A direct synthesis of 1,7-dioxaspiro[4.5]decanes from the new 3-methylidenepentane-1,5-dianion synthon

4-Phenylsulfanyl-2-(2-phenylsulfanylethyl)but-1-ene (2) has proved to be an appropriate and new 3-methylidenepentane-1,5-dianion synthon. The reaction of 2 with an excess of lithium powder and a catalytic amount of DTBB (2.5%) in the presence of a carbonyl compound in THF at 0 °C, leads, after hydrolysis, to the expected methylidenic diols 3. These diols undergo double intramolecular iodoetherification promoted by a silver salt, to furnish the corresponding 1,7-dioxaspiro[4.5]decanes (4) in very high yields. The oxidation of compounds 4 to the corresponding lactones is also studied.