A Practical Synthesis of Indole-Based Heterocycles Using an Amidoaluminum-Mediated Strategy

A large number of biologically active compounds consist of an indole scaffolding. Because of this, chemists are continually searching for more efficient means through which to successfully synthesize the required alkaloids. In our recent effort to synthesize indole-based p38 inhibitors and gramines, we found that a series of indole-based indole-3-carboxamides could be efficiently synthesized from various indole-3-carboxylates using an amidoaluminum-mediated strategy. The treatment of ethyl indole-3-carboxylates bearing a range of substitution patterns on the indole ring with various amidoaluminum complexes, led to the corresponding 1H-indole-3-carboxamides in yields up to 75%. Reduction by diisobutylaluminum hydride afforded the corresponding gramines in 63-85% yield. This is the first reported example of amidoaluminum complexes of type Al2(CH3)4(NR2)2 promoting facile amidation of relatively inert indole esters. This particularly promising approach has resulted in the first strategy for generating medicinally important alkaloids of this type.