Concise and diversity-oriented synthesis of ligand arm-functionalized azoamides.

Azoamides, previously established as bioactive intracellular GSH-depleting agents, were decorated with a terminal alkyne moiety to 4 and then were transformed, by copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), into different ligand-arm functionalized azoamides 6. Azides 5 having ligand-arms amenable for binding to platinum(II) were selected for this study. Because, for the fragile azoamides 4, the typically employed reaction conditions for CuAAC failed, several alternative solvents and copper catalysts were tested. Excellent results were obtained with copper(II) sulfate pentahydrate/metallic copper and especially with heterogeneous catalysts, such as copper-in-charcoal, cupric oxide, and cuprous oxide. The heterogeneous catalysts were employed to obtain the desired products in almost quantitative yields by a simple three-step "stir-filter-evaporate" protocol with no or negligible contamination with copper impurities. This is of particular importance because compounds 6 have been designed for coordination.