Diels–Alder reaction of α-tropolone and electron-deficient dienophiles prompted by Et 3N or silica gel: a new synthetic method of highly functionalized homobarrelenone derivatives

Abstract

A Diels–Alder reaction of α-tropolone and electron-deficient dienophiles prompted by Et 3N or silica gel was performed. Reaction with the highly reactive dienophile, N-methylmaleimide, proceeded smoothly in the presence of Et 3N or silica gel to yield adducts as a mixture of endo and exo isomers. Both catalysts accelerated endo/ exo isomerization of the product, and detailed examination of the reaction using hinokitiol and N-methylmaleimide revealed that isomerization proceeds via an intramolecular path without retro Diels–Alder reaction. Successful cycloaddition reactions were established with six other dienophiles: acrylonitrile, methyl acrylate, ethyl vinyl ketone, dimethyl fumalate, dimethyl malate, and dimethyl acetylenedicarboxylate, and the corresponding adducts were obtained in good to moderate yields.