Direct bromination of ethyl 5-alkylthiophene-2-carboxylates

Approaches to brominated thiophene-2-carboxylic acids by electrophilic bromination of the corresponding acids and esters were compared and investigated. A synthetic route was developed involving direct bromination of ethyl 5-alkylthiophene-2-carboxylates followed by saponification of the resulting ethyl 5-alkyl-4-bromothiophene-2-carboxylates. The key bromination step is selective in dichloromethane solution at 0-5 °C and furnishes the corresponding ethyl 5-alkyl-4-bromothiophene-2-carboxylates in excellent yields. No migration or isomerization of the alkyl substituents was observed. © Georg Thieme Verlag Stuttgart New York.