The allylation of indigo results in the one-step synthesis of two unique complex heterocyclic systems: a spiroindoline–pyridoindolone arising from the addition of three allyl moieties and a fused pyridoindolo-azepinoindolone generated from the addition and subsequent cyclisation of two allyl moieties. The structures of these novel heterocycles are assigned unambiguously using extensive NMR experiments and by X-ray crystallographic analysis. The distribution of the products is influenced by the use of thermal versus microwave heating.
Abdel-Hamid, M. K., Bremner, J. B., Coates, J., Keller, P. A., Milander, C., Torkamani, Y. S., ... Willis, A. C. (2009). Novel spiro fused heterocycles from the allylation of indigo. Tetrahedron Letters, 50, 6947-6950. https://doi.org/10.1016/j.tetlet.2009.09.098