Synthesis of unsymmetrical arylthiophenes and bithienyls via oxidative cyclization of 1,3-butadiene-1-thiols

Oxidative cyclization of 2-mercapto-5-aryl-2,4-pentadienoic acids with iodine produced the respective 5-aryl-2-thenoic acids. The method was also suitable for the synthesis of unsymmetrical substituted 2,2′-bithienyls, and 2,3′-bithienyls. The synthesis of 5-carboxy-2′-bromo-5′-phenyl-2,3′-bithienyl from benzal-acetone demonstrated that oxidative cyclization of 1,3-butadiene-1-thiols is a useful procedure for preparing 5′-aryl-2,3′-bithienyls from simple carbonyl compounds. Preferential ring bromination of 2-phenyl-4-methyl-thiophene with N-bromosuccinimide and a radical catalyst was also observed.