alpha-phenylpropionic acid derivatives. Synthesis and dethiation of 5-benzoylbenzo[b]thiophene-3-carboxylic acid

The total synthesis of the title compound 8 started with p-thiocresol which was acylated with oxalyl chloride to give compound 1 . This product underwent a condensation reaction with chloroacetic acid under basic conditions yielding compound 2 . Two different synthetic pathways were used to convert compound 2 into the title compound 8 . The first consisted in decarboxylation of 2 to 3 , which was then converted to the ester 4 , which was brominated and the product 5 was subjected to a Friedel-Craft's reaction with benzene. The resulting benzyl derivative 6 was oxidized to the benzoyl stage i.e. compound 7 , which was finally hydrolyzed to 8 . The second pathway was similar to the first one so that the steps of esterification, bromination, Friedel-Craft's alkylation and oxidation started with the dicarboxylic acid 2 . Thus compounds 12–16 were obtained, and the last product was decarboxylated to 8 . The yields in both procedures were similar. Finally, the dethiation of compound 8 with Raney nickel afforded compounds 18, 19 and 20 .