I2-catalyzed base-free cyclization of 3-homoallylquinoline-2-thiones: facile synthesis of tetracyclic, furothiopyrano[2,3-b]quinolines

I2-catalyzed base-free reactions of 3-homoallylquinoline-2-thiones have been described for the synthesis of tetracyclic quinolines, tetrahydrofuro [2′,4′:4,6]thiopyrano[2,3-b]quinolines in excellent yields. Similarly, I2-catalyzed reactions could proceed to tricyclic quinolines from hydroxyl group protected 3-homoallylquinoline-2-thiones. However, deprotection of group in tricyclic quinoline with HI again transformed into tetracyclic quinoline. The sulfonium salt intermediate has been proposed to explain these reactions.