Sulfur-assisted ring contraction of polyfluoroalkylthiopyran derivatives as a route to functionalized fluorine-containing thiophenes

A synthetic route to functionalized alpha-(polyfluoroalkyl)thiophenes based on the reactions of ring contraction of polyfluoroallcyl substituted thiopyran derivatives has been explored. Potent 6-membered precursors for the preparation of thiophenes were obtained by the addition of bromine to the double bonds of 2-chloro-2-(trifluoromethyl)-3,6-dihydro-2H-tetrahydrothiopyran and 6-(polyfluoroalkyl)2H-thiopyrans. The reactions of the resulted 4,5-dibromo-2-chloro-2-(trifluoromethyl)tetrahydrothiopyrans and trans-3,4-dibromo-6-(polyfluoroalkyl)-3,4-dihydro-2H-thiopyrans with sodium acetate in acetic acid produced 2-(acetoxymethyl)-5-(polyfluoroalkyl)thiophenes. Base-catalyzed methanolysis of the acetoxymethyl derivatives allowed preparing the corresponding 2-(hydroxymethyl)-5-(polyfluoroalkyl)thiophenes. The mechanism, which includes formation and nucleophilic cleavage of thiiranium cations, has been suggested for the transformation of dibromothiopyran derivatives to substituted thiophenes. (C) 2015 Elsevier B.V. All rights reserved.