Dehalogenation of 1-halogenothienyl-di- and -tetra-hydroisoquinolines by sodium methoxide in dimethyl sulphoxide

On treatment with sodium methoxi-dimethylsulphoxide (NaOMe–DMSO) 1-(5-halogeno-2-thienyl)-6,7-dimethoxy-2-methyl-1,2,3,4-tetrahydroisoquinolines suffer loss of halogen and are converted into the related 1-hydroxytetrahydroisoquinolines. The reaction fails with comparable 1-bromophenyl- and 1 -(halogeno-3-thienyl)tetrahydroisoquinolines. A similar transformation takes place with (5-halogeno-2-thienyl)phenylmethoxymethanes, leading to the dimethyl acetal of the 5-dehalogenated-2-thienyl phenyl ketone. α-Halogenated-2- and 3-thienyl-3,4-dihydroisoquinolines undergo dehalogenation-aromatisation with NaOMe–DMSO. Mechanisms for these conversions are proposed.