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Acanthella cavernosa from three different dive site locations (Coral Gardens, Old Woman Island and Tani’s Reef) at Mooloolaba in South East Queensland Australia has been studied. A series of isocyanide and isothiocyanate metabolites, including axisonitrile-3 (22) and axisothiocyanate-3 (23) have been isolated from this sponge. The structures of all compounds were determined by GC/MS analysis and 1D and 2D NMR spectroscopy. A specimen of Acanthella cavernosa from Coral Gardens yielded 9-aristolene (66), the aromadendrane compound (69), the eudesmane compound (73), axisonitrile-3 (22) and 1-isocyanoaromadendrane (75). These compounds were identical to metabolites of this sponge previously reported from other Mooloolaba collections. Collections of Acanthella cavernosa from Tani’s Reef yielded a great diversity of sesquiterpene isocyanide and isothiocyanates metabolites including a rare isocyanate compound, axisocyanate-3 (95). Axisothiocyanate-2 (79) is reported for the first time from an Australian sample of Acanthella cavernosa. The novel 10-isothiocyanatoguai- 6-ene (78) was isolated and was fully characterised by a complete assignment using 1D and 2D NMR spectroscopy. The pair of metabolites, 7-isothiocyanato-7,8-dihydro-F- bisabolene (82) and 7-isocyano-7,8-dihydro-F-bisabolene (94), were reported for the first time in an Australian specimen of the marine sponge Acanthella cavernosa and were fully assigned by 1D and 2D NMR analysis. Furthermore, the literature NMR assignments for 1-isothiocyanatoaromadendrane (67) and 7-isocyano-7,8-dihydro-F- bisabolene (94) were revised using 2D NMR techniques. The structural relationship between the bisabolene NC/NCS pair was confirmed by conversion of the bisabolene isocyanide to the bisabolene isothiocyanate by reaction with sulphur. The presence of NCO functionality was confirmed by conversion of the isocyanate to its urea derivative. Sesquiterpene isocyanides were found to be the major metabolites from the Old Woman Island collection. The sponges also showed some chemical variations in their isothiocyanate metabolites.
The presence of axisonitrile-3 (22) and axisothiocyanate-3 (23) in the sponge Acanthella cavernosa has been investigated by comparison of the chemical constituents within samples across three different locations and within individual samples from the same location. The chemical association between nudibranchs and their sponge prey was investigated using Acanthella cavernosa and the nudibranch Phyllidia ocellata collected from the same location at Old Woman Island. Specimens of Phyllidia ocellata contained axisonitrile-3 (22), 1-isocyanoaromadendrane (75) and 10-isocyano-4- amorphene (36) in which compound 22 and 75 were also found in their food source.
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