Fries rearrangement of some 3-acetoxy- and 3-propionyloxy-thiophenes

Abstract

The Fries rearrangement of ten 3-alkanoyloxythiophenes has been studied in dichloromethane using aluminium chloride as catalyst. An intermolecular component of the mechanism has been demonstrated by the observation of all possible mixed products in approximately equal proportions in a crossover experiment. 3-Alkanoyloxythiophenes were prepared from the corresponding 3-hydroxythiophenes and the rearrangement proceeded generally at ambient temperature to give 3-hydroxy-2-alkanoylthiophenes in good yields. This synthetic route provides a useful alternative to the Friedel-Crafts alkanoylation. The structures of both the acyl and 3-thiophenoxy moieties were found to exert an influence on the rearrangement. Acetyl esters rearranged at a faster rate than propionyl esters. An ester or cyano group in the 4-position did not interfere with the rearrangement whereas an acetyl group prevented it; two ester groups in the thiophene ring also prevent rearrangement occurring yielding 3-hydroxythiophenes in almost quantitative yield.