The Structure-Directed Isolation of New Secondary Metabolites from South Pacific Marine Sponges

Abstract

This study describes the NMR-directed isolation and structure elucidation of nine new and several known compounds from New Zealand and Tongan marine sponges. An examination of the New Zealand marine sponge Hamigera tarangaensis resulted in the isolation of five new congeners of the hamigeran family (60, 62, 64–66) and a new 13-epi-neoverrucosane (67). The nature of the new hamigeran congeners in particular has allowed a diterpenoid biogenesis to be proposed that has been missing from the chemical literature. Of these compounds, hamigeran F (60) possesses an unusual fourth carbocycle which forms a cage-like structure, hamigeran H (64) is the only congener that retains its full complement of isoprenyl carbons, and hamigeran J (66) is a regioisomer of hamigeran A (52).

An improvement on NMR-based screening using the HMBC experiment is discussed. Semi-purified extracts from New Zealand and Tongan sponges were screened in this manner. From this, the known compounds plakinidines A and B were isolated from a collection of the Tongan sponge Plakortis quasiamphiaster. The isolation of guaiazulene (143) from an unidentified Tongan sponge is believed to be the first instance of its occurence from an organism of this kind. Examination of an undescribed Tongan marine sponge produced a new polyketide-derived γ-lactone (144). A spectroscopic investigation into the Tongan marine sponge Cacospongia mycofijiensis afforded several known, yet biologically interesting secondary metabolites. Among these, the re-isolation of zampanolide (194) resulted in the discovery of its microtubule-stabilising activity. Attempts to procure additional quantities of 194 also yielded two new latrunculin congeners (196 and 197). These two compounds feature substitution of a hydroxyl group at C-18 in place of the typical methine proton. A diastereomeric relationship is proposed for these compounds. Using the HMBC spectra of 66 sponges from New Zealand and Tonga, a rudimentary analysis was undertaken in an attempt into compare the secondary metabolite profile of temperate and subtropical sponges. The results show a definite bias towards Tongan sponges with respect to the amount of secondary metabolites they contain, but no discernible favour towards either location in regards to the diversity of their correlation data.