Cyclic dienol ether 5-(benzyloxy)-2,8-bi(methylene)-3,4,9,10-tetrahydro-2H,8H-benzo[1,2-b; 3,4-b']dipyran (3) has been synthesized by reacting the corresponding lactone with the Tebbe reagent. Compound 3 was very susceptible to hydrolysis and isomerization in the presence of a proton source. The isomerization of exocyclic enol ether 3 to endocyclic enol ether 4 was finished in 10 min, whereas long reaction time gave only the hydrolyzate and decomposi-tion product rather than compound 4.

Key words: isomerization, Tebbe reagent, synthesis, cyclic enol ether