Indium-Mediated Atom-Transfer and Reductive Radical Cyclizations of Iodoalkynes:  Synthesis and Biological Evaluation of HIV-Protease Inhibitors

Novel indium-mediated radical cyclization reactions of aliphatic iodoalkynes have been studied. Treatment of iodoalkynes with a catalytic amount of In (0.1 equiv) and I2 (0.05 equiv) promotes atom-transfer 5-exo cyclization to give five-membered alkenyl iodides. In contrast, reaction with In (2 equiv) and I2 (1 equiv) yields reductive 5-exo cyclization products via the same 5-exo cyclization. Both processes are most likely initiated by low-valent indium species. To demonstrate versatility of these reactions, optically active HIV protease inhibitors were synthesized by this reductive cyclization method. Among them, several products, which contain a hydroxyethylamine dipeptide isostere as a transition state-mimicking substructure, proved to possess potent activity (IC50 = 5−39 nM) against a wide spectrum of HIV strains, including multidrug-resistant variants.