Imidazoquinolinethiones from 8-aminoquinolines by a novel peri-participation

8-Aminoquinolines and 4,5-dichloro-1,2,3-dithiazolium chloride 1 give N-(quinolin-8-yl)iminodithiazoles 6 and 15a-c which undergo a novel thermal rearrangement to give imidazo[4,5,1-ij]quinoline-4-thiones 13 and 16a-c respectively. Normal cyclisation of the dithiazolo group onto the carbocyclic ring to form a benzothiazole (e.g. 6 --> 5) is averted by peri-participation of the quinoline ring nitrogen. This participation results in formation of the imidazole ring and delivery of a sulfur atom to the quinoline 2-position; delivery of this sulfur appears to be intramolecular and possibly involves a [1,3] sigmatropic shift of a carbon-sulfur bond (Scheme 4). The same overall reaction is observed, at much lower temperature, on treatment of the quinolinyliminodithiazoles (15a and c) with sodium hydride in THF (Scheme 5), thus providing a ready route to imidazo[4,5,1-ij]quinoline-4-thiones. These thiones are rapidly oxidised to the corresponding 4-ones, such as 25; 25 was also formed, rapidly and quantitatively, from the analogous iminodithiazole derivative 23 of 8-aminoquinolin-2-one in boiling ethanol (Scheme 6). Mechanisms are proposed for all the new rearrangements reported.