Some derivatives of [1]benzothieno[2,3-c]pyridine and [1]benzothieno[3,2-c]pyridine

Abstract

1,6-Disubstituted 3,4-dihydro[1]benzothieno[2,3-c]pyridine derivatives (I) have been prepared by Bischler–Napieralski cyclisation of the N-acetyl or N-benzoyl derivative of the appropriate 2-(5-substituted 3-benzo[b]thienyl) ethylamine. Attempts to prepare 3,4-dihydro-1-(3,4-dimethoxybenzyl)[1]benzothieno[2,3-c] pyridine and its 6-chloro-analogue by a similar method gave the 1-(3,4-dimethoxybenzoyl) derivative in each case. The 3,4-dihydro-derivatives (I) were reduced with sodium borohydride to the corresponding 1,2,3,4-tetrahydroderivatives (II), and were dehydrogenated with palladised charcoal to the fully aromatic compounds (III). The tetrahydro-compounds (II) could also be prepared by Pictet–Spengler reaction of the appropriate 2-(5-substituted 3-benzo[b]thienyl) ethylamine with either acetaldehyde or benzaldehyde.

2-(5,6-Methylenedioxy-2-benzo[b]thienyl)ethylamine and the 5-methyl analogue were prepared by reaction of the appropriate 2-carbaldehyde with nitromethane, and reduction (LiAlH₄) of the resulting 2-nitrovinyl compound. Bischler–Napieralski cyclisation of the N-acetyl or N-benzoyl derivatives of these amines did not give the expected 3,4-dihydro[1]benzothieno[3,2-c]pyridines; instead, the dehydrogenated products (V) were obtained.