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Chemistry of thienopyridines 32. direct introduction of c substituents gamma to the heteronitrogen atom in the thieno 2 3 b pyridine system


, : Chemistry of thienopyridines 32. direct introduction of c substituents gamma to the heteronitrogen atom in the thieno 2 3 b pyridine system. Journal of Heterocyclic Chemistry 21(4): 1135-1140

Treatment of the N-oxide 1a of thieno[2,3-b]pyridine (1) with either (a) acetic anhydride and ethyl cyanoacetate or (b) benzoyl chloride and an enamine of cyclohexanone (Hamana reactions) serves to introduce a C-substituent at the 4-position of 1. In case (a) a yellow, isolable vinylogous N-ylide 5 (23% yield) is obtained, which undergoes facile transformation into ethyl 2-(4-thieno[2,3-b]pyridyl)cyanoacetate (1e) (88-93%). Acetic anhydride reconverts 1e to 5 (95%). Case (b) produces 2-(4-thieno[2,3-b]pyridyl)cyclohexanone (39%), hydrolyzable to 6-(4-thieno[2,3-b]pyridyl)hexanoic acid (45%). The hydrochloride of 1a is also reported. Structural formulations are based on spectral studies (including 13C NMR data for 1 and 1e) and chemical transformations. Major contrasts (plus some similarities) between the systems 1 and quinoline in the Hamana reactions are discussed.

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