Highly stereoselective syntheses of spiroacetal enol ethers, (E)- and (Z)-methoxycarbonylmethylene-1, 6-dioxaspiro[4.5]decanes

(E)- and (Z)-2-methoxycarbonylmethylene-1,6-dioxaspiro[4.5]decanes have been synthesized from an acyclic keto alcohol possessing an alkynoate part via intramolecular conjugate addition. Under thermodynamically controlled conditions using t-BuOK in THF, the (E)-isomer could be obtained in 52:1 ratio. When a catalytic amount of Pd(OAc)2 was used in benzene, the (Z)-isomer could be obtained in 95:1 ratio.