Azoles. Part 7. A convenient synthesis of thieno[2,3-d]imidazoles

Abstract

A number of 1-mono- and 1,2-di-protected 4-bromoimidazole-5-carbaldehydes, (7), (8), (11), and (13)–(15), reacted with ethyl thioglycollate in ethanol in the presence of sodium ethoxide to give the corresponding title compound, (16), (17), or (19)–(22), respectively (65–70% yield). Shorter reaction times allowed intermediates to be isolated, e.g.(13) gave (20)(69%) and (23)(12%). Several attempts to obtain the parent system, 1H- or 3H-thieno[2,3-d]imidazole, failed. Attempts to prepare the title compounds from imidazoles by several classical syntheses used for annelated thiophene systems (due to Tilak, Campaigne, and Krollpfeiffer) failed as did attempts to prepare imidazolethiols through chlorosulphonation or thiocyanation of imidazoles or via their reaction with 2,4-dinitrobenzenesulphenyl chloride. Several unsuccessful approaches to the title compounds from thiophenes are also described.