Synthesis of Polycyclic Ethers by Two-Directional Double Ring-Closing Metathesis

trans-Fused tricyclic ethers containing combinations of six-, seven-, eight-, and nine-membered rings are constructed using two-directional double ring-closing metathesis reactions (see scheme). Such double cyclizations of precursors bearing alkenes, allylic ethers, enol ethers, or alkynyl ethers offer a new strategy for the synthesis of brevetoxins and ciguatoxins. PMB=(p-MeOC(6)H(4)CH(2)).