Natural products from the Southern Great Barrier Reef

Marine organisms are a source of a diverse range of secondary metabolites. This thesis describes the isolation and structure elucidation of novel alkaloid and terpenoid metabolites from marine invertebrates which were collected from the Mackay - Capricorn Section of the Great Barrier Reef Marine Park (Keppel Bay and the Capricorn Bunker Group).

B-Carboline, its N,N symmetrical dimer and a series of novel asymmetrical dimers of B-carboline, were isolated from a didemnid ascidian (genus Didemnum). The asymmetrical dimers, however, were isolated in such low yield from the ascidian that their structures could not be conclusively determined. Derivatization of B-carboline allowed the preparation of these compounds in sufficient quantities to allow the elucidation of their structures by NMR spectroscopy. Two other new asymmetrical dimers of B-carboline, not observed in the ascidian, were also prepared and their structures elucidated.

Tetronic acids are commonly isolated from sponges of the genus Ircinia. A novel sesterterpene tetronic acid was isolated from the sponge Ircinia (= Psammocinia) wistarii. This novel compound (a sulfate ester) was highly unstable; rapid decomposition of the sulfate ester resulted in the formation of the known compounds ircinianin and wistarin. The isolation and structure elucidation of the novel sulfate ester is described in Chapter 3 of this thesis.

Alcyonolide 5, a novel diterpene triacetate, was isolated from two soft corals of the order Alcyonacea. This compound, is one of a series isolated from corals of the genera Alcyonium and Efflatounaria.

The bastadins are a series of (typically macrocyclic) tyrosine derivatives, commonly isolated from the sponge lanthella basta. During these investigations a new bastadin was isolated from lanthella quadrangulata.

The norcembrenolides, a series of norditerpenes, are commonly isolated from the soft corals of the genus Sinularia. The soft coral Sinularia numerosa was found to contain two of these compounds. One was the known compound norcembrenolide (the first representative of this class), the second was a stereoisomer of norcembrenolide. The chemical literature reports several compounds of this type, however, there appears to be a degree of confusion surrounding the structures of these stereoisomers. This thesis resolves the confusion surrounding the stereochemistry of these isomers, and assigns a structure to the minor metabolite of S. numerosa.

Approximately 200 marine invertebrates were collected during the course of these investigations. Many known compounds were isolated from the marine invertebrates that were collected from the Mackay - Capricorn Section of the Great Barrier Reef Marine Park. This afforded an opportunity to survey the natural products which may be found in this region, and highlights some of the difficulties associated with finding novel compounds within this region.