Preparation and properties of some 3-acetyl- and 3-formyl-5-halogenobenzo[b]thiophens

Abstract

5-Chloro- and 5-bromo-benzo[b]thiophen-3-carbaldehydes were prepared from the corresponding 3-bromomethyl compound, either by the Sommelet reaction (65% yield) or by the Kröhnke reaction (30% yield). They had the usual properties of an aromatic aldehyde; in particular, they underwent the Doebner reaction with malonic acid, and formed crystalline condensation products with rhodanine.

Friedel–Crafts acetylation of 5-chloro- or 5-bromo-benzo[b]thiophen gave predominantly the 3-acetyl derivative. This reacted with dimethylamine hydrochloride and formaldehyde (Mannich reaction) to give the corresponding keto-amine, the keto-group of which was reduced with sodium borohydride. The resulting alcohols were treated with thionyl chloride to give the corresponding substituted 3-chloropropylamines.