C-11 was shown to be (S), opposite to that reported for furospongin-1 (5). Compound 4a was found to be quite labile in acidic media and was converted, at room temperature, to 1 (HClO4 in dioxane) or to a mixture of 1, 2 and 3 (HClO4 in MeOH). These results suggest that 1, 2 and 3 are at least partly artefacts formed from 4a during the isolation process.