Thieno[2,3-b][1] benzothiophen and thieno[3,2-b][1]benzothiophen. Part I. Preparation

Abstract

Benzo [b]thiophen-2-thiol and its 5- and 7-chloro-derivatives were prepared by treatment of the approriate 2-benzo[b]thienyl-lithium derivative with sulphur. Benzo [b]thiophen and its 5-bromo- and 7-chloro-derivatives were sulphonated in the 3-position. The resulting sulphonic acids were converted into the sulphonyl chlorides, which were reduced (LiAlH₄) to the corresponding 3-thiols. The 2- and 3-thiols thus prepared condensed readily with chloroacetaldehyde diethyl acetal, chloroacetone, or 3-chlorobutan-2-one, and the products were cyclised with polyphosphoric acid to give thieno[2,3-b or 3,2-b][1] benzothiophen derivatives. Thieno[2,3-b][1]benzothiophen-2-carboxylic acid, its 5-bromo-derivative, and thieno[3,2-b][1]benzothiophen-2-carboxylic acid were prepared by cyclisation of the appropriate β-(3- or 2-benzo[b]thienyl)-α-mercaptoacrylic acid with iodine in either tetrahydrofuran or nitrobenzene. The formation of thieno[2,3-b or 3,2-b][1]benzothiophen SS-dioxides probably involves oxidation of the sulphur atom adjacent to the benzene ring in each case.