Tandem radical cyclizations on iodoaryl azides:: Synthesis of the core tetracycle of Aspidosperma alkaloids

A new stereoselective approach to the tetracyclic core of Aspidosperma, alkaloids is described. Selective attack by tristrimethylsilylsilyl radicals on the aryl carbon-iodine bond of iodoaryl azides was first demonstrated on the simple model 15, thus both extending the recent discoveries of Kim and co-workers on aliphatic C-I bonds and demonstrating that the selectivity can be exploited in cascade radical cyclizations. Extension to the more complex substrate 25 afforded the core ABCE tetracyclic skeleton of the alkaloids in excellent yield and with efficient control of relative stereochemistry.