Syntheses of the [4+2] Adducts of 2-Pyrone-5-carboxylates with Substituted Ethylenes and Their Benzene Derivatives

Abstract

The Diels-Alder (DA) reactions of three kinds of 2-pyrone-5-carboxylates with acyclic vinyl ethers gave six stable endo-8-alkoxy-3-oso-2-oxabicyclo[2.2.2]oct-5-ene-6-carboxylates (major) 1D and six exo isomers [2]. The analogous reaction of methyl 2-pyrone-5carboxylate with cyclic vinyl ether, 2, 3-dihydrofuran, afforded methyl 9-oxo-3, 8-dioxatricyclo [5.2.2.0 2.6]undec-10-ene-11-carboxylate (endo adduct) [3] and its exo isomer [4]. On the other hand, reaction with fumaronitrile at high temperature gave methyl endo-7, exo-8dicyano-3-oxo-2-oxabicyclo[2.2.2]oct-5-ene-6-carboxylate [5], whereas reaction with tetracyanoethylene gave no adducts. As 2-pyrone-5-cfa. rbox ylates readily reacted with electron-rich olefins and poorly reacted with electron-deficient olefins with difficulty, these DA reactions seem to be of inverse electrondemand type. The reactivity of methyl 2-pyrone-5-carboxylate with ethylenes diminished in the order; acyclic vinyl ethers (ethyl vinyl ether >2-chloroethyl vinyl ether) >cyclic vinyl ether (2, 3-dihydrofuran). These facts suggest that steric influences and ionic effects are exerted at the reaction points. The DA adducts were hydrolyzed to give isophthalic acid derivatives [7] [8], [9] or 1, 3, 5-benzenetricarboxylic acid derivatives.