Thieno[2,3-b][1]benzothiophen and thieno[3,2-b][1]benzothiophen. Part III. Substitution reactions of derivatives with substituents in the thiophen ring

Abstract

3-Methylthieno[2,3-b and 3,2-b][1]benzothiophen undergo bromination, Vilsmeier–Haack formylation, and Friedel–Crafts acetylation in the 2-position. The corresponding 2-methyl compounds are brominated in the 3-position. The course of the acetylation of 2-methylthieno[2,3-b][1]benzothiophen depends on the reaction conditions: either the 3- or the 6-acetyl compound may be the major product. We could not distinguish unambiguously between the 6-acetyl compound and the 5-substituted isomer by n.m.r. spectroscopy. However, authentic 5-acetyl-2-methylthieno[2,3-b][1]benzothiophen and the corresponding 5-carboxylic acid were prepared, and they were different from the acetylation product and its hypohalite oxidation product respectively. When 2-methylthieno[3,2-b][1]benzothiophen was acetylated by use of the two different sets of conditions used for the [2,3-b]-isomer, the 3-acetyl, 6-acetyl, and 3,6-diacetyl compounds were formed in each case. 2,3-Dimethylthieno[2,3-b or 3,2-b][1]benzothiophen each underwent acetylation in the 6-position; monobromination of the [2,3-b]-isomer gave the 3-bromomethyl compound, but bromination was not confined entirely to the side-chain for the [3,2-b]-isomer. However, dibromination of either isomer gave the 6-bromo-3-bromomethyl compound. Bromination of thieno[2,3-b or 3,2-b][1]benzothiophen-3-carbaldehyde readily gave the 2-bromo-compound. Nitration of the corresponding 2-carbaldehydes caused replacement of the formyl substituent, to give the 2-nitro-compound as the major product; bromination gave mainly 2,3-dibromothieno[2,3-b][1]benzothiophen or 2,6-dibromothieno[3,2-b][1]benzothiophen.