ON THE 1ST MARINE STIGMASTANE STEROLS AND STEROLS HAVING A 24,25-DOUBLE BOND - ISOLATION FROM THE SPONGE STELLETTA SP OF DEEP CORAL SEA

The sponge Stelletta sp. (Astrophorida, Stellettidae), collected at - 700 m in the Coral Sea, is shown to contain sterones and sterols of the stigmastane type with a C(24) = C(25) bond for which there is no precedent in the sea. Structure elucidation of the second abundant of these steroids, stigmasta-4,24(25)-dien-3-one((+)-1), is based on 1D and 2D NMR spectra and chemical transformation to acetate (-)-5. Stigmasta-4,24(25)-diene-3,6-dione ((-)-3), present in trace amounts in the sponge, was obtained in sufficient quantity for NMR study by oxidation of the also present, inseparable, and abundant 4:1 mixture of stigmasta-5,24(25)-dien-3-beta-ol (6) and its 5,6-dihydro derivative 7 (Scheme 1). This oxidation also afforded the ketone analogues (+)-8 and (+)-9, which could be separated, thus making structure elucidation possible. The 6-beta-hydroxystigmasta-4,24(25)-dien-3-one ((+)-4), also present in trace amounts in the sponge, was obtained in sufficient amount for NMR study, together with its C(6) epimer (+)-11, by hydroperoxidation of (+)-8 followed by deoxygenation (Scheme 2). The last trace steroid of the sponge, stigmasta-4,6,24(25)-trien-3-one ((-)-2), was structurally elucidated using limited NMR data and comparison with the other stigmastanes. These stigmastanes, as the only steroids of this sponge, are likely to function as stabilizers of its cell walls; their phytosteroid structure, for a sponge which lives in the dark of deep waters, suggest origin through a complex food chain, possibly followed by bioelaboration in the sponge.