Thyrsiferol and venustatriol are cytotoxic and antiviral natural products isolated from red algea belonging to the genus Laurencia. The research proposed herein has, as its overall objective, the synthesis of these compounds and the development of flexible routes to structurally modified analogues whose biological evaluation may lead to an increased understanding of the mechanism of action of thyrsiferol and venustatriol and, perhaps, to the development of clinically useful substances. The various synthetic approaches to these natural products have in common a construction of the left half which involves a novel sequence of iterative mercuri-cyclizations and oxidations. Early work has focused on the synthesis of a left half model compound. Potential routes to the right halves are also discussed.