Rearrangement Reactions in Aza‐Vinylogous Povarov Products: Metal‐Free Synthesis of C3‐Functionalized Quinolines and Studies on their Synthetic Application

Abstract

Several types of C4‐functionalized 4‐alkyl‐2‐aryl‐1,2,3,4‐tetrahydroquinolines underwent rearrangement of their functional groups to C3, with concomitant aromatization, by simple reflux in 1,2‐dichlorobenzene. The functional groups that were shown to undergo the C4 to C3 migration were –CH=CH‐Z (where Z = CO2Et, CN, NO2, COCH3, CH2OH) and –CH=C(Y)–Z (where Y = CN and Z = CO2Et or Y = Z = CN). On the other hand, the dimethylhydrazono group failed to migrate under thermal conditions but was shown to undergo a smooth dehydrogenation/C4 to C3 rearrangement/ dehydrogenation sequence at room temperature in the presence of DDQ, with a broad scope that includes 4‐alkyl‐2‐aryl‐ and 2‐acyl‐1,2,3,4‐tetrahydroquinolines. We also report a computational and experimental study of the mechanism of both reactions, which supports an unusual intramolecular aza‐ene pathway. The ready availability by this method of 2,3‐difunctionalized quinolines allowed the simple preparation of fused heterocyclic systems derived from the pyrrolo[3,4‐b]quinoline framework, using both reductive and non‐reductive domino processes.