Heating with excess thionyl chloride and a catalytic amount of pyridine gives a benzo[b]thiophene derivative, formed by electrophilic addition of thionyl chloride across the double bond of cinnamoyl chloride, Pummerer rearrangement of the resulting sulfinyl chloride and final cyclization. ɑ-Chlorocinnamoyl chloride is formed as a by-product by elimination of SCl2: J. Org. Chem., 40, 3037 (1975). Under similar conditions, 3-phenylpropionic acid gave the same products in lower yield; the reaction was thought to occur via an initial ɑ-enolization of the acid chloride: Tetrahedron Lett., 5149 (1968).
