Azoles. Part 12. Synthesis of thieno-extended purines: First example of the imidazo[4′,5′:4,5]thieno[3,2-d]pyrimidine ring system

Article abstract of DOI:10.1016/0040-4020(95)00845-Y

1-Benzyl-4-bromoimidazole-5-carbonitriles were synthesised by several methods and shown to be surprisingly resistant to nucleophilic displacement of the bromine atom (e.g. by sulfanylacetamide under various conditions). 1-Benzyl-4-bromo-2-(memylsulfanyl)imidazole-5-carbonitrile underwent Br → Li exchange with butyllithium (THF/-100°C) and the resulting 4-lithiated imidazole was treated successively with elemental sulfur and chloroacetamide, to give 2-[1-benzyl-5-cyano-2-(methylsulfanyl)-imidazol-4-ylthio]acetamide. With sodium ethoxide in ethanol this cyclised, to give 6-amino-1-benzyl-2-(methylsulfanyl)thieno[2,3-d]imidazole-5-carboxamide which gave the first reported derivative of the title compound on treatment with triethyl orthoformate in acetic anhydride. 1-Benzyl-4-bromoimidazole-5-carbaldehyde was synthesised from 1-benzyl-2,4,5-tribromoimidazole by its initial conversion into a 2-TBDMS-protected derivative followed by introduction of the 5-formyl group via Br → Li exchange and an N-formylpiperidine quench of the resulting 5-lithiated imidazole and, finally, removal of the 2-TBDMS-group (Bu4NF/THF).