Polyfluorinated oxetanes are prepared in high yields by an electrophilic [2+ 21 cycloaddition reaction between hexafluoroacetone and fluorinated ethylenes that is catalyzed by an anhydrous aluminum chlorofluoride Lewis acid. The reaction is regiospecific with hydrofluoroethylenes CHX= CFz (X= H, F, C1, Br), whereas halotrifluoroethylenes CFX= CFz (X= C1, Br) give nearly equal amounts of the isomeric oxetanes. Hexafluoropropylene ...