Condensed thiophen ring systems. Part 20. Synthesis of 5-arylthieno-[3,2-b]pyrroles and 5-arylthieno[3,2-c]pyrazoles

Abstract

Reductive cyclisation of the alkenes (5)–(8)(prepared by condensation of methyl 2-methyl-3-nitrothiophen-5-carboxylate with an aromatic aldehyde) with triethyl phosphite gave thieno[3,2-b]pyrroles (1)–(4). Thieno[3,2-c]pyrazoles (12)–(14) were prepared similarly from the anils (15)–(18) or the nitrones (22) and (23). Condensations between alkyl 2-methyl-3-nitrothiophen-5-carboxylates or 2-methyl-3-nitrobenzo[b]thiophen and NN-dimethyl-4-nitrosoaniline have been shown to give mixtures of nitrones [(22), (23), and (27)(major products), respectively] and the corresponding anils [(16), (17), and (26)]. A procedure is given for alkylation of 2-formyl-3-nitrothiophen-5-carboxylic acid (19) which avoids deformylation and involves formation of its sodium or potassium salt with an alkali metal hydrogen carbonate in hexamethylphosphoramide followed by addition of an alkyl iodide.