This program is directed towards the completion of the total synthesis of brevetoxin A and ciguatoxin. Both compounds are potent neurotoxins isolated from marine organisms. Their total syntheses are expected to be accompanied by discoveries in the area of mew synthetic technologies and strategies and may have implications in the neurosciences, the understanding and protection of the marine environment, and medicine. The key steps in the syntheses involve the application of novel methods for the construction of 6-, 7-, 8- and 9-membered cyclic ethers using thionoesters, thionolactones, hydroxyepoxides, hydroxyketones and hydroxydithioketals as precursors.