New Sesterterpenes from Marine Sponges from the Tropical Waters of the Kingdom of Tonga

Over the course of this study, various species of Tongan marine sponges were investigated using an NMR-based screening method and has resulted in the discovery of three new sesterterpenes and 11 known compounds. Examination of the sponge Fascaplysinopsis sp. resulted in the isolation of two novel sesterterpenes, isoluffariellolide (46) and 1-O-methylisoluffariellolide (47). Compounds 46 and 47 share the same backbone pattern as the known luffariellolide (45) and 25-Omethylluffariellolide (107) respectively, and differ only in the substitution pattern of the butenolide rings. Isoluffariellolide (46) was found to be approximately six times less cytotoxic than 1-O-methylisoluffariellolide (47). Interestingly, these results suggested that the 1-O-methyl group in compound 47 plays an important role in the cytotoxicity of the compound. Secothorectolide (49), a new ring-opened and geometric isomer of the known compound thorectolide (48), was obtained from a sponge of the order Dictyoceratida. This ring closure and opening relationship was also observed between manoalide (109) and secomanoalide (110), as well as luffariellins A (141) and B (142). Despite the different carbon skeleton, the functional groups in 141 and 142 are similar with those in 109 and 110, respectively, and not surprisingly the biological properties are almost identical. The biological activities of compounds 48 and 49 were almost the same, which would give an insight into the structure-activity relationship (SAR) between these types of compounds.