Base-Catalyzed Intramolecular 5-exo-dig Cyclization of 2-Propynyl-1,3-dicarbonyl Compounds: An Atom-Economic Route to Stereodefined 2-Methylene-2,3-dihydrofurans

A new and straightforward base‐catalyzed 5‐exo‐trig cyclization of 2‐propynyl‐1,3‐dicarbonyl compounds leads to the corresponding stereodefined 2‐methylene‐2,3‐dihydrofurans in excellent yields. A regio‐ and stereoselective 5‐exo‐dig cyclization of 2‐propynyl‐1,3‐dicarbonyl compounds under metal‐free conditions has been developed. This reaction constitutes a new and straightforward method for the efficient synthesis of stereodefined 2‐methylene‐2,3‐dihydrofurans. Spiro‐dihydrofurans can also be prepared in excellent yields by this approach.