Reformatsky reactions with N-arylpyrrolidine-2-thiones: synthesis of tricyclic analogues of quinolone antibacterial agents

A convenient synthesis of 5-oxo-1,2,3,5-tetrahydropyrrolo[1,2-a]quinoline-4-carboxylic acids, tricyclic analogues of the quinolone antibiotics, is described. Key steps in the route are a novel zinc-mediated Reformatsky reaction between diethyl bromomalonate and N-arylpyrrolidine-2-thiones 18, and cyclisation of the resulting diethyl pyrrolidinylidenemalonate intermediates 19 in polyphosphoric acid. The products proved to be devoid of biological activity.