Investigation of bis(tributyltin)-initiated free radical cyclization reactions of 4-pentenyl iodoacetates

Bis(tributyltin)-initiated atom transfer cyclization reactions of 4-pentenyl iodoacetates 1) at 80 degreesC led to the formations of 5-(3-iodopropyl)-substituted dihydro-2(3H)-furanones 3) in high yield. With BF3.Et2O as the catalyst, the reactions were run at room temperature to afford the corresponding gamma-iodoheptanolactones (2), which could be further transformed into 3- tetrahydro-2-furyl)propanoic acids (6) upon treatment with aqueous NaHCO3. The reaction mechanism was postulated to be the 8-endo free radical cyclization to generate gamma-iodoheptanolactones which easily underwent intramolecular nucleophilic substitution to form bicyclic acylium species (7) as the key intermediate. Subsequent attack by iodide ion furnished gamma-lactones while attack by hydroxide ion gave the tetrahydrofuran derivatives.