BiX3 and FeX3‐Promoted Prins Cyclization of Enol Ethers in CH2Cl2

Abstract The Prins cyclization of enol ethers has been realized by employing BiX 3 (or FeX 3 ) as catalyst and TMSX (X=Br, Cl) as the halogen source. The presence of a tiny amount of water in the solvent dichloromethane played a key role for the reaction to proceed. The reaction is believed to be catalyzed by Lewis acid‐assisted Brønsted acids, which were generated in situ from MX 3 and water in the solvent.