An investigation of the allylation cascade reactions of substituted indigos

Abstract

In a continuation of the exploration of indigo cascade reactions, a series of –OMe, –Ph, –Br and –NO₂ substituted indigos 1a–i were synthesised to probe electronic effects upon the outcome of allylation cascade reactions. When indigos 1a–i in the presence of base were reacted with allyl bromide, spiroindolinepyridoindolones 17–25 (36–75%) were obtained as the major products in each case, marking a shift in outcome relative to that previously reported for unsubstituted indigo. In electron-rich derivatives (–OMe, –Ph), C-allylspiroindolinepyridoindolediones 26–29 (3–11%) were also isolated, which are most likely formed via a Claisen rearrangement of the respective spiroindolinepyridoindolones 18–21. Additionally, the isolation of diallylbiindolone 16, oxazinobiindole 30 and N,N′-diallyl-3,3′-bis(allyloxy)biindole 31 each represented novel polyheterocyclic derivatives, providing intriguing new mechanistic insights, reaction pathways and in the case of 30 the first common heterocyclic skeletal outcome shared in both allylation and propargylation cascade reactions of indigo.