Allylic and Retro-Allylic Rearrangements upon Bromination of 8,9-Substituted 4,4,6-Trimethyl-4H-Pyrrolo[3,2,1-ij]Quinoline-1,2-Diones. New Aspects and Synthetic Applications

Bromination of 4,4,6-trimethyl-4H-pyrrolo[3,2,1-ij]quinoline-1,2-diones containing an allylic moiety with N-bromosuccinimide (NBS) in carbon tetrachloride in the presence of benzoyl peroxide (BPO) and in DMF solution were studied. When using an equimolar amount of NBS in the BPO-CCl4 system, the reaction proceeds regioselectively and standardly at the methyl group at position 6 with the formation of 6-bromomethylene derivatives. In the NBS-DMF system, monobromination proceeds at position 5 with the migration of a multiple bond by the allylic rearrangement. With a twofold excess of NBS, in both cases a (Z)-5-bromo-6-bromomethylene derivative is formed. The 5-monobromine isomers undergo retro-allylic rearrangement in N- and S-alkylation reactions, resulting exclusively in 6-N-alkyl and 6-S-alkyl derivatives, respectively. Similar products are formed by the same reactions with the use of 6-bromomethylene derivatives.