Indole–Indole Ullmann Cross‐Coupling for CAr–N Bond Formation: Total Synthesis of (–)‐Aspergilazine A

Abstract

The copper‐catalyzed cross‐coupling reaction of indole and protected haloindoles (X = Br, I) by using CuI, N,N′‐dimethylethylenediamine, and K2CO3 in dioxane at 130 °C afforded the corresponding bis‐indole products in generally high yields. Starting from the N1′–C6 bis‐indole positional isomer obtained by CAr–N Ullmann coupling, a new synthesis of (–)‐aspergilazine A was completed. A bidirectional Negishi‐catalyzed cross‐coupling of the C3,C3′‐diiodo N1′–C6 bis‐indole derivative was used to construct the corresponding bis‐tryptophan. Amide formation with l‐proline and thermally induced deprotection/cyclization completed the diketopiperazine units of (–)‐aspergilazine A.