The Stobbe condensation. Part VIII. The cyclisation of trans-3-methoxycarbonyl-4-(2-thienyl)but-3-enoic acid and αβ-dithenylidene-succinic anhydride to the corresponding benzothiophen derivatives
The condensation of thiophen-2-carbaldehyde with dimethyl succinate in the presence of potassium t-butoxide or sodium hydride gave predominantly trans-3-methoxycarbonyl-4-(2-thienyl)but-3-enoic acid, whose configuration is revealed by cyclisation with sodium acetate in acetic anhydride to the corresponding benzothiophen derivatives. Alcoholysis of the derived trans-3-caboxy-4-(2-thienyl)but-3-enoic anhydride yields the half-ester which is isomeric with the above mentioned half-ester. Condensation also gave the αβ-dithenylidenesuccinic acid in small amounts; its ratio depending upon the nature of the condensing agent. The derived anhydride yields on pyrolysis 4-(2-thienyl)benzothiophen-5,6-dicarboxylic anhydride.