Lewis Acid-Catalyzed [3+4] Annulation of 2-(Heteroaryl)- cyclopropane-1,1-dicarboxylates with Cyclopentadiene

A novel Lewis acid-catalyzed [3+4] annulation of 2-(heteroaryl)cyclopropane-1,1-dicarboxylates with cyclopentadiene is reported. This reaction proceeds via an electrophilic attack of the Lewis acid-activated donor-acceptor cyclopropane onto cyclopentadiene followed by Friedel–Crafts intramolecular alkylation of the heteroarene substituent. This is the first general example of reactions of donor-acceptor cyclopropanes wherein the donor substituent serves as a nucleophile. The described annulation represents a convenient approach to bicyclo[3.2.1]octa-2,6-dienes with heteroarenes annulated to C(2)-C(3) bond. Its efficiency was demonstrated for a series of furyl, thienyl, pyrrolyl, benzofuryl, benzothienyl, and indolyl substituted cyclopropanes. Additionally, in the case of 2-(5-methyl-2-furyl)cyclopropane-1,1-diester we observed the predominant formation of product of the [3+4] annulation or the tetracyclic 5,8-methanocyclopenta[a]azulene derivative, depending on the reaction conditions.