SYNTHESIS OF FURANS BY BASE-CATALYZED CYCLIZATION ISOMERIZATION OF BETA-ALKYNYL AND GAMMA-ALKYNYL ALLYLIC ALCOHOLS

Beta- and gamma-alkylyl allylic alcohols 3, 13, 26, available through Pd-mediated coupling of appropriate vinylic halides and terminal alkynes, cyclize and subsequently isomerize to furans 4, 17, and 32 upon treatment with KO-t-Bu in t-BuOH-THF at 25-60-degrees-C. The methodology has been used to prepare 2,3-, 2,4-, and 2,3,5-substituted furans. Reactions in t-BuOD as cosolvent lead to deuterium incorporation consistent with concurrent pathways in which direct 5-exo-dig or 5-endo-dig cyclization of the alkynyl allylic alcohol competes with prior isomerization to an allene intermediate which subsequently cyclizes by 5-exo- or 5-endo-dig pathways.