Annelated furans. XI. The reactions of 2-vinylfurans with acetylenic esters

Abstract

In general dimethyl acetylenedicarboxylate and methyl propiolate have been found to react with 2-vinylfurans to give the corresponding dimethyl benzofuran-4,5-di-carboxylates and methyl benzofuran-4- carboxylates respectively. In several instances dimethyl acetylenedicarboxylate did not react exclusively with the exocyclic diene system of the 2-vinylfuran and here the corresponding dimethyl 3,6-epoxy-3-vinyl-3,6-dihydrophthalates were obtained by Diels-Alder addition to the furanoid diene system. At 80° 2-vinylfuran and dimethyl acetylenedicarboxylate formed the 1 : 2 adduct (11), the formation of which can be interpreted as involving an ene addition of a second mole of the acetylenic ester to the initially formed Diels-Alder adduct.