Lewis Acid Promoted Reactions of Ethenetricarboxylates with Allenes: Synthesis of Indenes and γ-Lactones via Conjugate Addition/Cyclization Reaction

Indenes are important core structures in organic chemistry. Few simple arylallenes have been used to construct indene skeletons by Friedel−Crafts reaction. Lewis acid catalyzed reaction of ethenetricarboxylates 1 and arylallenes has been examined in this study. The reaction of arylallenes and ethenetricarboxylate triesters with SnCl4 gave indene derivatives efficiently, via a conjugate addition/Friedel−Crafts cyclization reaction. On the other hand, the reactions of 1,1-diethyl 2-hydrogen ethenetricarboxylate and arylallenes or alkylallenes with SnCl4 at −78 °C or room temperature and subsequent treatment with Et3N gave γ-lactones. The reactions of triethyl ethenetricarboxylate and 1,1-dialkylallenes with SnCl4 at room temperature also gave γ-lactones.