BASE-CATALYZED ISOMERIZATION OF ALKYNYLOXIRANES - A GENERAL-SYNTHESIS OF FURANS

Alkynyloxiranes 5, available through coupling of vinylic halides with terminal alkynes followed by epoxidation with-MCPBA, are isomerized to furans 6 upon treatment with KO-t-Bu in t-BuOH-18-crown-6. The reaction has been employed for the synthesis of furans with substituents at the 2,2,4, 2,5, and 2,3,5 positions. A pathway involving initial 1,4-elimination to a cumulenyl alkoxide B, which then cyclizes to a vinylic anion C, is proposed. Support for the proposed pathway includes deuterium incorporation when t-BuOD is employed as the solvent and isolation of vinylacetylene products when furan formation is structurally prevented.