Cyclization strategies for the synthesis of macrocyclic bisindolylmaleimides

Three new approaches to the synthesis of macrocyclic bisindolylmaleimides 1-4 have been identified. Two strategies afford 8, the penultimate intermediate for the synthesis of 1-4, in 73% and 32% yield by intramolecular cyclization of 31 and 40, respectively. The optimum synthesis of 1 was achieved in nine steps and 15% yield by intramolecular formation of the macrocycle and maleimide in one step by reaction of the sodium indolate of 12 with methyl indole-3-glyoxylate 47. The mechanism of this reaction has been elucidated, using the trityl-protected derivative, to involve initial formation of the tricarbonyl imide 48, followed by irreversible alkylation of the indole nitrogen to generate the 17-membered macrocycle 49. Cyclization of 49 to hydroxymaleimide 50 and subsequent dehydration afforded 8a. This approach eliminated the problem of dimerization observed in the intramolecular cyclization reactions.